EXPLORING

E-Activity 17

2008-06-26T23:08:00.001-07:00

The relay switch

2008-06-25T05:44:00.000-07:00

Sometimes it is useful to be able to control the current flowing in one circuit by using a second circuit. This is especially true if the current flowing in the first circuit is large. This can be done by using a relay switch.
In figure 6, when switch A is closed, the iron core of the coil becomes magnetized and attracts the iron armature. As the end of the armature (B) is attracted, its other end pushes the wires at C together, turning the second circuit on.
If switch A is opened, the electromagnet loses its magnetism, the armature returns to its original position and the second circuit is turned off.

The electric bell

2008-06-25T05:42:00.000-07:00

When the button is pressed, current flows around the circuit and the electromagnet at E becomes magnetized. The striker arm is attracted towards it and the striker hits the bell. While this is happening, a gap appears at the contacts. The circuit is now incomplete, the electromagnet loses its magnetism and the striker arm moves back to its original position.
The whole process then begins again. The bell continues to ring as long as the button is pressed.

Electromagnetism

2008-06-25T05:37:00.000-07:00

When a current flows through a wire, a circular magnetic field is created around it. This field can be seen using either iron filings or compasses.
The field only exists when the current is flowing. If it is turned off, the field disappears if the current flows in the opposite direction, the field changes direction.
Field strength
The field created when current flows in a single piece of wire can be quite weak, but its strength can be increased by
• Increasing the current through the wire,
• Increasing the number of pieces of wire, i.e. making the wire into a coil.

Field shape
The magnetic field around a long coil of current-carrying wire is the same shape as that around a bar magnet.
Electromagnets
If a piece of iron is inserted down the centre of a long coil of wire, this increases the strength of its magnetic field. This combination of iron core and coil is known as an electromagnet. When current flows around the coil, the iron becomes magnetized. When current stops flowing, the iron loses its magnetism
Electromagnets are very useful because
• They can be turned on and off,
• They can be made stronger and weaker.

Magnetic shielding

2008-06-25T05:36:00.000-07:00

When the magnet below the wooden table in figure 14 is moved, the footballer above is made to move. If you tried to play this game on a table with a steel or iron top, moving the magnet would have no effect on the footballer. Magnetism is able to pass through non-magnetic materials such as wood, but cannot pass through magnetic materials such as iron or steel. Magnetic materials are often used to shield objects such as sensitive electrical circuits from stray magnetic fields.

Magnets and Magnetism

2008-06-25T00:49:00.000-07:00

Making a magnet
If a steel rod is stroked 15-20 times with one end of a bar magnet, the domains within the rod can be made to line up so that they are all pointing in the same direction. The rod is then magnetized.
Magnetic fields
When an object made of iron or steel is placed close to a magnet, it is attracted towards it. This happens because the object is inside the magnet’s magnetic field.
Discovering the shape of a magnetic field
1. Using iron filings
A piece of paper is placed over the magnet. Iron filings are gently sprinkled over the paper. The pattern formed by the filings shows the shape of the magnetic field.
2. Using a compass
A magnet is placed on top of a piece of paper and drawn around. A compass is placed next to the magnet, a circle drawn around it and the direction of the compass needle marked inside the circle. The compass is then moved so that its tail is next to the compass point just drawn. A circle is again drawn around the compass and its direction recorded. This process is repeated all over the paper. The compass needles on the paper will show the shape and direction of the magnetic field.
Magnetic field pattern around a bar magnet
Both methods show that the field pattern around a bar magnet is as given in figure 11Where the field is strong, the lines are drawn close together. Where the field is weak, the lines are drawn well apart.
The earth’s magnetic field
If a bar magnet is suspended so that it is free to rotate, it will eventually come to rest with its north pole pointing northwards and its south pole pointing south wards. The magnet is therefore behaving like a simple compass. Magnets and compasses do this because they are inside the earth’s magnetic field.
It is this magnetic field which has allowed travelers to navigate using a compass.

The domain theory of magnetism

2008-06-25T00:30:00.000-07:00

We believe that magnetic materials such as iron and steel have inside them small molecular magnets. These small magnets are contained in tiny cells called domains. Within each domain all the molecular magnets point in the same direction.

Magnets and Magnetism

2008-06-25T00:25:00.000-07:00

Magnetic materials
Magnets are able to attract objects which are made from certain materials, e.g. iron, steel, nickel and cobalt. These are called magnetic materials.
Non-magnetic materials
Magnets are unable to attract objects which are made from materials such as paper, plastic and copper. These are called non-magnetic materials.
Poles of a magnet
If iron filings are sprinkled over a bar magnet or a horseshoe magnet, most of them will stick to the two ends. These are the strongest parts of the magnets and are called the poles. Magnets have two poles, a north pole and a south pole.
Attraction and repulsion between poles
If two opposite poles are placed close together they attract
If two similar poles are placed close together they repel

Magnetism 1

2008-05-10T06:21:00.001-07:00

Danger from sparks

2008-05-08T21:41:00.000-07:00

Making a spark
Look at the diagram. The belt on the electrostatic generator transfers charge to the dome. As the belt turns, the charge on the dome increases.
1. Copy and complete the sentences.
The greater the charge on the dome, the ____________ the potential difference between it and ____________.
When the potential difference is high enough, a _________ jumps through the air to a nearby ___________.
Sparks big and small
Static electricity can produce sparks. Often these sparks are very small. But sometimes they are huge.
2. Look at the pictures.
television screen,lightning and taking off a sweater
Then complete the following
Size of sparks (large or small)and How the static charge is produced in each case.

Preventing explosion at flour mills
Flour is made by grinding wheat to a very fine powder. The bits of flour make the air very dusty. This mixture of flour and air can be very dangerous. Just a tiny spark can make it explode.
3. Look at the diagrams.
There is a danger of a spark between the pipe and the container. Explain why.
4. How can such a spark be avoided?
Fire hazards with petrol tankers
Petrol is very flammable. A tiny spark can cause a serious fire when petrol or petrol vapor is open to the air.
At filling stations, petrol is transferred from tankers to underground tanks. There must be no sparks while this is being done.
5. In what two ways can petrol tankers produce static electricity?
6. Why does static electricity create a hazard when pumping petrol?
7. Write down two things that are done to prevent sparks.

Handle chips with care
Technicians who put electronic chips into circuits have special mats on their workbenches. These mats conduct electricity and are connected to earth. The technicians sometimes also wear wrist straps connected to earth.
8. Why are these precautions needed?
9. Explain how these precautions prevent damage to microchips.

E-Activity 16

2008-05-08T21:35:00.000-07:00

Making use of static electricity

We can use static electricity in many different ways to do useful jobs.
1. Write down two different ways that static electricity is often used in schools and factories.
How a photocopier works
We take photocopiers for granted, but schools and offices could not manage without them. The diagrams above show, step by step, how a photocopy is made.
How to remove the dirt from smoke
Smoke contains lots of tiny bits of dirt. This dirt falls on houses and gardens. If people breathe in the dirt, it can damage their lungs.
The diagram shows how the dirt can be removed from factory chimneys using a smoke precipitator.
3. Copy and complete the sentences.
A metal grid and antenna are connected to the positive side of a high _______ supply.
The metal chimney is connected to the ______ side of the supply.
Tiny bits of dirt pick up a _______ charge as they pass through the grid. They are then repelled by the antenna and _____ by the chimney.
So the dirt collects (precipitates) on the inside of the __________ and is removed every so often.
How to make a spray hit its target
When a liquid is sprayed out of a nozzle, it becomes electrically charged. Sprays can be made to charge the droplets as much as possible. The diagrams show how this helps the spray to find its target.
4. Explain why a pesticide spray works better if it charges the droplets of pesticide.
5. Car makers want most of the paint from a spray to end up on a car body. What can they do to the car body to make sure this happens?

E-Activity 15

2008-05-08T21:34:00.000-07:00

1. When you rub two different materials together, they become charged. Why?
2. What type of electrical charge does each electron carry?
3. Copy and complete the sentence.
When you rub a piece of polythene with a cloth, you make electrons move from the ____________ to the _______________.
4. Explain the following, as fully as you can.
(a) The polythene ends up with a negative charge.
(b) The cloth ends up with a positive charge.
5. The negative charge on the polythene is exactly equal to the positive charge on the cloth. Why is this?
6. (a) What type of charge do you get on a cloth when you rub apiece of acetate with it?
(b) What happens to the electrons as you rub the acetate with a cloth?
7. Copy and complete the sentence.
When you peel different materials apart, one of them becomes__________ charged and the other becomes _____________ charged.
8. When you peel some sticky tape from a roll, it often tends to go back on again. Explain, as fully as you can, why it does this.

Why rubbing things together produces electricity

2008-05-08T21:30:00.000-07:00

It’s all down to electrons
When you rub two different materials together, electrons are rubbed off one material and onto the other. These electrons are very tiny. Each electron carries a small electrical charge.
If you rub two pieces of the same material together, electrons don’t move.
The materials that you rub together must not only be different. At least one of them, and usually both of them, must be an electrical insulator.

The diagrams show what happens when you rub polythene with a cloth and when you rub acetate with a cloth.
Peeling two different materials apart has the same effect as rubbing them together. Electrons are transferred from one material to the other.

E-Activity 14

2008-05-08T21:28:00.001-07:00

1. Combing your hair can produce static electricity.
(a) Why does this happen?
(b) Write down two ways in which you can tell when it happens.
2. Copy the sentences. Then use the words attract and repel to complete them.
Two charged strips made from the same plastic __________each other.
A charged polythene strip and a charged acetate strip ______________ each other.
Both charged strips will also ___________ light objects that do not have a charge.
Charged objects sometimes attract and sometimes repel. This means that there must be different types of electrical charge.
3. Copy and complete the sentences.
Objects that have the __________ type of electrical charge repel each other. So if two charged objects attract each other they must have ___________ electrical charges.
4. You can use an electrostatic machine to make hair really stand on end. Explain how this happens, as fully as you can.

electrostatic induction

2008-03-12T16:00:00.002-07:00

Static Electricity Detection by Water

2008-03-12T16:00:00.001-07:00

What is Static Electricity ? ( Animation )

2008-03-01T19:37:00.001-08:00

Electricity that can make your hair stands on end:

2008-02-27T04:47:00.000-08:00

Electricity doesn’t always flow through circuits. Electricity can also stay just where it is. This is called static electricity.
You can produce static electricity by rubbing together two different materials. We say that we have charged the materials with electricity.

Is all static electricity the same?

You can rub two strips of plastic with a cloth. This charges them with static electricity. The diagrams show what the charged strips of plastic will then do.
Charged objects sometimes attract and sometimes repel. This means that there must be different types of electrical charge.

Two types of charge

Two polythene strips that are rubbed with the same cloth must have the same kind of charge. These charges repel. Two acetate strips rubbed with the same cloth must have the same kind of charge. These charges also repel.
Here is a simple way to remember what happens:
Like charges repel, unlike charges attract.
The charge you get on a polythene strip when you rub it is called negative (-).
The charge you get on an acetate strip when you rub it is called positive (+).

STATIC ELECTRICITY

2008-02-25T21:33:00.000-08:00

ELECTRIC CHARGE:

You may have noticed that after combing your hair on a fine, dry day, the comb can attract small pieces of paper. Have you also ever experienced a mild electric shock when you touched a metal door knob in an air-conditioned room?

In general, when two different materials (especially insulating materials) are rubbed against each other, negative charges (Electrons) will transfer from one object to the other so that one object becomes positively charged and the other becomes negatively charged.

For instance, when a polythene strip is rubbed with wool the friction causes a certain amount of negative charge (electrons) to be transferred from the wool to the polythene strip. Hence the strip becomes negatively charged. On the other hand, the wool has lost the same amount of electrons and therefore become positively charged. This method is known as charging by friction.

What Factors Affect the Resistance of a Wire? (2)

2008-02-19T19:53:00.000-08:00

Nicola Tregay
Mill Chase School, Bordon, Hampshire, UK

http://www.sci-journal.org/index.php?template_type=report&id=26&htm=reports/vol3no1/../vol3no2/v3n2a3.html&link=reports/home.php

Hypothesis:

Does the Diameter of the wire affect the Resistance?
Ohms law states that if the cross section of the wire is uniform then the resistance is proportional to the length and inversely proportional to the area of the cross section. The diameter of the wire will have an affect on the resistance of a wire. This is because the electrons have to squeeze together more to pass through a thin wire than they do to pass through a thick wire.
I can predict that the thinner the wire the higher the resistance. If I had a piece of wire 1 mm in diameter and a piece of wire 2 mm in diameter, how much would the resistance increase by? Would the resistance double because the diameter has doubled? Would it increase four times because the cross section has increased by four? Or does it do something else?
π r2 is how to calculate the area of the cross section.
0.52 = 0.25, 0.25* π = 0.79cm2 (Area of cross section of a 1mm wire)
12 = 1, 1* π = 3.14cm2 (Area of cross section of a 2mm wire)
I will need 5 x wires, an ammeter, a voltmeter, 30cm of 32swg nichrome wire, 30cm of 26swg nichrome wire, 30cm of 22swg nichrome wire, 2 crocodile clips, and a power pack.
The diameters of the wires are:
32swg = 0.28mm
26swg = 0.45mm
22swg = 0.71mm
The area of the cross section of each of these are:
0.142 = 0.0196* π = 0.06cm2 (32swg)
0.2252 = 0.05* π = 0.16cm2 (26swg)
0.3552 = 0.126* π = 0.39cm2 (22swg)
To make this test fair I will keep the length of the wire the same and I will also keep the voltage the same. I will do the same as I did before except this time I will be using different wires.

The Plan

1. Collect apparatus: A voltmeter, an ammeter, 5 x wires, 2 crocodile clips, 30cm 32swg nichrome wire, 30 cm 26swg nichrome wire, 30 cm 22swg nichrome wire and a power pack.
2. Set the apparatus up as shown.
3. the power pack on as a low a setting as possible. (So that the volts aren't too high for the voltmeter to read.)
4. Place the 32swg nichrome wire between the two crocodile clips to complete the circuit.
5. Replace the 32swg nichrome wire with the 26swg nichrome wire. Remember to keep the voltage the same, or as near as you can get it.
6. Record what both the ammeter and voltmeter say.
7. Change the wire to the 22swg nichrome wire and repeat the experiment.
8. Work out the resistance for all your results by using Ohms law.
V=I*R or R=V/I
9. Record the results in a graph.

Results

Nichrome Wire
Diameter/mm Voltage/volts Current/amps Resistance/ohms
0.28 0.6 0.14 4.29
0.45 0.6 0.45 1.33
0.71 0.6 0.74 0.81

Conclusion

My hypothesis was correct. The thinner the wire, the higher the resistance and the thicker the wire, the lower the resistance. This is because there are electrons in a circuit. These electrons travel around the circuit at an even pace. When they come to the wire I placed in the circuit the electrons have to slow down in order to be able to pass through it. This causes a resistance. The electrons will bump together and the more they bump the higher the resistance of the wire. In a thin wire these electrons have to squeeze tightly together in order to pass through, however in a thick wire these electrons do not have to squeeze together as much to be able to pass through. So the more they squeeze together, the higher the resistance.
After collecting my results I drew three graphs. The graph which compares the diameter of the wire to the resistance of the wire travels in a curve. The graph shows that the resistance in Ohms is inversely proportional to the diameter of the wire. (This means that if you double the diameter you halve the resistance). The graph which compares the area of the cross section and the resistance of the wire also travels in a curve.
This proves Ohm's Law as he states that if the area of the cross section of the wire is uniform then the resistance of the wire is inversely proportional to the cross section. The third graph which compares the diameter of the wire to the current flowing through the wire travels a straight line through the origin. This means that the diameter of the wire is directly proportional to the current flowing round the circuit. If you double the diameter, the current also doubles because there is less resistance as the diameter increases.
The gradient of this graph is (Height divided by width) 1.5/0.12 = 12.5.
My working method was a good one although there were a few wayward results. This could have been avoided if I had had the time to repeat the experiment and work out an average. They could have been slightly out because I did not measure the length of wire correctly or because I did not read the ammeter and voltmeter accurately. For this investigation I used 32swg, 26swg and 22swg nichrome wire because they were the only ones that I had. (It might have been better if I'd used more thicknesses, because then the graphs would have been smoother).
Ohms law says that to find the resistivity of the wire / specific resistance of the cross sectional area you do:
ρ = RC/L
(ρ = resistivity of the wire, R = resistance of the wire, C = cross sectional area, L = length of the wire)
Summary of Ohm’s law
The current flowing through the circuit is directly proportional to the voltage applied to the circuit. (If you double one, you double the other.)
The cross sectional area is inversely proportional to the resistance of the wire. (If you double the cross sectional area, you halve the resistance.)
Ohm’s law states that:
V = I*R and
ρ = RC/L
V = voltage applied, I = current
R = resistance of the circuit, C = cross sectional area
L = length of the wire, and ρ = resistivity of the wire.

What Factors Affect the Resistance of a Wire? (1)

2008-02-13T21:31:00.000-08:00

Nicola Tregay
Mill Chase School, Bordon, Hampshire, UK

http://www.sci-journal.org/index.php?template_type=report&id=26&htm=reports/vol3no1/v3n1k44.html&link=reports/home.php

Hypothesis

I am going to investigate what factors affect the resistance of a wire. There are three main factors which affect the resistance of a wire:
The material of the wire. (What the wire is made out of)
The length of the wire.
The thickness of the wire. (The diameter of the wire)

Resistance is a force which opposes the flow of an electric current around a circuit so that energy is required to push the charged particles around the circuit. The circuit itself can resist the flow of particles if the wires are either very thin or very long.
e.g. The filament across an electric light bulb.

Resistance is measured in ohms. The symbol for an ohm is Ω . A resistor has the resistance of one ohm if a voltage of one volt is required to push a current of one amp through it.

George Ohm discovered that the emf of a circuit is directly proportional to the current flowing through the circuit. This means that if you triple one, you triple the other He also discovered that a circuit sometimes resisted the flow of electricity. He called this resistance. He then came up with a rule for working out the resistance of a circuit:

V/I = R

V - volts
I - current
R - resistance

To begin with I am going to investigate which materials put up the highest resistance and I will combine it with the investigation about which length puts up the greatest resistance.

The type of material will make a difference because the electrons have to pass through the material. These electrons find it easier to pass through some materials than others. In this experiment I am going to use copper and nichrome wire. I predict that the nichrome wire will have a higher resistance than the copper wire. I can say this because I know that the electrons have to squeeze together more in order to be able to pass through nichrome wire than they do in order to pass through copper wire. (The more the electrons bump together, the higher the resistance.)

The length of the wire will make a difference. This is because when you have a long wire, the electrons have to squeeze together for longer to be able to pass through the wire than they do in order to be able to pass through a short wire. I predict that the longer the wire, the greater the resistance. If I had a 30 cm wire and a 60 cm wire, the 60 cm wire would have a resistance twice that of the 30 cm wire.

For this experiment I will be using a voltmeter, an ammeter, five wires, two crocodile clips, some nichrome and copper wire and a power pack. The wire will be: 26 swg of copper and 26 swg of nichrome. The diameter of the wire will be kept the same so that it is a fair test. The voltage will also be kept the same, although the readings may not be exactly the same each time. For the lengths I will be using 10, 20 and 40 cm wires, this is because I can then see if my prediction about doubling the length is correct.

To make this test fair I should take more than one result so that I can work out an average, this will help prevent any wayward results.

Method
Collect apparatus: a voltmeter, an ammeter, 5x wires, 2 crocodile clips, 10, 20 and 40 cm of both nichrome and copper wires and a power pack.
Set apparatus up as shown:

Set the power pack on as low a voltage as possible. (So that there is not too high a current passing through the circuit.)
Place the 10 cm of nichrome between the two crocodile clips to complete the circuit.
Turn on the power pack and record what the ammeter and voltmeter read.
Replace the 10 cm of wire with the 20 cm of nichrome remembering to keep the voltage the same. Turn on your power pack and record what the ammeter and voltmeter say.
Change the wire to the 40 cm of nichrome wire and repeat the experiment.
Do the same for the copper wires.
Work out the resistance for all the results using Ohm's law. V = I x R
Record your results in a graph.

Conclusion
My hypothesis was correct. The nichrome wire has a higher resistance than the copper wire and the longer the wire, the higher the resistance.

Ohm's law states that the current flowing through the circuit is directly proportional to the voltage applied. (If you double one, you double the other.)

I worked out the resistance of the wires by using the formula:
V/I = R

This happens because of the electrons that flow through the wire. These electrons travel at a steady pace, when they come to a different piece of wire, they have to slow down in order to be able to pass. (This is why the current differs). While moving through the wire, the electrons need to squeeze together. This is because there is not enough room/space for them to pass evenly through. The more the electrons have to bump together then the higher the resistance. This is because it will take longer for them to pass from one side of the wire to the other side. This is because the current is slowed down. (The longer the wire, the longer the electrons have to stay squashed together, and so the longer they take to pass through the wire and the higher the resistance. The material of the wire makes a difference because it is harder for electrons to pass through some materials than it is for them to pass through others. (Some wires cause the electrons to bump together more than others.)

I put my results into graphs although they are not on the same axes because the resistance for copper wire is so low. I had to draw a line of best fit on all the graphs because there were a few wayward results.

The graphs which compare the length of the wire to the resistance it gives travels in a straight line through the origin. This means that the size of the length is directly proportional to the resistance it gives. I can work out the gradient of this line by dividing the height of the line by the width. (Gradient = height/width)
The gradient of the graph for copper wire is: 0.01/9 = 0.001
The gradient of the graph for nichrome wire is: 1/16 = 0.0625
As the gradient of the second graph is higher. I can say that the resistance for the nichrome wire increases more rapidly as the length of the wire increases than the resistance for the copper wire. The results for the copper wire have a pattern to them. The result for 10 cm is 0.011, and the result for 20 cm is 0.022, so I can predict that the result for 50 cm will go 0.055 and so on.
The other graphs which compare the current of the wire to the length of the wire also travel in a straight line. These graphs however travel downwards, this means that they will have a negative gradient. As the current decreases, the length of the wire increases.
The gradient of the copper graph is: 30/-12 = -2.5
The gradient of the nichrome wire graph is: 0.2/-8 = -0.025
As the gradient of the second graph is lower I can say that as the length increases the flow of current decreases faster in copper wire than in nichrome wire.
There were a few wayward results. This could have been because the wires were not exactly the correct length or because I did not read the voltmeter and ammeter accurately. The wires might have been over-heated The temperature of the wire will affect the resistance. Hotter wires will have a higher resistance than cold wires. To improve my results I should have obtained more than one result for each wire and length. I could have then worked out an average for more accurate results.

The method I used was fine, although it was extremely hard to collect any results for copper wire. This is because copper has such low resistance. It has a low resistance there will be a large current. The ammeters we have in school cannot read such a high current. Without the current I cannot work out the resistance of the wire. I ended up using some results I had already collected the last time I did this experiment.

E-Activity 13

2008-02-08T02:32:00.000-08:00

Q-1: In figures 12a and 12b, what will be the readings on the meters X, Y and Z?
Q-2: Calculate the combined resistance in each case of the resistors in figures 13 a, b, c, and d.
Q-3: In figure 14, which resistor arrangement, A or B, has the lower combined resistance?
Q-4: In figure 15, what is the P.D across the 6Ω resistor?
Q-5: In figure 16, calculate
(a) The current in the main circuit
(b) The P.D across the 4 Ω resistors
(c) The current through the 12 Ω resistors.
Q-6: In figure 17, the switch is open. What will be the reading on the ammeter? What would the ammeter reading be if the switch were closed?

Example on combined Series and Parallel Circuit

2008-02-05T21:52:00.000-08:00

Example:What is voltage on A, B, and D?
What is current on A, B, C, and D?
What is resistance on C?
What is total current and resistance?
First of all, we have to look at the diagram very carefully (The order of the questions also help us from where we have to start), we know that the voltage on D is equal to C, which is 80 V. We also know A and B have the same voltage. Using the voltage law, we can find out the voltage on A and B, which is 230 - 80 = 150 V each.
Now we get all the voltages on each component. Using ohm's law, we can find out the current on A, B, C, and D. IA= 150/30 = 5 A; IB = 150/30 = 5 A; ID = 80/40 = 2 A; IC = 10-2 = 8 A. The sum of the current on A and B is equal to that of C and D (eq. 3). A+B = C+D.
The resistance of A+B is 15 ohm The resistance on C is R = 80 [V] /8 [IC] = 10 ohm. Therefore, the resistance of C+D is 8 ohm.
We can find out total resistance of this circuit.
R = 15+8 = 23 ohm; I = 230 [V] /23 [R] = 10 A

Examples on Series and Parallel Circuits

2008-02-05T21:41:00.000-08:00

Example 1

We have a series circuit like this. What is the total voltage, resistance and current?
First, we have to find out the total voltage , and then resistance , and finally you can find out the current.
Total voltage is 9 + 1 + 16 + 4 = 30 V
Total resistance is 30 + 10 + 40 + 20 = 100 ohm
Using ohm's law, I = V / R, then we can find out the total current.
I = 30 / 100 = 0.3 A

Example 2

If you have a parallel circuit like this, what is the total resistance and voltage? And voltage and current on A, B, and C?
In order to find out the total voltage, we have to find out the total resistance. Using equation 5, we can find out the total resistance.
1/R = 1/15 + 1/15 + 1/30 = 5/30, R = 6 ohm
Then using ohm's law,V = I R, we can find out the total voltage.
V = 5 * 6 = 30 V
Using equation 4, we now know the voltage on A, B, and C, which is 30 V each.
Using ohm's law again, we can find out the current on A, B, and C.
IA = 30/15 = 2 A,
IB = 30/15 = 2 A,
IC = 30/30 = 1 A .
When you add up all the current (using equation 6), we get 5 A which is the total current.

E-Activity 12

2008-01-28T04:20:00.000-08:00

Q-1:
(a) What do we call several cells connected together?
(b) Explain why it is important that the cells are connected the right way around.
(c) What is given to the electricity as it flows through the cells?
Q-2:
(a) Draw a circuit diagram for a series circuit containing a cell, a switch and four bulbs.
(b) Draw a circuit diagram for a parallel circuit containing a cell and four bulbs.
(c) Explain why no electricity flows in the above circuit when the switch S is open.
When switch S is closed a current of 0.2 A flows through ammeter A.
(d) What current flows through ammeter B?
(e) What current flows through ammeter C?
Q-3:
Katy built the circuit as shown above.
(a) Why did the bulb not glow?
(b) Katy placed each of the objects shown above in turn, across the gap AB.
(c) Which two objects made the bulb glow? Tick the appropriate boxes.
(d) Why did the bulb glow when either of these two objects was placed across the gap AB?
Q-4:
In the circuits shown here all the cells and all the bulbs are identical
(a) in which circuit will the bulb(s) glow the brightest?
(b) In which circuit will the bulb(s) glow least?
(c) In which two circuits will the bulbs have the same brightness?
(d) What energy changes are taking place when electricity flows through each of the bulbs?
Q-5:
The diagram below shows two similar circuits.
Explain why bulb A glows but bulb B does not glow.

Equivalent resistance 3

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Equivalent resistance 2

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Equivalent resistance 1

2008-01-22T19:50:00.001-08:00

Series and Parallel Circuits

2008-01-21T20:47:00.001-08:00

Series circuits

A series circuit is a circuit in which resistors are arranged in a chain, so the current has only one path to take. The current is the same through each resistor. The total resistance of the circuit is found by simply adding up the resistance values of the individual resistors:
equivalent resistance of resistors in series : R = R1 + R2 + R3 + ...

A series circuit is shown in the diagram above. The current flows through each resistor in turn. If the values of the three resistors are:
R1=8 ohms, R2=8 ohms and R3=4 ohms
The total resistance is R1 + R2 + R3= 20 ohms

With a 10 V battery, by V = I R the total current in the circuit is:
I = V / R = 10 / 20 = 0.5 A. The current through each resistor would be 0.5 A.

Parallel circuits

A parallel circuit is a circuit in which the resistors are arranged with their heads connected together, and their tails connected together. The current in a parallel circuit breaks up, with some flowing along each parallel branch and re-combining when the branches meet again. The voltage across each resistor in parallel is the same.
The total resistance of a set of resistors in parallel is found by adding up the reciprocals of the resistance values, and then taking the reciprocal of the total:
equivalent resistance of resistors in parallel: 1 / R = 1 / R1 + 1 / R2 + 1 / R3 +...
A parallel circuit is shown in the diagram above. In this case the current supplied by the battery splits up, and the amount going through each resistor depends on the resistance. If the values of the three resistors are:
R1=8 ohms, R2=8 ohms and R3=4 ohms
The total resistance is found by 1/R = 1/8+1/8+1/4 = 1/2
that gives R = 2 ohms

With a 10 V battery, by V = I R the total current in the circuit is: I = V / R = 10 / 2 = 5 A.
The individual currents can also be found using I = V / R. The voltage across each resistor is 10 V, so:
I1 = 10 / 8 = 1.25 A
I2 = 10 / 8 = 1.25 A
I3=10 / 4 = 2.5 A
Note that the currents add together to 5A, the total current.

Ohm's Law Part 2: Ohm's Law Applied to Simple Circuits

2008-01-13T21:24:00.001-08:00

Ohm's Law Part 1: Units and Quantities

2008-01-13T21:22:00.001-08:00

Resistance in Circuits, Part 2 or 2

2008-01-13T03:13:00.001-08:00

Resistance in Circuits, Part 1 of 2

2008-01-13T03:12:00.001-08:00

Current and resistance

2008-01-13T02:46:00.000-08:00

When the switch in this circuit is closed, there is a good flow of electricity through the bulb so it glows brightly.
When a second or third bulb is included in the circuit, they each glow less brightly i.e. the current in the circuit is smaller.
The size of the current in any circuit is affected by the number of components in the circuit and what they are. The components oppose the flow of electricity. They have resistance.
The effect of including a component which has resistance into a circuit can be explained by picturing the current as runners on an athletics track, and the component’s resistance as an obstacle such as a set of step ladders. Without the component, the electricity flows freely around the track. When the component is included in the circuit, the flow of electricity, i.e. the current, is reduced.
Resistors:Components called resistors are included in some circuit to control the electricity that flows. Some resistors have a resistance that can be charged or altered. They are called variable resistors and are extremely useful for altering the size of current in a circuit.
The variable resistor in the circuit in figure 5, is being used as a dimmer switch to control the brightness of the bulb. Variable resistors are also used to alter the loudness of the music from a stereo system, the colour and brightness of the picture on a TV set, and the speeds of electric motors.
We measure the size of a resistor in ohms Ω for example a 10Ω resistor will offer half the resistance to the flow of current of a 20Ω resistor

Series and parallel circuits

2008-01-12T00:20:00.000-08:00

There are two kinds of electrical circuit. These are called series circuits and parallel circuits.
In series circuit current has only one path to follow, i.e. there are no branches.
In a series circuit the same current passes through all parts. If one of the bulbs in the circuit below is turned off, they are all turned off.
In a parallel circuit there are several paths electricity can follow, i.e. there are branches.
In a parallel circuit it is possible to switch off some parts of the circuit and yet leave others on, as shown in figure 4.
Measuring current:
The size of a current is measured using an instrument called an ammeter. Current is measured in units called amps (A). The ammeter is placed in series with the part of the circuit being investigated.
Currents in series circuits:
• Ammeter A is measuring how much current is leaving the cell.
• Ammeter B is reading how much current is flowing into the resistor.
• Ammeter C is reading how much current is flowing into the bulb.
• Ammeter D is reading how much current is flowing back into the cell.
From these readings it is clear that:
1. The current leaving the cell is the same size as the current returning to it. Current is not used up as it flows around a circuit.
2. The size of the current is the same in all parts of a series circuit.

Current in parallel circuit:• Ammeter A is measuring how much current is leaving the cell.
• Ammeter B is reading how much current is flowing through the bulb.
• Ammeter C is reading how much current is flowing through the resistor.
• Ammeter D is reading how much current is flowing back into the cell.

From these readings it is clear that:
1. The size of the current in different parts of a parallel circuit is not the same.
2. The current leaving the cell is the same size as the current returning to it.
3. The current entering a junction is equal to 0.5 A= 0.2 A + 0.3 A.

Energy in circuits:
As current passes through a cell or a battery it receives electrical energy which it carries around the circuit. This energy ischanged into other forms as the current passes through the various components. For example when current passes through a bulb, some of the electrical energy it is carrying is changed into heat and light energy. If the current passes through a buzzer, some of the energy ischanged into sound.
We can measure how much energy is given to the electricity as it passes through the cell or battery, and how much electrical energy is transformed in the various components in a circuit using a voltmeter. The voltmeter is connected in parallel with the part of the circuit we are interested in. large voltage readings indicate large energy transfers by the components.
Note that the voltmeter reading across the cell is equal to the sum of the voltmeter readings across the bulb and the buzzer. This indicates that all electrical energy received by the current as it passes through the cell is converted to other forms of energy by the components in the circuit.

Current Electricity

2008-01-09T05:25:00.000-08:00

Electric Current and Charge
• We consider electric current as the rate of flow of charge. In other words, an electric current flowing determines the amount of charge passing a given point in one second.
• When one coulomb of charge passes a given point in one second the corresponding current is one ampere (A)
• If the electric current is I (in amperes), then the amount of charge Q (in coulombs) which flows past a point in time t (in seconds), is given by
• Q = I t
• I = Q/t
Example-1• When the starter motor of a car is switched on for 0.5 s, 16 C of charge passes through the wires in the motor. Assuming the charge is uniformly transmitted over the time, how large is the electric current?
Solution• Data: Q = 16 C, t = 0.5 s
• Formula: I = Q/t = 16/0.5 = 32 A
• Answer: Hence the current is 32 A
Measuring Electric Current
• To measure the size of an electric current, an ammeter can be used. The ammeter must be connected in series to the circuit.
• For proper connection, note that the electric current (not the flow of electrons which is in the opposite direction to that of conventional current) must flow into the ammeter by the positive (+ or red) terminal and leave by the negative ( - or black) terminal.
• If it is connected the other way round, the pointer will deflect slightly below the zero mark. Check your connections if this happen during an experiment.
• If the circuit consists of only one loop, it does not matter where the ammeters are placed in the circuit. Since the same current flows through the simple circuit, they will all measure the same current.
• Some ammeters allow us to measure different ranges of current. Figure shows such an ammeter. Study the diagram carefully. Can you see how the ammeter is connected and read?
Summary• An electric current is the rate of flow of charge.
• Current = charge/time
• I = Q/t
• One coulomb is that amount of charge which flows past a given point in one second when the current is one ampere.
• An ammeter is used to measure current and must be connected in series in the circuit.

Simple circuits

2008-01-09T05:17:00.000-08:00

The diagram shows the workings of a simple torch. When the button B is pressed, electricity flows around the circuit and the torch bulb glows. When the button is released, electricity stops flowing around the circuit and the bulb no longer glows.
Like water flowing through pipes, electricity needs something to travel through. The ‘pipes’ for electricity are metal wires. The wires, batteries and bulbs form circuits around which currents flow.
Cells and batteries:
A cell is a kind of pump which makes electricity move. This movement is called an electric current. If a larger current is need several cells can be connected together to form a battery.
Both circuits shown in figures 2 and 3 are complete circuits. Electricity can flow all the way around them. But if a gap is created in the circuit, electricity will not flow and the bulb will not glow. We describe a circuit like this as being incomplete. The position of the gap in the circuit is unimportant. Current will only flow if the circuit is complete.
Switches:
A switch turns a circuit on and off. It behaves like a draw bridge which can make the circuit complete when closed, or incomplete when open.
Circuit diagrams:
Drawing diagrams such as those shown opposite is not easy. To simplify things scientists and electricians use circuit diagrams. These simple diagrams use symbols to represent the various bits and pieces (more properly called components) of the circuit. A list of some of the most common components is shown above.

How electricity works

2008-01-08T23:45:00.001-08:00

Current Electricity

E-Activity 11

2008-01-08T23:38:00.000-08:00

1. The diagram shows a man playing a guitar.
(a) Suggest two ways in which the guitarist could alter the note being produced by one of the strings.
(b) Draw a picture of what you would expect to see on an oscilloscope screen if the sound produced by the guitarist is
• High pitched but quiet
• Low pitched but loud
2. The diagram shows a young girl watching a firework display.
• Why does she see the flash of an exploding rocket before she hears the bang?
3. The gong in the diagram below is vibrating gently and producing a quiet sound.
(a) What would you do to the gong to make it produce a loud sound?
(b) Which of the two bells in the diagram would you shake if you wanted to produce a lower-pitched sound?
4. The table below contains several sources of sounds. In the second column enter an approximate decibel level for these sounds.
Source of sound Loudness on decibel scale
• A normal conversation between two people
• Someone shouting as loud as they possibly can
• A firework rocket exploding
• A small alarm clock ringing
5. Richard likes to listen to loud music
(a) What may happen to Richard’s hearing if he does this too often?
(b) What two things could Richard do to avoid this problem?
6. What is noise? What effect might noise have on a person?
7. Suggest two ways in which noise levels can be reduced.
8. Give one example of someone who should wear ear protectors.

Wave Motion

2007-12-26T05:02:00.001-08:00

Longitudinal Waves

2007-12-25T09:53:00.001-08:00

Different kinds of sounds

2007-12-25T07:36:00.000-08:00

Frequency of vibration and pitch:

Large objects, like one of the strings of a double bass, vibrate slowly when plucked. Its frequency of vibration is low and only a small number of sound waves are produced each second. The note produced by the string will have a low pitch.
It is possible for you to see a picture that represents these waves using a piece of apparatus called a cathode ray oscilloscope (CRO).
Small objects, like a violin string, vibrate more quickly when plucked. Its frequency of vibration is higher so more sound waves are produced each second. The note produced by this string will have a higher pitch.
The frequency of an object or wave is the number of complete vibrations it performs each second. It is measured in hertz (Hz) where 1 Hz is one vibration per second.

Musical instruments:

Like all musical instruments, the double bass and the violin can produce notes with a wide range of frequencies and pitch. The changes in frequency produced by the vibrating strings are achieved by altering.
• The length of the string by changing the positions of the fingers on the frets board. The longer the vibrating string, the lower the pitch of the note produced.
• The thickness of the string. The thicker the string, the lower the pitch of the note produced.
• The tension of the string. The greater the tension, the higher the pitch of the note produced by the string.

Changes in the frequencies of the notes produced by wind instruments are achieved by altering the lengths of vibrating air columns.

SOUND

2007-12-25T06:11:00.000-08:00

We live in a world filled with a great variety of sounds. We communicate with each other using sounds. Sometimes we like to relax to the playing of music. At other times, we become irritated because of excessive noise. Like light, sounds travel in the form of waves.

Producing sounds:

All sounds are produced by objects that are vibrating. These sounds travel outwards from the source as sound waves.
For example:
The skin on a drum vibrates to produce the sound of the drum beat.
When the clanger hits the inside of a bell, the vibrations are heard as a ringing sound.
The buzzing sound you hear from a bee is created by the vibration of its wings.
In a loudspeaker, electrical energy is changed into vibrations which we hear as sounds or music.

Creating and hearing sound waves:

Sound waves travel from vibrating objects to our ears by means of sound waves. Figure 1 show how a sound wave is created in air.
As the object vibrates to the right, it pushes the air particles close together, creating a compression. As the object vibrates to the left, a region of more spread out particles is created. This is called a rarefaction. When the object has vibrated several times, a series of compressions and rarefactions have been created that are moving away from the object. This is a sound wave. A model of a moving sound wave can be created by pushing and pulling one end of a slinky.

Sound waves must have something to travel through:

Sound waves can travel through solids, liquids and gases. But they cannot travel through space where there are no particles, i.e. they cannot travels through a vacuum.
When there is air in this bell jar we are able to hear the bell ring when the switch is closed. If the air is pumped out of the jar and the switch closed, we can see the bell ringing, but we cannot hear it. This experiment proves that light waves can travel through a vacuum but sound waves cannot.

Speed of sound:

Sound waves travel much more slowly than light waves. This is why sometimes we see an event before we hear the sound, e.g. thunder and lightning or the flash of an exploding rocket followed seconds later by a loud bang.
Sound waves travel at different speeds in different materials. In air the speed of sound is approximately 340m/s, in water is approximately 1500 m/s and in steels it is 6000 m/s. sounds travel most quickly through solids and least quickly through gases.

ELECTROMAGNETIC SPECTRUM - NASA

2007-11-13T20:45:00.001-08:00

E-Activity 10

2007-11-13T20:07:00.000-08:00

Copy and complete the following sentences.
1. The case of a microwave oven is made of _____ this ____microwaves.
2. Food contains ____molecules. These _____microwaves and become hot.
3. The food is put into containers made of _____, _____ or _____. These materials allow microwaves to pass through them very easily. We say that they _____ the microwaves.
4. We use a _____ dish to collect enough microwaves for a strong signal. This _____ the microwaves on to an aerial.
5. The aerial transfer’s energy from the microwaves as an ______signal.
6. X-rays can pass easily through skin and flesh but not through _____ or _____.
7. Photographic _____ absorbs any X-rays that fall on it. These parts of the film then go _____ when the film is developed.
8. Gamma radiation can kill living _____.
9. It is used to kill harmful _____ or _____ cells inside people’s bodies.
10. Radiation from the sun, or from a sun bed, can give pale skins a _____. But it can also damage skin cells and cause skin _____. These things happen because of _____ radiation.
11. Some substances _____ ultraviolet radiation and use the energy to produce _____. We say that these substances are _____.
12. Toasters and grills cook food using _____ radiation. Foods become hot when they _____ this radiation.
13. Metal things _____ microwaves, even if they are full of small holes. Some microwaves can pass easily through the Earth’s _____. These microwaves are used to carry information to and from _____.
14. Infrared rays are used to control a _____ set or a _____ player, and to send telephone messages along _____ fibers.
15. In microwave ovens, the microwaves are strongly absorbed by _____molecules in food. The energy from the microwaves makes the food_____.

E-Activity 9

2007-11-13T19:49:00.000-08:00

1. What substance will radio waves pass through easily?
2. What happens when radio waves are absorbed by an aerial?
3. Why can’t you send a radio message to or from a submarine?
4. Why is it useful to be able to reflect long wavelength radio waves?
5. What is the wavelength of these radio waves?
6. What wavelengths of microwaves are used for satellite television? And why are these wavelengths used?
7. How is your instruction carried to the television set or video player?
8. Why must you point the remote control at the television set or video player?
9. Doctors can use X-rays to see whether your lungs are healthy. How do they know if there is diseased tissue in your lungs?
10. Why can X-rays and gamma rays cause cancer?

Gamma Rays

2007-11-08T00:00:00.000-08:00

Gamma rays are electromagnetic waves emitted by radioactive nuclei. They are also released during nuclear reactions.
Wavelengths of gamma rays range from about 10-10 m to less than 10-14 m.
They are very penetrating and cause serious damage when absorbed by living tissues.

X-Rays

2007-11-07T23:59:00.000-08:00

X-rays are electromagnetic waves with wavelengths ranging from about 10-8 m (10 nm) down to 10-13 m (10-4 nm).
X-rays are used as a diagnostic tool in medicine and dentistry.
If you have experienced traveling by air, you would have noticed that your luggage are inspected by the security officer present using x-rays.

Ultra-Violet Radiation

2007-11-07T23:58:00.000-08:00

Ultra-violet radiation is the radiation beyond the violet end of the visible spectrum.(wavelengths range from 10-8 to 10-7 m)
The main source of ultra-violet radiation is sunlight and it is this radiation which gives rise to suntans.
Ultra-violet radiation from the sun also stimulates our body to produce vitamin D, which we need for healthy bones.
However overexposure to ultra-violet radiation can cause skin-cancer and damage to the retina.
Ultra violet radiation also kills bacteria and viruses. Thus it is used to sterilize hospital operating rooms and surgical instruments.
Low intensity ultra-violet lamps are sometimes placed above grocery meat counters to reduce spoilage.

Infra-Red Radiation

2007-11-07T23:57:00.000-08:00

The name infra-red means beyond red. There are waves just beyond the red end of the visible spectrum. In fact, all objects with temperatures above 0 K emit infra-red radiation.
When objects absorb infra-red radiation, they become hotter. This property of infra-red radiation is used to provide heat treatment for various illnesses.
We are not able to see in the dark because our eyes are sensitive only to the visible part of the electromagnetic spectrum. However, an infra-red camera, with special photographic films which are sensitive to infra-red radiation, can be used to take pictures in the dark without using a flashlight.
Infra-red radiation can also be used in intruder alarms. An intruder would unknowingly block the rays and set off an alarm.
One popular use of infra-red radiation today are the remote control devices for many electrical appliance via infra-red radiation which is produced by light emitting diodes (LEDs) inside the unit.

Microwaves

2007-11-07T23:56:00.002-08:00

Microwaves are very similar to UHF radio waves. They have wavelengths of a few centimeters.
Microwaves are produced by special electronic devices such as the klystron tube.
Microwaves are used to transmit television signals and are also used in telecommunications.
Due to its short wavelength, relative to radio waves, the microwave signal can travel in a straight line without losing much of its energy.
Radar systems use microwaves to find the direction and distance of objects which reflect the microwaves back to a large receiving aerial mounted near the transmitter.
Nowadays, another common use of microwaves is in microwave ovens.

Radio Waves

2007-11-07T23:56:00.001-08:00

Radio waves have the longest wavelengths of the waves in the electromagnetic spectrum. The wavelengths range from several hundred meters (long wave, LW) to a few centimeters (ultra high frequency, UHF).
They are used in radio and television communication to transmit sound and picture information over long distances.

ELECTROMAGNETIC SPECTRUM

2007-11-04T23:34:00.001-08:00

Electromagnetic waves

2007-11-04T23:19:00.000-08:00

All electromagnetic waves have certain fundamental properties in common. They basically differ from each other in their wavelengths and in the effects produced.
Properties of Electromagnetic Waves
An electromagnetic wave is produced by the simultaneous vibration of electric and magnetic fields.
Features common to all the electromagnetic waves include the following:
They transfer energy from one place to another.
They are transverse waves.
They can travel through a vacuum.
They travel through a vacuum at 300 000 000 meters per second (3 x 108 m/s). This speed is commonly known as the speed of light.
They all show wave properties like reflection and refraction.
They obey the wave equation  = ƒ 
Application of Electromagnetic Waves
Electromagnetic waves have wavelengths ranging from several kilometers (in the case of radio waves) to less than a picometer(10-12 m) (in the case of gamma rays).
The shorter the wavelength, the higher the frequency.
As the frequency gets higher, the energy also increases. This causes different electromagnetic waves to have different properties and applications.

E-Activity 8

2007-11-04T19:54:00.000-08:00

1. What is the velocity of a wave of frequency 600 Hz and wavelength 0.5m?
2. What is the frequency of a wave of wavelength 0.9m and velocity 300 ms-1 ?
3. What is the wavelength of a wave of velocity 300 ms-1 and frequency 1000 Hz?
4. A source of frequency 500 Hz emits waves of wavelength 0.2 m. how long does it take the waves to travel 400m?
5. A wave of frequency 500 Hz travels between two points P and Q with a velocity of 300 ms-1 . How many wavelengths are there in PQ if the length of PQ is 600 m?
6. To understand the wave motion of a transverse wave or a longitudinal wave, we have to examine how the wave particles vibrate as the wave propagates through the medium.
7. Surf the Internet to locate information on transverse and longitudinal waves. Choose the web sites that show animation of the waves. Write down the similarities and the differences of the two types of waves and give examples of each.

Waves

2007-11-04T19:51:00.000-08:00

There are different kinds of waves. All waves have one thing in common: they transfer energy from one place to another.
• A wave is a phenomenon in which energy is transferred through vibrations. The wave carries energy away from the wave source. You can see the effect of rope waves if you fix one end of a rope by tying it round a rod and move the other end up and down. A series of crests and troughs can be seen to pass along the rope. Each section of the rope is set into an up-and-down motion by the previous section as the wave passes along the rope.
• The rope is the medium through which the wave propagates. Note that the particles in the rope itself do not move forward with the wave.
• A similar effect is obtained with water waves. A small cork on the water surface will bob up and down (or vibrate) as the wave passes, but will not travel forward with the wave. In this case, water is the medium through which the energy is transmitted.
Types of Waves
1. Transverse waves
2. Longitudinal waves
Transverse waves• Water waves and rope waves are examples of transverse waves. Transverse waves are waves which travel in a direction perpendicular to the direction of the vibrations. If you move the end of a slinky coil from side to side, transverse waves are set up. Light waves and other electromagnetic waves are also transverse waves.
At home you produce transverse waves when you shake the dust from a blanket or rug. Watch closely the motion of your hand, flipping the edge of your blanket, and the waves that are produced.
Longitudinal waves
• Another type of wave is the longitudinal wave. Longitudinal waves travel in a direction parallel to the direction of vibrations.
• If a slinky coil is pushed and pulled at one end, longitudinal waves are set up. Watch the way the compression travels along the coil. The compressed coils themselves do not travel. They just vibrate forward and back. Sound waves are examples of longitudinal waves.
Wave Terms• Some of the terms and quantities used to describe transverse wave motion are as follows:
1. The high points are called crests or peaks while the low points are called troughs.
2. The amplitude is the maximum displacement from the rest position. It is the height of a crest or depth of a trough measured from the normal undisturbed position.
3. The wavelength, l, is the distance between two successive crests or two successive troughs. It is also equal to the distance between any two identical points on successive waves, for example points A and B, and points C and D in figure
4. The frequency, ƒ, is the number of crests (or troughs) that pass a point per second. This is equivalent to the number of complete waves generated per second. Frequency is measured in hertz (Hz). A frequency of 1 hertz means that one wave cycle or one oscillation is completed per second.
5. The period, T, is the time taken to generate one complete wave. It is also the time taken for the crests, or any given point on the wave, to move a distance of one wavelength. T = 1 / ƒ
6. The speed, n, of the wave is the distance moved by a wave in one second. Since the wave crest travels a distance of one wavelength in one period, the wave speed, n = l/T or n = ƒ l
7. In longitudinal waves, the part where the particles of matter are closest together is called the compression. The part where the particles are spread apart is the rarefaction.

Three Laws of Motion

2007-11-01T20:38:00.001-07:00

E-Activity 7

2007-10-31T06:16:00.000-07:00

1. Copy and complete the table using the results of all six experiments shown in the diagrams.
2. What do you notice about the first and last columns in the table?
3. Look at the examples in the picture.
Use the formula to work out the missing items.
The first one is done for you.

Force, mass and acceleration

2007-10-31T06:09:00.000-07:00

Applying a force
Students investigate the relationship between force, mass and acceleration in the laboratory.
• How do the students apply a constant force to accelerate the trucks?
• When they apply the same force to different masses, which accelerates faster-the larger mass or the smaller mass?
• If they keep the mass constant but increase the force, how does the acceleration change?
• Give one reason why it’s difficult to do the experiment accurately.
Experiments in space
A spaceship is a better place to measure the accelerations produced by different forces.
• Why is a spaceship a good place to do experiments on force and acceleration?
How force affects acceleration
The diagrams show some experiments in a spaceship.
The same 1 kg mass is accelerated using different forces.
• How much acceleration does a 1 N force give tgo the 1 kg mass?
• How much acceleration does a 2 N force give?
Copy and complete the sentence.
• When the force on an object is doubled, its acceleration is _______________
How mass affects acceleration
A 1 N force is then used to accelerate different masses.
• How much acceleration do you get when the mass is twice as big?
• How much acceleration do you get when the mass is three times as big?
Copy and complete the sentence.
• If the force stays the same, an object with twice as much mass is given half as much __________
A force of1 N acting on a mass of 1 kg produces an acceleration of 1 m/s2.

Friction V-Speed and Safety

2007-10-27T22:34:00.000-07:00

Friction and the motor car

In order that a car can change its speed or direction, there must be friction between its tyres and the road surface. A new tyre with lots of tread has a rough surface and will provide lots of grip. A worn tyre with a smoother surface will provide less grip and the cart will be much more difficult to control and stop.
Having tyres which are in food condition is important if you need to stop quickly, but there are several other important factors which will affect how quickly a car can stop. These are:
• The reaction time of the driver,
• The efficiency of the breaking system of the car,
• The speed of the car,
• The weather/road conditions,
Figure illustrates how the speed of a car affects the total distance it will travel before it stops. The total stopping distance is equal to the thinking distance + the breaking distance.
Braking force
When a car driver brakes, the braking force between the brakes and the wheels slows the wheels down.
Explain why you must brake harder to stop in the same direction when you are traveling at higher speed.
Tyres must grip
Friction between the tyres and the road makes the tyres grip the road. When you brake, the wheels slow down. The friction with the road must then increase to slow the whole car. The friction force depends on the tyre design and the road surface.
• Why does the car skid if you brake too hard?
• Why must the driver brake carefully when the road is wet?
• Why must the driver brake very carefully when there is ice on the roads?
• Why is it safer to have a rough road surface before a pedestrian crossing?
Why do tyres have tread?
You need good tyres to stop quickly. Tyres can grip the road only if they are touching it. They lose their gripwhen the road is wet. The tread on a tyre is designed to push away the water. In dry conditions, the tread doesn’t help. In dry weather, racing cars use tyres with no tread.
Why do racing drivers stop to change their tyres when it starts raining?
Why do worn tyres increase the chance of a skid?
You can’t stop instantly
If someone steps out in front of a car, it takes time for the driver to react. This is called the reaction time. The distance the car travels during the reaction time is called the thinking distance.
Look at the diagram. Why does it take time to react?
Look at the table.
• What is the thinking distance when traveling at 30 miles per hour?
• What happens to the thinking distance if the speed is doubled?
When the driver presses the brake pedal, it takes time for the brakes to slow the car down. During this time, the car travels a distance called the braking distance.
Look at the table.
• What is the braking distance for a speed of 30 miles per hour?
• What happens to the braking distance if the speed is increased?
• Copy and complete the formula.
• Stopping distance = thinking distance + _________ distance
After drinking alcohol, people may feel perfectly normal but their reactions are actually much slower.
• Why is it a bad idea for people to drive after drinking alcohol?
• What other factors could affect the reaction time?

Friction IV-Speed time graph for a skydiver

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Friction III-Terminal velocity

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Friction II-Drag

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Drag

Whenever an object moves through the air it experiences frictional forces or drag. Which try to prevent its motion. The faster the object moves, the greater the drag. To reduce these forces objects such as cars, trains and aircrafts are shaped so that they cut through the air. They are streamlined to reduce air resistance.
Animals such as dolphins, sharks and penguins have streamlined shaped so that when they move through the water, frictional forces are as small as possible.

Friction I

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One of the most common forces which can act upon an object is friction. Whenever an object moves or tries to move, friction is present. Friction is a force which opposes motion.
On some occasions friction can prove very useful. For example, when you walk or run, you push yourself forward by pushing backwards on the ground. Friction between your foot and the floor helps you to do this. If there was no friction, i.e. like on a slippery ice rink, your feet would slip!
Smooth surfaces reduce the friction between objects while rough surfaces increase the frictional forces. Trainers and football boots are designed to prevent your foot from slipping by increasing the frictional forces between you and the ground. In contrast, skates and skis are designed with smooth surfaces which keep friction to a minimum.
Where there is contact between surfaces, friction can be reduced using a lubricant such as oil, friction between two surfaces can cause the surfaces to wear away and become hot.

E-Activity 6

2007-10-09T09:46:00.000-07:00

1. Copy and complete the sentences.
(a) When a bicycle moves through the _______________, a force of friction acts in the opposite _____________.
This force ______________ the bicycle down.
When a canoe moves through the water, a force of friction acts in the _______ direction.
This ___________ the canoe down.
(b) An ice rink is almost _________________ free. If there was no friction at all, a curling stone would carry on moving at ____________ speed in a ______________ line. It would _____________ stop.

2. What is the resultant force on the sailboard when it is traveling at constant speed?
3. What force balances the air resistance on the moving jet?
4. What is the resultant force on the jet?
5. When was the space probe Voyager 1 launched?
6. How fast is Voyager traveling away from the Sun?
7. Voyager has no engine, so why does it keep moving?
8. Explain why Voyager is now moving in an almost straight line.

6-Moving at constant speed

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Why do things slow down?
If you stop pedaling your bicycle, air resistance and other friction forces slow you down. Friction forces always act in the opposite direction to movement. A similar thing happens if you stop paddling a canoe-the main friction force on a canoe is water resistance.

Keeping going
When there is friction, you need another force to keep going at constant speed.
When a sailboard is moving, water resistance acts against it. To move at a steady speed, the force of the wind on the sail must balance the friction force.
These examples show that if the resultant force acting on a moving body is zero, the body continues to move at the same speed and in the same direction.

Life without friction
Without friction, a cyclist could freewheel forever in a straight line. She would not need to pedal to keep going at a constant speed.
You can get some idea of what life would be like without friction at an ice rink.

Motion in space
Space is an almost perfect vacuum. There is no air to cause friction. In deep space, where the gravity of stars and planets is negligible, a spacecraft will travel in a straight line at constant speed.

Mass

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Inertia

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Acceleration Part II

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Acceleration Part I

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Speed

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Gravity

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Weight vs. Mass

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Mass and weight are different. You have the same mass wherever you are because you have the sameamount of body. But your weight depends on the strength of gravity where you are.

Mass and weight

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The mass of an object is the amount of matter it contains and is measured in kilograms (kg). The weight of an object is the size of gravitational attraction between the object and the planet (or moon) it is on. We measure weight in Newton (N).
The table below gives some examples of the masses and weights of different objects on the Earth and on the Moon.
We can see from this table that gravity on the Moon’s surface is only 1/6th of that on the Earth. So objects here only weigh 1/6th of what they would weigh on Earth. The masses of all the objects do not change. They are the same here on the Earth as they are on the Moon.
Working out weight
Weight depends on how much mass an object has. It also depends on the strength of gravity.
We say that the strength of the Earth’s gravity is 10 Newton per kilogram (10N/kg). We call this the Earth’s gravitational field strength. We can work out the weight of an object as follows.
Weight = mass x gravitational field strength
(Newton, N) (Kilograms, kg) (Newton/kilogram, N/kg)

E-Activity 5

2007-09-29T05:35:00.000-07:00

How can a helicopter hover?
1-When people are lifted from a boat; the helicopter has to keep very still. This is difficult because the weight of the helicopter is always pulling it downwards.
(a) What force is acting upwards on the helicopter?
(b) Explain how the helicopter can keep still.

2-What would happen to the helicopter if
(a) The uplift was greater than the weight?
(b) The weight was greater than the uplift?
Give reasons for your answers.

3-Find the resultant force (the forces all act along the same line)

Applying forces

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Equal and opposite
When two objects interact, there are always two forces. The forces are equal in size and opposite in direction. One force acts on one object and an equal and opposite force acts on the second object.

More than one force
The horse in the first diagram is pulling the tree trunk with a 1000 N force.
In the second diagram, two horses are pulling. Their forces are in the same direction so they add together. They have the same effect as if a single force equal to both the forces added together was acting.

Forces that balance
In this diagram, the horses pull with forces that are equal in size but in opposite directions.
The forces cancel each other out.

Balanced and unbalanced forces

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In everyday life it is rare for an object to be acted upon by no forces or a single force. It is much more likely that it will experience several forces. These forces may be balanced or unbalanced.
If the two tug of war teams in figure pull with the same force, the forces are balanced and there is no movement. (Figure 1)
If one of the teams pulls with a force which is greater than that of the opposition, the forces are unbalanced and there is movement. (Figure 2)
The ship above is floating in water and is stationary. There are several forces acting upon it, so these forces must be balanced.
Gravitational forces, i.e. the weight of the ship, are pulling it downwards but a second force called the up thrust from the water is pushing upwards. (Figure 3)
If too much cargo is loaded onto the ship, the weight may become larger than the up thrust and the ship may sink.
An object placed on a table also experiences balanced forces. (Figure 4)
If an object is moving and balanced forces are applied to it, the object will continue to move in the same direction and at the same speed. The aero plane below is experiencing four forces. Its weight, a lifting force from its wings, thrust from its engines driving it forwards and drag from the air trying to resist the forward motion. (Figure 5)
If the lift and weight forces are balanced, there is no vertical motion. The aero plane stays at the same height.
If the thrust and drag forces are balanced, the aero plane will travel at a constant speed in a straight line.

5-Effects of forces

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There are many different types of force. These include pushes, pulls, twists and stretches.
If you apply a force to an object it may:
• Make it start to move
• Make it move faster
• Slow it down
• Make it stop
• Change the direction in which it is moving
• Change its shape

Sometimes it is not necessary to be in contact with an object in order to apply a force to it.
The steel nails in figure are lifted by a force. This attractive force exists between magnets and magnetic materials such as iron and steel.
The bungee jumper in figure has just jumped out of the basket and is feeling the force of gravity pulling him downwards. The common name given to this force is weight.
There are gravitational forces between all objects, but they are often very weak. They are only noticeable when one or both of them are massive, such as a planet, moon or star. It is gravitational forces of attraction that hold the Moon in orbit around the Earth and hold the planets in orbit around the Sun.

E-Activity 4

2007-09-29T01:08:00.000-07:00

1-
(a) Make a copy of the graph (Example 1) but mark the changes for part C instead of part B.
(b) Calculate the speed for part C of the graph. Show your working.
2-
(a) Make a copy of the graph (Example 2) but mark the changes for part R instead of part Q.
(b) Calculate the acceleration for part R of the graph. Show your working.
3-
(a) Make a copy of the graph (Example 3). Mark the area under the graph for between 5 and 10 seconds. Then calculate the distance traveled during this period. Show your working.
(b) Find the total distance traveled by the object between 0 and 15 seconds.

4-More about motion graphs

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Calculating speed from a distance-time graph
Speed = distance/time
So, for part A of the graph, speed=20m÷5s=4m/s
To calculate the speed for other parts of the graph, you need to use the slope o0r gradient of the graph.
Example 1 shows you how to do this for part B of the graph.

Calculating acceleration from velocity-time graph
Acceleration=change in velocity/time taken
So, for part P of the graph, acceleration=2m/s÷2s=1m/s2
To calculate the acceleration for other parts of the graph, you need to use the slope or gradient of the graph.
Example 2 shows you how to do this for part Q of the graph.

Calculating distance traveled from a velocity-time graph
The area beneath a velocity-time graph tells you the distance that an object has traveled.
Example 3 shows how you can work out from the graph the distance traveled in the first 5 seconds.

E-Activity 3

2007-09-23T06:47:00.000-07:00

Rocket launch

1-The data in the table are for a rocket launch.
(a) Plot a velocity-time graph for the rocket as it lifts off.
(b) how does the velocity-time graph show the rocket is accelerating as it leaves the launch pad?
(c) Draw a line of best fit through the points. Measure the gradient of this line to find the rocket's acceleration.

Performance figures

2-Two sports cars are tested against each other from a standing start.
Look at their velocity-time graphs.
(a) Which car accelerates more quickly to 60 mph?
(b) Which car has the greater top speed?
(c) Which car decelerates faster when it brakes?
Give reasons for your answers.

3-velocity-time graphs

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The figure shows three cars moving in different ways. The velocity-time graph for each car is shown as well.
1-What does a horizontal line on a velocity-time graph tell you about the velocity?
2-What feature of a velocity-time graph tells you that the object is accelerating?
3-The velcity-time graph for an object is a horizontal line through the origin. What can you say about the object's motion?
Velocity-time graphs look very similar to distance-time graphs but beware, they are quite different. Do not confuse the two.

A cyclist's journey

A cyclist is riding along a straight road. Look at the velocity-time graph for the cyclist. The slopeof the velocity-time graph tells you how the velocity is changing.
1-In which parts of the journey is the velocity steady?
2-What is the cyclist's velocity
(a) in section A?
(b) in section C?
3-How does the graph show an acceleration?
4-How does the graph show that the cyclist slows down quicker than she speeds up?
A steeper slope on a velocity-time graph means a bigger acceleration (or deceleration)

E-Activity 2

2007-09-23T01:50:00.000-07:00

1-Look at the aircraft flying in a circle.
(a) Is its speed constant?
(b) Is its velocity constant?
Give a reason for your answers.

2-Calculate the change of velocity for each of the examples in the figure.

3-Look at the examples in the picture and work out the missing items in each one.

2-Velocity and Acceleration

2007-09-23T01:27:00.000-07:00

Same speed but different velocity
The diagram shows three aircraft taking part in a display.
1. What is the same about the motion of the planes?
2. What is different?
All three aircraft have the same speed, but they are moving in different directions. We say that they have differnt velocities. Velocity is speed in a given direction.
Acceleration
When your velocity changes, we say that you accelerate. Racing drivers want a very large acceleration. This means they want to go from a low speed to a high speed in a very short time.
How to calculate acceleration
The racing car in the figure takes 5 seconds to reach a velocity of 50 metres per second. So every second, its velocity increases by 10 metres per second. This is its acceleration. You can work out acceleration like this:
acceleration = change in velocity/time taken
So for the racing car, acceleration = 50m/s / 5s = 10m/s2
The change in velocity is measured in metres per second. the time taken is measured in seconds. So the units of acceleration are metres per second, per second.

E-Activity 1

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This example shows how we can describe a journey on a distance-time graph. Look at it carefully and use it to answer the questions.

1. Look at stage I of the journey, when Jane is walking.
a. How many metres does Jane walk?
b. How long does it take Jane to walk this distance?
c. What is her speed for this part of her journey?

2. Look at stage II of the journey.
a. What is Jane's speed while she chats?
b. Describe the shape of the graph for this stage.
c. Write down a rule for telling when something is stationary on a distance-time graph.

3.
a. Which part of the graph is steeper, I or III?
b. Which stage of the journey is faster, I or III?
c. What is the connection between speed and slope on a distance-time graph?

The slope of a distance-time graph tells you about the speed.
If the object is stationary, the graph is a horizontal line.
If the object has a higher speed, the graph has a steeper slope.

A distance-time graph

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A man is running at a steady speed in a straight line. This graph shows the distance he travels against time taken, so it is called a distance-time graph.
You can read from the graph the distance he travels in a period of time. For example, it takes him 4 seconds to run 20 metres.
What speed is this?

1-Travelling at speed

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Calculating speed:
A sprinter runs 10 metres in 1 second. His speed is 10 metres per second (m/s). Speed in metres per second tells you how many metres you travel in 1 second.
You can work out speed like this:
speed (metres persecond) = distance travelled (metres)÷ time taken (seconds)

Example:
On a motoway, a car travels 300 metres in 10 seconds.
Distance traveled = 300 metres (m)
Time taken = 10 seconds (s)
Speed = ?
So speed = 300 m ÷ 10s= 30 metres per second (m/s)

INTRODUCTION

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Since the aim is to integrate ICT with Physics, it is important to note that the focus will be on the application of various tools and technologies in teaching and learning. As such we expect that you are a competent user of commonly used tools, such as a word processor, spreadsheet, presentation software, web browsers, e-mailing and so on, and will be able to learn other tools quite easily.

The course will be taught by e-learning approaches through Online-Learning will be a process of constructing personal meaning through interation with the content, tutor and peers, and reflection on these interactions.

A large part of your study will be independent. You will be able to interact with tutors and peers through e-mail and e-activities on Online. Participation in e-activities is an integral part of learning in this course. Throughout the term there will be different types of activities to help you deepen your understanding, reflect on new ideas and apply new knowledge and skills to practical examples.

It is hoped that this will make the learning of Physics at this level more interesting and exciting and will give a good foundation for higher learning.