E-Activity 17

The relay switch

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

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

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

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

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

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

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

Danger from sparks

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

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

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

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

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

Static Electricity Detection by Water

What is Static Electricity ? ( Animation )

Electricity that can make your hair stands on end:

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

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)

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.