E-Activity 12

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

Equivalent resistance 2

Equivalent resistance 1

Series and Parallel Circuits

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

Ohm's Law Part 1: Units and Quantities

Resistance in Circuits, Part 2 or 2

Resistance in Circuits, Part 1 of 2

Current and resistance

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

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

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

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

Current Electricity

E-Activity 11

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.