© Boardworks Ltd 2004 1 of 20 © Boardworks Ltd 2006 1 of 45 KS3 Physics 7J Electrical Circuits

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20061 of 45

KS3 Physics

7J Electrical Circuits



Contents

Circuits

Series circuits

Parallel circuits

Energy in circuits

Summary activities


An electrical circuit is like a central heating system:

There is a pump that pushes water around the system.

boilerand

pump

high pressurelow pressure

radiator

What is an electrical circuit?

The water everywhere starts to move at the same time.

There are pipes that carry the water.

In the pipes the water is flowing.


What is an electrical circuit?

Instead of a flow of water, electricity flows in an electrical circuit.

The electrical current is pushed by the cell (or battery), which has the same function as the pump and boiler. The strength of push provided by the battery is called its voltage.

The bulb in the circuit is like a radiator. An electrical device uses electrical energy supplied by the circuit.

The wires are like pipes; they carry the flow of electricity (called current) around the circuit.


Components in an electrical circuit


Match the circuit symbols


Parts of a plug



Contents

Circuits

Series circuits

Parallel circuits

Energy in circuits

Summary activities


This is a simple series circuit.

What is a series circuit?

In a simple series circuit, everything is connected in one loop across the terminals of the battery. There are no points where the current can split or join (these are called junctions).

This circuit has two lamps connected in series. Circuits are always drawn using straight lines.


Here are some other simple series circuits:

2. A variable resistor (or rheostat) and a bulb connected in series:

R1 R2

Examples of series circuits

1. Two resistances (resistors) connected in series:


The unit of measure for current is the amp, which has the symbol A.

A

Measuring current

AA

Current is measured using a device called an ammeter. In a circuit diagram, an ammeter is shown by the symbol .

When measuring the current through a component, the ammeter is always connected in series (in the same loop) with that component.


1

2

1. Set up the circuit as shown above.

R1

A

A

Experiment 1: Current in series circuit

Circuit 1

2. Measure the current using the ammeter at positions 1 and 2.


3

1. Add another resistor into the circuit (R2) and another ammeter after it.

R1

A

A

A

R2

Experiment 1: Current in a series circuit

Circuit 2

1

2

2. Now measure the current using the ammeter at positions 1, 2 and 3.


Circuit 2 results:

Current at position 1 =



Circuit 1 results:



Experiment 1: Current in a series circuit – results

R1A

A

Circuit 1 Circuit 2

R1 R2A

AA1

2

1

2

3


Experiment 1: Current in a series circuit – summary

Circuit 1The current at different positions in the circuit, before and after the resistor, was the _______.

Current is _____ used up by the components in the circuit.

Circuit 2Increasing the number of components in the circuit ________ the current.

The current at all points in a series circuit is the _______.

same / same / decreased / not

Circuit 1 Circuit 2

R1A

A R1 R2A

AA


When measuring the voltage across a component, the voltmeter is always connected in parallel with (or across) the component.

Voltage is measured using a device called a voltmeter. In a circuit diagram, a voltmeter is given the symbol . V

This is still a series circuit.

Measuring voltage

The voltage supplied by the battery is shared between allthe components in a series circuit.

V3

V2

V1


A

V

componenthere

Voltage is measured by connecting the voltmeter across (or in parallel with) the component.

V

Measuring voltage across a resistance or a bulb

R

Components

Voltage is measured in volts and the symbol for this is V.



R1

V

V

Circuit 1

Experiment 2: Voltage in a series circuit

2. Connect the voltmeter across the power supply (battery) and measure the supply voltage.

3. Then connect the voltmeter across the resistor (R1) and measure this voltage.


1. Add another resistor (R2) to the circuit as shown.

V2

Experiment 2: Voltage in a series circuit

Circuit 2

R1 R2

V

V1

2. Connect the voltmeter across the power supply (battery) and measure the supply voltage.

3. Then measure the voltage across each of the resistors.


Circuit 1 results:

Voltage (supply) =

Voltage (R1) =

R1 R2

V

V1 V2

R1

V

V

Circuit 1 Circuit 2

Experiment 2: Voltage in a series circuit – results

Circuit 2 results:

Voltage (supply) =

Voltage (R1) =

Voltage (R2) =


Experiment 2: Voltage in a series circuit – summary

The current is the _______ of electricity around the circuit. The _________ is the amount of push.

When two components were put into Circuit 2, the voltage of the supply was the _______ as Circuit 1. However, the voltage across R1 __________ .

The voltage across both components in Circuit 2 added to be equal to the ________ voltage.

R1 R2

V

V1 V2

R

V

V

Circuit 1 Circuit 2

supply / decreased / voltage / flow / same


Current or voltage?



V

V

R A

Experiment 3: Cells in a series circuit

Circuit 1

2. Connect the voltmeter across the power supply (battery) and measure the supply voltage. Then measure the voltage across the resistance. Also measure the current.


1. Add an additional battery to the circuit.

V

V

R A

Experiment 3: Cells in a series circuit

Circuit 2

2. Connect the voltmeter across the power supply and measure the supply voltage. Then measure the voltage across the resistance. Also measure the current.


Circuit 1 results:

Voltage (supply) =

Voltage (R) =

Current =

Circuit 2 results:

Voltage (supply) =

Voltage (R) =

Current =

Experiment 3: Cells in a series circuit – results

R

V

V

A R

V

V

A

Circuit 1 Circuit 2


Experiment 3: Cells in a series circuit – summary

Delete the wrong answer:

Increasing the number of cells increases/decreases the current that flows in the circuit.

The current/voltage depends on the current/voltage.

Circuit 1 Circuit 2

R

V

V

A R

V

V

A


1. In a series circuit the current is the same in all parts of the circuit.

Series circuits – key ideas

2. The supply voltage is shared between the components in a series circuit. (The sum of the voltage across each component is the same as the total supply voltage.)

3. The current depends on the voltage in any circuit.


Make your own series circuit



Contents

Circuits

Series circuits

Parallel circuits

Energy in circuits

Summary activities


What is a parallel circuit?

A B

This means that there is more than one path around the circuit.

A parallel circuit is one which contains a point (a junction) where the current can split (point A) or join (point B).


A1

A2

A3

A4

1. Place the ammeter, in turn, at positions 1, 2, 3 and 4.

Measuring current in a parallel circuit

Ammeter Current (A)

A1

A2

A3

A4

2. Record the ammeter readings in the table.


In a parallel circuit, the current that leaves the cell is the same as the current that returns to the cell.

Current in a parallel circuit

The current does not get used up by the circuit, just the energy that the electrons are carrying.

A1

A2

A3

A4

A1 = A4


The current splits up at the first junction and then joins together at the second junction.

Current in a parallel circuit

If the bulbs are identical then the current will split evenly.

The following is always true for this type of parallel circuit:A1

A2

A3

A4

A1 = (A2 + A3) = A4

If the bulbs are not identical, then the current will not split evenly.


V2

Connect up this circuit and measure, in turn, the voltage at V1, V2 and V3. Record your results in the table.

Measuring voltage in a parallel circuit

What do you notice about the results?

How can you explain this?

Voltmeter Voltage (V)

V1

V2

V3

V1

V3


Make your own parallel circuit



Contents

Circuits

Series circuits

Parallel circuits

Energy in circuits

Summary activities


Energy transfer in circuits

Energy cannot be created or destroyed.

In all devices and machines, including electric circuits, energy is transferred from one type to another.

When this circuit is connected, chemical energy stored in the battery is transferred via electrical energy to heat and light energy in the bulbs.

The total amount of heat and light energy is the same as the amount of chemical energy lost from the battery.


Most of the energy from the battery does not produce light – most of it is wasted as heat!

chemical energy from battery (e.g. 100 J)

heat energy of bulb

95 J transferred to

5 J transferred to bulb as light energy

Energy transfer in electrical circuits


Batteries can power many electrical devices.

What sort of energy is electrical energy transferred into in these electrical devices?

What’s the energy transfer?



Contents

Circuits

Series circuits

Parallel circuits

Energy in circuits

Summary activities


Glossary

ammeter – A device that measures electric current.

circuit – A complete loop of conducting components that electricity flows around.

current – The flow of electricity, measured in amps (A).

parallel – A branched circuit – it has components connected on separate branches.

resistor – A component that makes it difficult for electricity to flow and so reduces the current.

series – A circuit without any branches – it has components connected in a row.

voltage – The amount of ‘push’ that a cell gives a circuit, measured in volts (V).

voltage – A device that measures voltage.


Anagrams


Compare a series and a parallel circuit


Electrical circuits – true or false?


Multiple-choice quiz

Documents

© Boardworks Ltd 2004 1 of 20 © Boardworks Ltd 2006 1 of 45 KS3 Physics 7J Electrical Circuits