7 j electrical circuits (boardworks)

KS3 Physics

7J Electrical Circuits
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© Boardworks Ltd 2004
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Contents

Introducing circuits
Series circuits
Parallel circuits
Energy in circuits
Summary activities
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What is an electrical circuit?
An electrical circuit is like a central heating system in a house.
There is a pump
that pushes water
around the system.
The water
everywhere starts
to move at the
same time.
There are pipes
that carry the
water.
In the pipes the
water is flowing.
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3

low
pressure

boiler
and
pump

high
pressure

radiator
radiator

flow of
water
2005

What is an electrical circuit?
The bulb in the circuit is like a radiator. An electrical device
uses electrical energy supplied by the circuit.
Instead of a flow of
water, electricity flows
in an electrical circuit.
The wires are like
pipes; they carry the
flow of electricity
(called current)
around the 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.
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Components in an electrical circuit

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Contents

Series circuits
Parallel circuits
Energy in circuits
Summary activities
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What is a series circuit?

This is a simple series circuit.
In a simple series circuit, everything is
connected in one loop across the terminals
of the battery. So there aren’t any 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.

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Examples of series circuits
Here are some other simple series circuits:
1. Two resistances (resistors) connected in series:

R1

R2

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

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Measuring current
The unit of measure for current is the amp, which has the
symbol A.
Current is measured using a device called an ammeter.
In a circuit diagram, an ammeter is shown by the symbol A .
When measuring the current through a component, the
ammeter is always connected in series (in the same loop)
with that component.
A

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Experiment 1: Current in series circuit
Circuit 1
1

A
R1

A
2

1. Set up the circuit as shown above.
2. Measure the current using the ammeter at
positions 1 and 2.

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Experiment 1: Current in a series circuit
Circuit 2
1

A

A
R1

A

3

R2

2

1. Add another resistor into the circuit (R2) and another
ammeter after it.
2. Now measure the current using the ammeter at
positions 1, 2 and 3.
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Experiment 1: Current in a series circuit – results
Circuit 1
1

Circuit 2

A

1

R1

A
2

A

A 3
R1

A
2

R2

Circuit 1 results:

Circuit 2 results:

Current at position 1 =




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Experiment 1: Current in a series circuit – summary
Circuit 1

Circuit 2

A

A
R1

A

A
R1

A

R2

Circuit 1
The 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 2
Increasing the number of components in the circuit
________ the current.
The current at all points in a series circuit is the _______.

same / same / decreased / not
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Measuring voltage
Voltage is measured using a device called a voltmeter.
In a circuit diagram, a voltmeter is given the symbol V .
When measuring the voltage across a component, the
voltmeter is always connected in parallel with (or across)
the component.
V1

This is still
a series circuit.

V2

V3
The voltage supplied by the battery is shared between all
the components in a series circuit.
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Measuring voltage across a resistance or a bulb
Voltage is measured by connecting the voltmeter
across (or in parallel with) the component.

V

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

Components
component
here

R

V

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Experiment 2: Voltage in a series circuit
Circuit 1
V

R1
V
2. Connect the voltmeter across the power supply
(battery) and measure the supply voltage.
3. Then connect the voltmeter across the resistance (R)
and measure this voltage.
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Experiment 2: Voltage in a series circuit
Circuit 2
V

R1

R2

V1

V2

1. Add another resistor (R2) to the circuit as shown.
2. Connect the voltmeter across the power supply (battery)
and measure the supply voltage.
3. Then measure the voltage across each of the resistor.
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Experiment 2: Voltage in a series circuit – results
Circuit 1

Circuit 2

V

V

R1

R1

R2

V

V1

V2

Circuit 2 results:

Circuit 1 results:
Voltage (supply) =

Voltage (supply) =

V

Voltage (R1)

V

Voltage (R1)

=

V

Voltage (R2)

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V

=

V

=

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Experiment 2: Voltage in a series circuit – summary
Circuit 1

V

Circuit 2

V

R

R1

R2

V

V1

V2

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.

supply / decreased / voltage / flow / same
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Experiment 3: Cells in a series circuit
Circuit 1

V

R

A

V
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.

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Experiment 3: Cells in a series circuit
Circuit 2

V

R

A

V
1. Add an additional battery to the circuit.
2. Connect the voltmeter across the power supply and
measure the supply voltage. Then measure the voltage
across the resistance. Also measure the current.
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Experiment 3: Cells in a series circuit – results
Circuit 1

R

V

A

V

Circuit 2

R

V

A

V

Circuit 1 results:
Supply voltage =

Supply voltage =

Voltage R =

Voltage R =

Current =

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Circuit 2 results:

Current =

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Experiment 3: Cells in a series circuit – summary
Circuit 1

R

V

A

V

Circuit 2

R

V

A

V

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.

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Series circuits – key ideas
1. In a series circuit the current is the same in all parts
of the circuit.

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.

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Make your own series circuit

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Contents

Series circuits
Parallel circuits
Energy in circuits
Summary activities
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What is a parallel circuit?
A parallel circuit is one which contains a point (a junction)
where the current can split (point A) or join (point B).
This means that there is more than one path around the
circuit.

A

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B

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Measuring current in a parallel circuit
1. Place the ammeter, in turn, at positions 1, 2, 3 and 4.

A1

A2

A4

A3
2. Record the ammeter readings in the table.
Ammeter

Current (A)

A1
A2
A3
A4
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Current in a parallel circuit
For a parallel circuit, the current that leaves the cell is the
same as the current that returns to the cell.

A1

A2

A4

A1 = A4

A3
The current does not get used up by the circuit, just the
energy that the electrons are carrying.

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Current in a parallel circuit
The current splits up at the first junction and then joins
together at the second junction.

A1

A2
A3

A4

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

A1 = (A2 + A3) = A4

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

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

Voltmeter
V2

Voltage(V)

V1
V2

V3

V3

What do you notice about the results?
How can you explain this?
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Make your own parallel circuit

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Contents

Series circuits
Parallel circuits
Energy in circuits
Summary activities
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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.

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Energy transfer in electrical circuits
5 J transferred to bulb
as light energy

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

95 J transferred to

heat energy
of bulb

Most of the energy from the battery does not produce light
– most of it is wasted as heat!
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What’s the energy transfer?
Batteries can power many electrical devices.

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

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Contents

Series circuits
Parallel circuits
Energy in circuits
Summary activities
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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.
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Anagrams

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Compare a series and a parallel circuit

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Multiple-choice quiz

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7 j electrical circuits (boardworks)

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