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KS3 Physics
7J Electrical Circuits
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7J Electrical Circuits
Contents
Introducing circuits
Series circuits
Parallel circuits
Energy in circuits
Summary activities
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boilerand
pump
radiator
highpressure
lowpressure
flow ofwater
An electrical circuit is like a central heating system in a house.
There is a pump that pushes water around the system.
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.
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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.
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Components in an electrical circuit
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7J Electrical Circuits
Contents
Introducing circuits
Series circuits
Parallel circuits
Energy in circuits
Summary activities
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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. 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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Here are some other simple series circuits:
2. A rheostat (or variable resistor) and a bulb connected in series:
R1 R2
Examples of series circuits
1. Two resistances (resistors) connected in series:
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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.
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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.
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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.
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Circuit 2 results:
Current at position 1 =
Current at position 2 =
Current at position 3 =
Circuit 1 results:
Current at position 1 =
Current at position 2 =
Experiment 1: Current in a series circuit – results
R1A
A
Circuit 1 Circuit 2
R1 R2A
AA1
2
1
2
3
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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
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Make your own series circuit
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7J Electrical Circuits
Contents
Introducing circuits
Series circuits
Parallel circuits
Energy in circuits
Summary activities
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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).
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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.
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For 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
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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.
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Make your own parallel circuit
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7J Electrical Circuits
Contents
Introducing circuits
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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Most of the energy from the battery does not produce light – most of it is wasted as heat!
chemical energy from battery (e.g. 100J)
heat energy of bulb
95 J transferred to
5 J transferred to bulb as light energy
Energy transfer in electrical circuits
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Batteries can power many electrical devices.
What sort of energy is electrical energy transferred into in these electrical devices?
What’s the energy transfer?
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7J Electrical Circuits
Contents
Introducing circuits
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