Download What is an electrical circuit?

KS3 Physics
7J Electrical Circuits
1 of 20
41
© Boardworks Ltd 2005
2004
Contents
7J Electrical Circuits
Introducing circuits
Series circuits
Parallel circuits
Energy in circuits
Summary activities
1
2 of 20
41
© Boardworks Ltd 2005
2004
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.
1
3 of 20
41
low
pressure
boiler
and
pump
high
pressure
radiator
radiator
flow of
water
© Boardworks Ltd 2005
2004
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.
1
4 of 20
41
© Boardworks Ltd 2005
2004
Components in an electrical circuit
1
5 of 20
41
© Boardworks Ltd 2005
2004
Contents
7J Electrical Circuits
Introducing circuits
Series circuits
Parallel circuits
Energy in circuits
Summary activities
1
6 of 20
41
© Boardworks Ltd 2005
2004
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.
1
7 of 20
41
© Boardworks Ltd 2005
2004
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:
1
8 of 20
41
© Boardworks Ltd 2005
2004
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
1
9 of 20
41
© Boardworks Ltd 2005
2004
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.
1
10ofof20
41
© Boardworks Ltd 2005
2004
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.
1
11ofof20
41
© Boardworks Ltd 2005
2004
Experiment 1: Current in a series circuit – results
Circuit 1
Circuit 2
1 A
1 A
R1
A
2
A 3
R1
A
R2
2
Circuit 1 results:
Circuit 2 results:
Current at position 1 =
Current at position 1 =
Current at position 2 =
Current at position 2 =
Current at position 3 =
1
12ofof20
41
© Boardworks Ltd 2005
2004
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
1
13ofof20
41
© Boardworks Ltd 2005
2004
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.
1
14ofof20
41
© Boardworks Ltd 2005
2004
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
1
15ofof20
41
© Boardworks Ltd 2005
2004
Experiment 2: Voltage in a series circuit
Circuit 1
V
R1
V
1. Set up the circuit as shown above.
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.
1
16ofof20
41
© Boardworks Ltd 2005
2004
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.
1
17ofof20
41
© Boardworks Ltd 2005
2004
Experiment 2: Voltage in a series circuit – results
Circuit 1
Circuit 2
V
R1
R1
R2
V
V1
V2
Circuit 2 results:
Circuit 1 results:
1
18ofof20
41
V
Voltage (supply) =
V
Voltage (supply) =
V
Voltage (R1)
V
Voltage (R1)
=
V
Voltage (R2)
=
V
=
© Boardworks Ltd 2005
2004
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
1
19ofof20
41
© Boardworks Ltd 2005
2004
Experiment 3: Cells in a series circuit
Circuit 1
V
R
A
V
1. Set up the circuit as shown above.
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
20ofof20
41
© Boardworks Ltd 2005
2004
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.
1
21ofof20
41
© Boardworks Ltd 2005
2004
Experiment 3: Cells in a series circuit – results
Circuit 1
Circuit 2
V
R
A
V
1
22ofof20
41
R
V
A
V
Circuit 1 results:
Circuit 2 results:
Supply voltage =
Supply voltage =
Voltage R =
Voltage R =
Current =
Current =
© Boardworks Ltd 2005
2004
Experiment 3: Cells in a series circuit – summary
Circuit 1
Circuit 2
V
R
A
V
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.
1
23ofof20
41
© Boardworks Ltd 2005
2004
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.
1
24ofof20
41
© Boardworks Ltd 2005
2004
Make your own series circuit
1
25ofof20
41
© Boardworks Ltd 2005
2004
Contents
7J Electrical Circuits
Introducing circuits
Series circuits
Parallel circuits
Energy in circuits
Summary activities
1
26ofof20
41
© Boardworks Ltd 2005
2004
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
1
27ofof20
41
B
© Boardworks Ltd 2005
2004
Measuring current in a parallel circuit
1. Place the ammeter, in turn, at positions 1, 2, 3 and 4.
A4
A1
A2
A3
2. Record the ammeter readings in the table.
Ammeter
Current (A)
A1
A2
A3
A4
1
28ofof20
41
© Boardworks Ltd 2005
2004
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.
A4
A1
A2
A1 = A4
A3
The current does not get used up by the circuit, just the
energy that the electrons are carrying.
1
29ofof20
41
© Boardworks Ltd 2005
2004
Current in a parallel circuit
The current splits up at the first junction and then joins
together at the second junction.
A4
A1
A2
A3
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.
1
30ofof20
41
© Boardworks Ltd 2005
2004
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?
1
31ofof20
41
© Boardworks Ltd 2005
2004
Make your own parallel circuit
1
32ofof20
41
© Boardworks Ltd 2005
2004
Contents
7J Electrical Circuits
Introducing circuits
Series circuits
Parallel circuits
Energy in circuits
Summary activities
1
33ofof20
41
© Boardworks Ltd 2005
2004
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.
1
34ofof20
41
© Boardworks Ltd 2005
2004
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!
1
35ofof20
41
© Boardworks Ltd 2005
2004
What’s the energy transfer?
Batteries can power many electrical devices.
What sort of energy is electrical energy transferred into
in these electrical devices?
1
36ofof20
41
© Boardworks Ltd 2005
2004
Contents
7J Electrical Circuits
Introducing circuits
Series circuits
Parallel circuits
Energy in circuits
Summary activities
1
37ofof20
41
© Boardworks Ltd 2005
2004
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.
1
38ofof20
41
© Boardworks Ltd 2005
2004
Anagrams
1
39ofof20
41
© Boardworks Ltd 2005
2004
Compare a series and a parallel circuit
1
40ofof20
41
© Boardworks Ltd 2005
2004
Multiple-choice quiz
1
41ofof20
41
© Boardworks Ltd 2005
2004