Download 9I Energy and Electricity

KS3 Physics
9I Energy and
Electricity
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Contents
9I Energy and Electricity
Measuring current
Measuring voltage
Energy in circuits
Summary activities
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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
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 =
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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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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.
A4
A1
A2
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.
A4
A1
A2
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.
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.
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Contents
9I Energy and Electricity
Measuring current
Measuring voltage
Energy in circuits
Summary activities
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Measuring voltage
Voltage is the amount of push and is measured in ‘volts’
which has the symbol V.
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.
This is still a series circuit.
V1
The voltage supplied by the
battery is shared between
all the components in a
series circuit.
V2
V3
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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.
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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
R1
R1
R2
V
V1
V2
Circuit 2 results:
Circuit 1 results:
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V
Voltage (supply) =
V
Voltage (supply) =
V
Voltage (R1)
V
Voltage (R1)
=
V
Voltage (R2)
=
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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Make your own series circuit
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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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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.
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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
Circuit 2
V
R
A
V
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R
V
A
V
Circuit 1 results:
Circuit 2 results:
Supply voltage =
Supply voltage =
Voltage R =
Voltage R =
Current =
Current =
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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.
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Current and voltage – key ideas
Current
 In a series circuit, the current is the same in all parts of
the circuit.
 In a parallel circuit, the current splits up and recombines
when the branches of the circuit meet up. (The sum of
the current in the branches equals the total current.)
 The current depends on the voltage in any circuit.
Voltage
 In a series circuit, the supply voltage is shared between
the components. (The sum of the voltage across each
component is the same as the total supply voltage.)
 In a parallel circuit, the voltage across each component
is the same as the supply voltage.
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Contents
9I Energy and Electricity
Measuring current
Measuring voltage
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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Calculating energy efficiency
The efficiency of an energy transfer can be calculated using
this formula:
%Efficiency =
useful energy output
total energy input
x 100
This bulb converts 200 J of chemical energy
form battery into 10 J of useful light energy:
Efficiency of bulb =
10 x 100
200
( )
= 5%
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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
9I Energy and Electricity
Measuring current
Measuring voltage
Energy in circuits
Summary activities
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Glossary
 current – The flow of electricity, measured in amps (A).
 efficiency – A measure of how much energy is changed
from one form to another.
 potential difference – The amount of ‘push’ or electrical
energy there is in a circuit, measured in volts (V).
 power – The amount of energy that an electrical device
uses per second, measured in watts (W).
 power station – A place where an energy resource is
transformed into electrical energy.
 transfer – To move energy from one place to another.
 transform – To change energy from one type to another.
 voltage – Another name for ‘potential difference’.
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Anagrams
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Multiple-choice quiz
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