Transcript 4. 9I Energy and Electricity

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Measuring Current.
The unit of measure for current is the "amp" which
has the symbol A.
We measure the current using a device called an
ammeter. In a circuit this is given 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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Measuring Voltage.
We measure the voltage using a device called an
voltmeter. In a circuit this is given the symbol V
When measuring the voltage across a component, the
voltmeter is always connected in parallel with (or across)
the component.
V
1
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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Series circuit animation
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Measuring Voltage - across a resistance or a bulb
A
Components
component
here
R
V
Voltage is measured by connecting the voltmeter
(or in parallel) with the component.
V across
Voltage is measured in volts and the symbol for this is V.
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Experiment: Measuring Voltage
V
Circuit 1
R
V
1. Set up the circuit as shown above.
2. Connect the voltmeter across the power supply and
measure the supply voltage.
3. Then connect the voltmeter across the resistance (R)
and measure this voltage.
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V
Circuit 2
R1
R2
V1
V2
1. Add another resistance (R2) to the circuit as shown.
2. Connect the voltmeter across the power supply and
measure the supply voltage.
3. Then measure the voltage across each of the resistances.
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Circuit 1
V
Circuit2
V
R1
R1
R2
V
V1
V2
Voltage (supply)
=
V
Voltage (R)
=
V
Voltage (supply)
=
V
Voltage (R1)
=
V
Voltage (R2)
=
V
Record your results:
Circuit 1:
Circuit 2:
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Circuit 1
V
Circuit2
Circuit2
V
R1
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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Measuring Voltage in Parallel Circuits
V1
V2
V3
Connect together the circuit shown above
and measure, in turn, the voltage at V1, V2
and V3
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Write down your results in the table below :
Voltmeter
Voltage
(V)
V1
V2
V3
Explain anything you notice about the results
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Parallel circuits
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Experiment: Measuring Current
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: Measuring Current
Circuit 2
1
A
A
R1
A
3
R2
2
1. Add another resistor into the circuit.
2. Now measure the current using the ammeter at
positions 1, 2 and 3.
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Results
Circuit 1
Current Position 1
Current Position 2
Circuit 2
Current Position 1
Current Position 2
Current Position 3
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Conclusions
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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Experiment: Cells
V
Circuit 1
R1
A
V
1. Set up the circuit as shown above.
2. Connect the voltmeter across the power supply and
measure the supply voltage. Then measure the voltage
across the resistance. Measure the current.
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Experiment: Cells
V
Circuit 2
R1
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. Measure the current.
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Results
Circuit 1: one battery
Supply Voltage
Voltage R1
Current
Circuit 2: two batteries
Supply Voltage
Voltage R1
Current
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Conclusions
Delete the wrong answer:
Increasing the number of batteries / cells
increases/decreases the current that flows in the circuit.
The current/voltage depends on the current/voltage.
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Summary for Series Circuits.
1. In a series circuit the current is the same at
any point in the circuit.
2. The supply voltage is shared between the
components in a series circuit.
3. The current depends on the voltage in ANY
circuit.
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Parallel Circuits
A
B
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.
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Measuring Current in Parallel Circuits
1
A1
A2
A4
2
4
A3
3
Ammeter
A1
A2
1. Place the ammeter,
in turn, at positions
1, 2, 3 and 4.
2. Record the
ammeter reading
at the points in the
table shown.
Current (A)
A3
A4
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For a parallel circuit, the current that leaves the cell or battery is the
same as the current that returns to the cell or battery. The current does
NOT get used up by a circuit, just the energy the electrons are
carrying.
A1 = A4
The current splits up at the first junction and then joins together at the
second junction. If the bulbs are identical then the current will split
evenly. If the bulbs are NOT identical, then the current will NOT split
evenly. The following is always true for this circuit.
A1 = A2 + A3 =A4
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Energy in Circuits
This section deals with the energy transfers in electric
circuits.
The most important thing to understand about energy is
that it cannot be created or destroyed.
In all devices and machines, energy is transferred from
one type to another.
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Energy Transfer in Electrical Circuits
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.
lamps
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Energy Transfer in electrical circuits
5J transferred to bulb as
light energy
Chemical energy
lost from battery
(e.g. 100J).
95 J Transferred to Heat energy of bulb
Notice, most of the energy from the
battery does not produce light - most
is wasted as heat!
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Energy Efficiency
We can work out the efficiency of an energy
transfer:
%Efficiency =
useful energy output
x 100
total energy input
For this bulb
efficiency = (5/100) x 100 = 5%.
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Other energy transfers
Batteries can power many things
What sort of energy is the electrical
energy transferred into in these
examples?
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