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Electricity
The following formulae and symbols may be useful.
vaverage 
d
t
F  ma
Ep  mgh
Fgravity  mg
Work  Fd
P
g  10 m s –2
g  10 N kg –1
E
t
a
change in speed
change in time
Ek 
1 2
mv
2
V  IR
P  IV
QUESTION ONE: ELECTRICITY
Study the circuit diagram below. The two lamps are NOT identical.
When an ammeter is placed in this circuit a reading of 1.2 A is recorded.
(a)
Using the equation V = IR, calculate the voltage across the 4 Ω lamp.
Voltage =
(b)
V
Calculate the power output of the 4 Ω lamp. Give an appropriate unit.
Power =
(
)
unit
(c)
In terms of energy, explain what the power output in (b) means.
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(d)
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Calculate the resistance of lamp B.
Resistance =
Ω
(e)
Describe how the brightness of lamp B compares with the 4 Ω lamp.
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The circuit is now altered so that the same lamps are in parallel, as shown below.
(f)
On the diagram above, draw an ammeter in the correct place to record the total current
supplied by the power supply.
When this ammeter is placed in the circuit, a reading of 5 A is recorded.
(g) Explain why the current is larger in the parallel circuit than the series circuit.
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(h)
State which lamp will be brighter, the 4 Ω lamp or lamp B, and discuss the reasons for
this lamp being brighter. You should consider current, resistance and power output.
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QUESTION TWO:
TORCHES
This is a diagram of the inside of a standard torch.
Metal conductors
Light bulb
Metal slide
switch
Two “C” cells
Reflector
(a)
In the box below, draw a circuit diagram of the torch.
Metal conductors
(b)
Each battery in this torch delivers 1.5 V to the light bulb.
(i)
Determine the voltage across the light bulb.
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(iii) The current flowing through the light bulb is 6.5 A. Calculate the resistance of the bulb.
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__________________________________________ Resistance = __________________ Ω
LED (light emitting diode) torches use less power to generate the same brightness as a
regular light bulb. The following is a circuit diagram showing three identical LEDs in a
torch.
(c)
The LED lamps are connected in parallel. Give two reasons why the manufacturers of
the LED lamps chose to connect the led lamps in parallel rather than series.
Reason 1
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Reason 2
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(d)
The set of LED’s are connected to a 4.5 V battery. Determine the voltage across each
of the three LED’s.
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(e)
Each LED has a power rating of 1.5 W. Calculate the resistance of one of the LED’s.
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______________________________________ Resistance = ________________ Ω