P2 Revision Booklet

Energy in Circuits

Harder (Grade B-A*)

Q1.         The graph shows how the electric current through a 12 V filament bulb varies with the potential difference across the bulb.

(a)     What is the meaning of the following terms?
electric current
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potential difference
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(2)

(b)     The resistance of the metal filament inside the bulb increases as the potential difference across the bulb increases.
Explain why.
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(3)

(c)     Use data from the graph to calculate the rate at which the filament bulb transfers energy, when the potential difference across the bulb is 6 V.
Use the correct equation from the Physics Equations Sheet.
Show clearly how you work out your answer.
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                                       Rate of energy transfer = ................................... W
(2)
(Total 7 marks)
 Q2.         A homeowner has installed electric underfloor heating in the kitchen. When the heating is switched on, an electric current flows through wires running under the tiled floor surface.
(a)     What is an electric current?
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(1)

(b)     The graph shows how the power output of an underfloor heating system depends on the area of the floor that is heated.

The area of the homeowner’s kitchen floor is 9.0 m2.
Use the graph and the equation in the box to calculate the current drawn from the 230 V mains supply by the heating system.
 
power    =    current    ×    potential difference	

Show clearly how you work out your answer and give the unit.
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                          Current = ....................................................................
(4)
(Total 5 marks)

Q3.          When a mains lamp is switched on it takes 0.5 seconds for the filament to reach its normal operating temperature. The way in which the current changes during the first second after switching on is shown in the sketch graph below. Mains voltage is 240 V.

(a)     Calculate the resistance of the filament whilst the lamp is drawing the maximum current.
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(3)

(b)     Describe how the resistance of the lamp changes after the current has reached its maximum value.
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(2)

(c)     Calculate the maximum power taken by the lamp.
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(2)

(d)     Calculate the power of the lamp in normal use.
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(2)

(e)     Calculate the energy used by the lamp in six hours of normal use.
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(3)
(Total 12 marks)
 Energy in circuits- Mark scheme

M1.         (a)     electric current
(rate of) flow of (electric) charge / electrons
accept 
with Q and t correctly named
1

potential difference
work done / energy transferred per coulomb of charge (that passes between two points in a circuit)
accept 
with W and Q correctly named
1

(b)     metals contain free electrons (and ions)
accept mobile for free
1

as temperature of filament increases ions vibrate faster / with a bigger amplitude
accept atoms for ions
accept ions/atoms gain energy
accept vibrate more for vibrate faster
do not accept start to vibrate 
1

electrons collide more (frequently) with the ions
or
(drift) velocity of electrons decreases
do not accept start to collide
accept increasing the p.d. increases the temperature (1 mark)
and
(and) resistance increases with temperature (1 mark) if no other marks scored
1

(c)     7.8
allow 1 mark for obtaining value 1.3 from graph
or allow 1 mark for a correct calculation using an incorrect current in the range 1.2-1.6 inclusive
2
[7]
M2.          (a)     (rate of) flow of charge / electrons / ions
accept movement for flow
do not accept flow of electricity
1

(b)     7(.0)
accept 6.96 / 6.95 or an answer that would approximate to 6.96 if rounded
allow 1 mark for obtaining correct power and changing to watts ie 1600
or
allow 2 marks for correct substitution and transformation
ie 1600 ÷ 230
an answer 0.00696 / 0.007 gains 2 marks
allow 1 mark for 1.6 / 230 or 1.7 / 230
an answer 7.39 or 7.4 gains 2 marks
3

amp (ere)
accept A
1
[5]
M3.          (a)     Current = 0.4A (1) 
R = V/I or 240/0.4 (1)
R = 600 ohm (1)
3

(b)     Doubles
gets 2 marks
          OR gets bigger
gets 1 mark
2

(c)     P = V.I or 240 × 0.4
P = 96W
for 1 mark each
2

(d)     1 = 0.2A
P = 48W
for 1 mark each
BUT may get equation mark here if not in (c)
2

(e)     P = V.I.t (1)
P = 240 × 0.2 × 6 × 3600
OR P = 48 × 6 × 3600
gets 1 mark
P = 1036800 W
gets 1 mark
3
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