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5 a)
Full Answers to Review Questions: 12 Potential difference, electromotive force and power
resistance. Also heat will be generated in the coil of the motor and in
the connecting wires due to the current in them having to overcome
their electrical resistance.
copper wire
V
A
� Figure A.2
b) Rate of doing work = power = VI = 0.53 V × 1.8 A = 0.95(4) W
c) Rearranging I = nAvq:
v = I ____
nAq = ______________________________________
1.8 A
8.0 × 1028 m –3 × 0.059 × 10 –6 m2 × 1.6 × 1019 C
d) Using P = Fv (see page 48):
F = P
__
v =
_____________ 0.954 J s–1
–1 = 400 N (to 2 significant figures)
2.4 × 10 –3 m s
6 a) Power delivered by alternator = VI = 14 V × 70 A = 980 W
b) Current taken by starter motor is given by rearranging P = VI:
I = P __ = _______ 1500 W
= 125 A
V 12 V
c) For each headlight:
I = P __ =
60 _____ W
= 5 A
V 12 V
so next fuse up, i.e. 8 A, would be suitable.
d) Energy = VIt = 12 V × 1 A × 62 × 60 × 60 s ≈ 2.7 MJ
e) If the four sidelights are left on for 12 hours:
energy used = 4 × 5 W × 12 × 60 × 60 s = 864 000 J
battery capacity, from part d), is 2 678 400 J
fraction used =
864 000 J
_________
2 678 400 J
1
= 0.32 ≈ _
3
= 2.4 mm s−1
f) Using P = Fv (see page 48), power to drive at 90 km per hour up a
gradient of 10% is given by:
P = 0.1 × mg × v = 0.1 × 1740 kg × 9.8 N kg−1 × 90 × 103 _________ m
60 × 60 s
= 43 kW (to 2 significant figures)
Maximum power is stated as 180 kW, so fraction of maximum power
= 43 kW
_______
180 kW
1
= 0.24 ≈ _
4
7 a) Energy saved over 8000 hours = (60 − 18) W × 8000 h = 42 × 8 kWh =
336 kWh
cost of energy saved will be = 336 × 15 p = £50.40
Allowing for £2 cost of lamp, total cost saved will be £48.40, which
comfortably exceeds the manufacturer’s claim.
b) Apart from monetary cost, the saving in energy is important in
conserving the world’s energy resources as well as reducing the carbon
footprint (less electrical energy consumed means less carbon dioxide is
Edexcel Physics for AS © Hodder Education 2009
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