Teacher's notes and answers to questions in the book - Hodder Plus ...

WJEC GCSE Additional Science Teacher’s Notes
_ Electrical power (pages 162–63)_________________
Questions
14. Calculate the power of a 6 V torch bulb drawing a current of 0.8 A.
P = V × I = 6 × 0.8 = 4.8 W
15. A current of 5 A passes through a lamp with a resistance of 2.4 Ω, and then through a small cooling fan
of resistance 4 Ω. Calculate the power of each component and hence calculate the total power drawn
from the circuit.
Lamp: P = I 2 × R = 5 2 × 2.4 = 60 W
Fan: P = I 2 × R = 5 2 × 4 = 100 W
Total power = 160 W
16. Study the circuit diagram in Figure 14.14.
a Calculate the power of each bulb.
6 bulb: P = I 2 × R = 2.5 2 × 6 = 37.5 W
12 bulb: P = I 2 × R = 2.5 2 × 12 = 75 W
b Calculate the total power drawn from the power supply.
Total power = 37.5 + 75 = 112.5 W
c
Calculate the voltage of the power supply.
V
P 112.5 45 V
I 2.5
d Calculate the voltage across each bulb.
6 bulb: V = I × R = 2.5 × 6 = 15 V
12 bulb: V = I × R = 2.5 × 12 = 30 V
17. A mains hairdryer operates with a voltage of 220 V.
a Calculate the power when it is on its HIGH setting, drawing a current of 8 A.
P = V × I = 220 × 8 = 1760 W
b The LOW setting operates with a power of 1 kW (1000 W). Calculate the current flowing through the
hairdryer.
P 1000
I 4.5 A
V 220
c
The hairdryer can also be used in the United States where the mains voltage is different. The power
given out by the hairdryer is the same as in the UK (your answer to part a), but the current flowing
through the hairdryer is 16 A. Calculate the voltage of the mains in the USA.
P 1760
V 110 V
I 16
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