# Electric energy and power for OCR A-level Physics 1. Power, voltage and current
2. Power, current and resistance
3. Energy, power and time

The relationship between power, voltage and current is given by: Power = Voltage × Current. Ohm's Law can be used with the relationship of power, voltage and current to come up with a different relationship betweeen power, current and resistance, given by P = I²R. The energy transferred to an appliance can be related its power rating and amount of time for which it is switched on through the following formula: Power = Energy transferred/time. # 1

A light bulb has a power rating of 100 W with a potential difference of 240 V across it. Calculate the current flowing through it.

P = IV
I = P ÷ V
I = 100 ÷ 240 = 0.42 A (2 d. p.)

0.42 A # 2

A vacuum has a power rating of 1500 W. Given that there is a current of 12 A flowing through it, calculate the resistance flowing through it.

Using P = I²R, 1500 W = (12 A)²R, therefore R = 10.4 Ω (to 3 significant figures). # 3

Given that the current through a laptop is 0.3 A and a resistance of 500 Ω, calculate the power rating of the laptop.

Using P = I²R, Power = (0.3 A)²(500 Ω) = 45 W. # 4

The power rating of a kettle is 2.5 kW. Calculate the amount of energy in Joules transferred to the kettle in 2 minutes.

2.5 kW = 2500 W.
Using P = E/t, 2500 W = E/(2 × 60 s), so E = 300000 J. # 5

An electric motor is used in a lift. Given that the power rating of the lift is 6500 W and the lift is carried up in 50 s, calculate the energy transferred in the motor.

Using P = E/t, 6500 W = E/50 s, so E= 6500 W × 50 s = 325000 J. End of page