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Electric energy and power for OCR A-level Physics

Electric energy and power

This page covers the following topics:

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 ร— Current.

Power, voltage and 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 = IยฒR.

Power, current and resistance

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.

Energy, power and 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 รท V
I = 100 รท 240 = 0.42 A (2 d. p.)

0.42 A

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.

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 = IยฒR, 1500 W = (12 A)ยฒR, therefore R = 10.4 ฮฉ (to 3 significant figures).

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.

3

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

Using P = IยฒR, Power = (0.3 A)ยฒ(500 ฮฉ) = 45 W.

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

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 ร— 60 s), so E = 300000 J.

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

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 ร— 50 s = 325000 J.

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.

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