Electrical power

Power is the rate of transfer of energy between energy stores.

curriculum-key-fact
One watt (W) is equal to one joule per second (J/s).

Energy transferred

The energy transferred can be calculated using the equation:

power = \frac{energy~transferred}{time}

This is when:

  • power is measured in watts (W)
  • energy is measured in joules (J)
  • time is measured in seconds (s)

Example

When a lamp is switched on for 60 s, 3,000 J of energy are transferred. Calculate the power of the lamp.

power = \frac{energy~transferred}{time}

= 3,000 \div 60

= 50~W

The energy transferred can be calculated using the same equation but with different units:

  • energy is measured in kilowatt-hours (kWh)
  • power is measured in kilowatts (kW)
  • time is measured in hours (h)

This is normally used when considering domestic electricity usage.

Example

A 500 W television set is switched on for 4 hours. Calculate the energy transferred.

500~W = \frac{500}{1,000} = 0.5~kW

power = \frac{energy~transferred}{time}

Rearrange the equation:

energy~transferred = power \times time

= 0.5 \times 4

= 2~kWh

Energy, voltage and charge

When a charge moves through a potential difference, electrical work is done and energy transferred. The energy transferred can be calculated using the equation:

energy transferred (work done) (J) = charge (C) × potential difference (V)

E = V \times Q

This is when:

  • energy (E) is measured in joules (J)
  • potential difference (V) is measured in volts (V)
  • charge (Q) is measured in coulombs (C)

One volt is the potential difference when one coulomb of charge transfers one joule of energy.

Example

How much energy is transferred when 3 C of charge moves through a potential difference of 6 V?

E = V \times Q

E = 6 \times 3

E = 18~J

Question

What is the potential difference between two points if 2 C of charge shifts 4 J?

E = V \times Q

V = \frac{E}{Q}

V = \frac{4}{2}

V = 2~V