# Electrical power

is the rate of transfer of energy between .

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

## Energy transferred

The energy transferred can be calculated using the equation:

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.

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.

Rearrange the equation:

## Energy, voltage and charge

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

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

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?

Question

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