Electrical current depends on resistance and potential difference. Different electrical components have different characteristics. These can be investigated using suitable circuits and apparatus.

Current is the rate of flow of charge. In metal wires, electrons move and cause a current. The following conditions are needed for an electric current:

- a source of potential difference, such as a battery, cell or power pack
- a closed circuit, which provides a complete path for the charges to move through

Originally, current was defined as the flow of charge from positive to negative. Scientists later discovered that current is actually the flow of electrons, from negative to positive. The original definition is now referred to as 'conventional current', to avoid confusion with the newer definition of current.

To calculate current, use the equation:

charge flow = current × time

This is when:

- charge flow is measured in coulombs (C)
- current is measured in amperes (amps) (A)
- time is measured in seconds (s)

Each electron in a circuit carries a very small charge but there are many billions of electrons present. Many everyday currents for small household appliances will be measured in milliamps, mA: 1,000 mA = 1 A.

A current of 60 mA passes through a lamp for half an hour. Calculate the charge transferred.

60 mA = 60 ÷ 1,000 = 0.060 A

0.5 hours = 30 minutes

= 30 × 60 = 1,800 s

charge flow = current × time

= 0.060 × 1,800

= 108 C

- Question
A charge of 5.0 C is transferred through a wire in 20 s. Calculate the current in the wire.

First rearrange the equation to find current:

charge flow = current × time