Current

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:

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.

Calculating 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.

Example calculation

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

current = \frac{charge~flow}{time}

current = \frac{5.0}{20}

= 0.25~A