The energy transferred by an electric current depends on three things: how large the current is, the potential difference (p.d.) it is driven by, and how long it flows. The equation is:
energy transferred = current × potential difference × time
Energy is in joules (J), current in amperes (A), potential difference in volts (V) and time in seconds (s). If the time is given in minutes, change it to seconds first.
Example 1: a motor has a current of 2 A at 12 V for 10 s. Energy transferred = 2 × 12 × 10 = 240 J.
Example 2: a lamp has a current of 0.5 A at 6 V for 2 minutes. Two minutes is 120 s, so the energy transferred = 0.5 × 6 × 120 = 360 J.
The equation can be rearranged. For time, divide the energy transferred by the current multiplied by the potential difference. A 10 V device with a current of 2 A that transfers 600 J runs for 600 ÷ 20 = 30 s.