When a direct current of I amperes is flowing in an electric circuit and the voltage across the circuit is V volts, then
power, in watts P =VI
Electrical energy = Power × time
= VIt joules
Although the unit of energy is the joule, when dealing with large amounts of energy, the unit used is the kilowatt hour (kWh) where
1 kWh = 1000 watt hour
= 1000 × 3600 watt seconds or joules
= 3 600 000 J
A source e.m.f of 5V supplies a current of 3A for 10 minutes. How much energy is provided in this time?
Energy = power × time, and power = voltage × current. Hence
Energy = VIt = 5 × 3 × (10 × 60)
= 9000 Ws or J = 9 kJ
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