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Electrical energy and power
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Power tells you how quickly energy is transferred. Three equations link power, current, p.d., resistance and time.
Power equations
P = I × V: power (W) = current (A) × p.d. (V).
P = I2 × R: power = current squared × resistance.
E = I × V × t: energy transferred (J) = current × p.d. × time (s).
P = I2 × R: power = current squared × resistance.
E = I × V × t: energy transferred (J) = current × p.d. × time (s).
Power and energy
Power is energy transferred per second: P = E ÷ t.
1 watt = 1 joule per second. A 2 kW kettle transfers 2000 J every second.
1 watt = 1 joule per second. A 2 kW kettle transfers 2000 J every second.
Energy transfers in appliances
Appliances transfer energy from the mains to useful stores: a kettle heats water, a fan makes kinetic energy, a speaker makes sound.
Some energy is always wasted, usually by heating the surroundings.
Some energy is always wasted, usually by heating the surroundings.
A heater uses a current of 8 A from the 230 V mains. What is its power?
- P = I × V
- P = 8 × 230
Answer: 1840 W
P = I × V, P = I2R, E = I × V × t, P = E ÷ t. Always use seconds for time.
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