Electrical Power, Energy and Heating Effect

How circuits turn electrical energy into heat and light.

Power is the rate of energy transfer, and in a circuit it equals voltage times current. Combined with Ohm's law it can also be written using resistance. Energy used is power multiplied by time, and the everyday unit, the kilowatt-hour, equals 3.6 million joules.

When current passes through a resistance, collisions inside the material produce heat. This Joule heating is useful in heaters and fuses, but it is wasted energy in connecting wires. A fuse wire melts when the current goes past its rating and protects the circuit.

• Power in watts; energy in joules or kilowatt-hours.
• Heat produced H = I²Rt.
• Thin wires carrying large currents get hot.

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