Joule heating

When an electric field drives charge carriers through a resistive material, interactions with the material transfer electrical energy into internal energy. In a metal conductor, collisions of moving electrons with the lattice contribute to heating. The effect is called Joule heating, resistive heating, or the Joule effect. Electrical energy is not destroyed: it is converted into thermal energy, which may then be transferred to the surroundings.

For an ohmic component with current I, resistance R and voltage V, the rate of electrical energy converted is the power P = I²R. Using Ohm's law, the same power can be written P = VI or P = V²/R. If the current and resistance remain constant for a time t, the energy converted to heat is E = I²Rt. These relations apply to the component under the stated electrical conditions; in real devices, resistance and temperature can change during operation.

Joule heating is deliberately used in electric heaters, toasters, incandescent lamp filaments and fuses. A fuse is designed so that excessive current heats its wire until it melts and opens the circuit. In motors, wiring and other electrical equipment, resistive heating may instead be an unwanted energy loss that raises temperature. Engineers account for electrical resistance, current, cooling and material limits when designing circuits and appliances.

Joule heating describes electrical power dissipated through resistance, rather than every way electrical energy can be converted. Some devices also convert energy into motion, light, chemical change or other forms. For a component that does not behave ohmically, power is still given by the product of voltage and current at a given operating point, but simple constant-resistance substitutions may not describe its behavior. The amount of heating in a practical system depends on its current, electrical properties, temperature and operating time.