Heating Effect of Electric Current

A cell or battery supplies electrical energy to keep current flowing. In a device like an electric fan, most of that energy does useful mechanical work — but some is lost as heat (which is why a fan feels warm after running a while). In a purely resistive circuit — just resistors connected to a battery, doing no other work — *all* the source's energy eventually turns into heat. This is the heating effect of electric current, and it's put to deliberate use in heaters, irons, and toasters.

For a current I flowing through resistance R for time t, the energy supplied by the source is VIt. Since this all converts to heat in a purely resistive circuit, the heat produced is H = VIt. Substituting V = IR from Ohm's law gives Joule's law of heating: H = I²Rt.

This means heat produced is directly proportional to the square of the current, directly proportional to resistance, and directly proportional to time. It's why appliances like heaters and irons — which need a lot of heat — are deliberately built with high-resistance coils.

This same effect explains two everyday devices. A bulb filament (tungsten, very high melting point) gets so hot it glows and emits light — most of the energy still becomes heat, only a small fraction becomes visible light. A fuse is the opposite use: a thin wire of a carefully chosen melting point, placed in series with a circuit, that deliberately melts and breaks the connection the instant current exceeds a safe value — protecting the rest of the circuit from damage.