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.