Where the Energy Goes in a Circuit

Charge itself is not used up in a circuit. The same electrons that leave a battery eventually return to it, going around and around the loop. What runs down is the battery's stored chemical energy. Each electron that passes through the battery picks up a fixed amount of energy per unit charge, the voltage. As that charge flows through a resistor, it gives that energy up, turning it into heat, and sometimes light or motion. So the electrons are a delivery truck, not the cargo.

Picture a loop with a battery and two resistors in series. The battery raises the electric potential energy of the charge by an amount set by its voltage, and that is the total energy budget for one trip around the loop. Each resistor takes a cut, a voltage drop, and that energy leaves the circuit as heat. Add up the two drops and they equal the battery's voltage exactly, because every bit of energy handed out by the battery has to show up somewhere. Power, the rate of this energy transfer, is voltage times current, and multiplying power by time gives total energy used.

This is just conservation of energy wearing a different costume. In mechanics you track energy moving between kinetic, potential, and heat as an object moves. In a circuit you track energy moving from the battery's chemical store into heat and light at the resistors, with charge playing the role mass plays in mechanics. Next up: capacitors complicate this picture, since they store energy temporarily instead of dissipating it right away, charging and discharging as the circuit runs.