Electrical Circuits

Current is the flow of charge around a closed loop, driven by voltage and limited by resistance. Ohm's law ties the three together: V = IR, so current equals voltage divided by resistance. In a series circuit, there is only one path, so every charge that leaves the battery must pass through every resistor in turn. That means the current is the same at every point in the loop. In a parallel circuit, the path splits into branches at a junction, so current divides among them, with more current taking the branch of lower resistance, then the branches recombine before returning to the battery.

This difference explains the holiday lights. Old-style series lights share one path, so if one filament breaks, the loop is cut everywhere and every bulb goes dark. Modern parallel strings give each bulb its own branch, so one broken filament only kills that branch, and current keeps flowing through the rest. Adding resistors also behaves differently depending on wiring: putting a second identical resistor in series increases total resistance, since current now fights through more material, but putting it in parallel decreases total resistance, since current gains a second route and the pair resists less than either one alone.

A battery is not a reservoir that empties of electrons. It is a pump that uses chemical reactions to push the charge already present in the wire around the loop, spending chemical energy rather than spending electrons. When the chemicals run out, the pump stops, not because the circuit ran out of charge to move.