Electric Circuits and Ohm's Law

An electric circuit is a complete, closed path through which electric current can flow, for example from one terminal of a cell, through a bulb, and back to the other terminal. If the path is broken anywhere, such as by an open switch, it is an open circuit and no current flows.

Ohm's law says that the current through a conductor is proportional to the voltage across it, at constant temperature: V = I × R. V is the potential difference in volts, I is the current in amperes and R is the resistance in ohms (Ω).

In a series circuit, components are connected one after another on a single path: the same current flows through all of them, the voltages add up, and if one fails they all stop. In a parallel circuit, each component has its own path across the supply: each gets the full voltage, the currents add up, and each can work on its own.

In series, add the resistances: R = R₁ + R₂ + R₃. In parallel, add the reciprocals: 1/R = 1/R₁ + 1/R₂ + 1/R₃. Two equal resistors in parallel have half the resistance of one.

A fuse is a short piece of thin wire that melts and breaks the circuit if the current becomes dangerously large, for example in a short circuit or when too many appliances are switched on. It protects the wiring and appliances from overheating and fire. Many homes now use MCBs (miniature circuit breakers), which switch off instead of melting.

Adding bulbs in series adds their resistances, so the total current falls. With two identical bulbs, the current halves and each bulb gets only a quarter of the power one bulb would get alone, so each glows much more dimly.