Flip a light switch and current flows through a circuit. A circuit is a closed path through which electric current flows. It needs a source of voltage, like a battery, and a continuous path of conductive material, like copper wire. Break the path and the current stops. That is what a switch does. Close the path and the current flows again.

Circuits come in two basic arrangements. In a series circuit, components are connected one after another. The same current flows through all of them, and if one fails, the whole circuit stops. In a parallel circuit, components are connected across the same voltage. Each has its own path, and if one fails, the others keep working. Household wiring uses parallel circuits for that reason. Circuits can also include resistors, capacitors, inductors, diodes, and transistors. Each component shapes the current and voltage in predictable ways. Ohm's law relates voltage, current, and resistance, and Kirchhoff's laws describe how current and voltage behave at junctions and loops.

Circuit components

• Voltage source. Battery or power supply.
• Conductor. Wire that carries current.
• Load. Device that uses current, like a bulb or motor.
• Switch. Opens or closes the path.
• Resistor. Limits current.

A short circuit happens when current takes an unintended path with low resistance. The current surges, and the wire can overheat or catch fire.