A current needs a complete path. A battery pushes charge out of one terminal, and that charge must be able to travel all the way around and back into the other terminal. Such a closed path is a circuit.
Key idea
Charge flows only around a closed loop from one terminal of the battery, through the devices, back to the other terminal. A gap anywhere (an open circuit) stops the current everywhere in that loop. A switch is a gap you can open and close.
Why it works
The battery doesn't supply charges; the wires are already full of them. It pushes them around the loop, like a pump pushing water around a closed pipe. Break the pipe anywhere and the whole flow stops, because the water has nowhere to go. That is why one switch by the door can turn off a light across the room.
Worked example: Does it light?
A bulb has two connection points. One wire joins it to the + terminal of a battery. Nothing is connected to its other point. Does it light?
- For current, charge must go from one terminal, through the bulb, and back to the other terminal.
- Here there is no path back to the battery, so there is no loop.
Answer: No: the circuit is open
Worked example: Where the switch goes
Two bulbs are wired in one loop with a battery and a switch. The switch is opened. Which bulbs go out?
- All the parts share the same single loop.
- Opening the switch breaks that loop, so the current stops everywhere in it.
Answer: Both bulbs
Common mistake
Connecting a bulb to only one end of a battery and expecting it to light. Charge must leave the battery, pass through the bulb and return, so the bulb needs two connections: one to each terminal (directly or through other parts).
Build circuits with switches and bulbs in the Science Lab: Circuit Builder.