← Current, voltage, resistance, and circuit diagrams

Current needs a complete loop

In brief

Here is the loop from the video, kept as one card: complete, broken at one spot, and broken at a different spot.

complete loopthe bulb lightsswitch openthe bulb is darkgap moved, still openthe bulb is dark
One complete loop carries a current, and the bulb lights. A gap anywhere, on either side of the bulb, makes the current zero everywhere in the loop.
Current flows only when the circuit forms one complete, unbroken conducting loop from one terminal of the source (here, the battery) to the other. Opening a switch anywhere in the loop stops the current everywhere in that loop.

A lamp is wired to a battery through a switch, and the switch is open. What is the current in the wire next to the battery?

Name the state of the loop:

The open switch is a gap, so the loop is not complete.

Apply the rule:

In a broken loop, the current stops everywhere, not just at the gap.

The current next to the battery is 0 A.

The same lamp, battery, and switch. Now the switch is closed, and the metal path from one battery terminal through the switch and the lamp back to the other terminal is unbroken. Does a current flow?

Name the state of the loop:

The circuit now forms one complete, unbroken conducting loop from terminal to terminal.

Apply the rule:

A complete loop is exactly what a current needs.

Yes: a current flows around the loop, and the lamp lights.

Check what you know

A doorbell circuit has a battery, a push-button switch, and the bell in one loop. The button is not pressed, so the switch is open. Compared with the current while the button is pressed, what is the current through the bell?

A camping lantern stays dark although its battery is fresh and its bulb works. Which fault explains it?

A circuit has one battery, one switch, and one motor in a loop. The switch is opened. Where does the current stop?

A reading lamp's cord is damaged, and one wire inside it is snapped. The lamp is plugged in and switched on. Compared with an undamaged cord, what current flows through the bulb?

Which of these circuits will carry a current?

A toy robot runs on one loop: battery, switch, motor. The switch is closed and the motor runs. The switch is then opened. What happens to the current at a point between the battery and the switch?