What makes a circuit

A battery has two ends

Every battery has two ends, called terminals: a negative (−) one and a positive (+) one. The battery pushes electrons out of the negative end and pulls them back into the positive end.

That push only does something if the electrons have a way to get from one end to the other. On its own, sitting on a shelf, a battery has no path between its ends, so no current flows.

Closed loop = circuit

Run a wire from one end of the battery, through a bulb, and back to the other end. Now there's a complete path: a closed loop. That closed loop is a circuit. Electrons flow all the way around it, and the bulb lights.

Remember from earlier in the series: the battery doesn't supply the electrons. They're already in the wire. The battery just pushes them around the loop.

Any gap = open circuit

Now cut the wire anywhere, even with a tiny gap. The path is broken, so the flow stops all the way around the loop, not just at the cut, and the bulb goes dark. A loop with a break in it is called an open circuit.

It doesn't matter where the gap is: next to the battery, next to the bulb, or halfway along a wire. In a single loop, one break stops everything.

A switch is a gap you control

A light switch is just a gap you can open and close on purpose. Flip it on and the gap closes, so the loop is complete and current flows. Flip it off and the gap opens, so the loop is broken and the flow stops.

Try it yourself

Take a flashlight with a push button. Turn it on and off and think about what's happening inside: each press closes or opens a gap in the loop between the batteries and the bulb. If the flashlight ever flickers when you tap it, a loose contact is opening and closing the loop by accident.

Good to know

• Electrons leave the battery's negative terminal and return to the positive one. Conventional current is drawn the other way, from + to −.
• The battery doesn't supply the electrons; they're already in the wire. It pushes them around the loop.
• "The flow stops all the way around" is true for a single loop (a series circuit). In circuits with several branches, a break only stops current in its own branch. Parallel circuits come later in the series.
• A short circuit is a loop with nothing in it to limit the current, such as a bare wire straight across a battery. That's covered in a later episode.
• At high voltage, current can jump a small gap as a spark (an arc), so real switches are designed to open far enough apart.
• The 0.5 A on the meter in the video is just an example value for a small bulb.