A Complete Circuit: Source, Load, Conductors and Switch

0:00What does a bulb need to light up?

0:03A complete circuit: an unbroken loop that charge can flow all the way around.

0:09On the left is a real circuit; on the right, its circuit diagram.

0:14The source is a cell, or a battery of cells. It gives the charge energy.

0:20The long line is the positive terminal, the short line the negative terminal.

0:26The load transfers that energy. Here it is a bulb, drawn as a circle with a cross.

0:32Metal wires are conductors, with electrons free to move. A diagram draws them as straight lines.

0:40Electrons flow from the negative terminal, through the bulb, back to the positive terminal. This flow is a current.

0:49A switch opens or closes a gap. Open it, and electrons cannot cross the gap, so the current stops everywhere.

0:58Close it again, and the loop is complete. Electrons flow and the bulb lights up.

1:04Diagrams show conventional current, from positive to negative. It was chosen before electrons were discovered.

1:12Current is measured in amperes, symbol capital A. Here, 0.2 amperes flows into the bulb.

1:22A common mistake is to think the bulb uses up some current, so less flows out.

1:28In fact, 0.2 amperes flows out too. The current is the same all the way around this loop.

1:35The bulb does not use up current. It transfers energy from the charge into light and heat.

1:42A circuit must be complete, from the source through the load and back, for electrons to flow.