Electricity Explained

Getting Down To Basics

By Bruce W. Maki

Circuits - Completing The Loop

One fundamental concept of electricity is the circuit.

As the term implies, a circuit is a loop. Electric current will only flow if there is a complete circuit, a conductor that starts at the EMF source, goes all the way to the electrical device (e.g. light bulb) and then comes all the way back to the other side of the EMF source.

Note that this implies that all EMF sources have two sides, or two terminals, or two connections. As you learn more about electricity you will observe this all around you. For example, batteries have two terminals, always.

The complete circuit concept works in our favor. In order to stop the flow of electricity, all we need to do is break the circuit, and only in one spot.

Sometimes, for safety reasons, we chose to break the circuit in two spots, but only one break is needed to stop current flow. When you turn off a light, you are merely breaking the complete loop that starts at the power generating plant, runs all the way to your house, to the light bulb, and then all the way back to the plant.

The electrical outlets (also called receptacles) in your home are partial circuits. You complete the circuit when you plug in a lamp and switch it on.

Unplug the lamp and the circuit is broken, no current flows through the lamp filament, and the light goes out. If somehow the plug were to get bent and one prong no longer made contact with the outlet, the circuit would be broken, and the light would not work.