The simplest complete circuit is a piece of wire from one end of a battery to the other. An electric current can flow in the wire from one end of the battery to the other, but nothing useful happens.

There is a current of 2.0 A in a component with a resistance of 40 Ω. Calculate the potential difference across the component.

I hope that you are never in a situation in which you are in danger from a downed, but live, power line. However, if that ever happens, the recommended safety procedure is to move away with tiny, ...