Question

what are closed and open circuits?

Answer 1

closed circuit
the current will follow as the key plug in and the bulb will glow.
open circuit
the current will not flow and the bulb will ñot glow . vise versa to closed circuit...

Answer 2

An open circuit is one where the continuity has been broken by an interruption in the path for electrons to flow.

A closed circuit is one that is complete, with good continuity throughout. A device designed to open or close a circuit under controlled conditions is called a switch.