What happens whena switch in a circuit is opened?

Answer 1

When a switch is opened (turned off) the circuit is broken and the electricity stops flowing, lights and other devices cease to work. Once the switch is turned on (closed) the electricity will once more flow and lights and other devices will work once more. A good example is when you enter a dark room. You switch on the wall switch, which turns on the light in the room, and you can see what you are doing.

Answer 2

Same thing that would happen if you disconnected one of the wires.