Explain why both a power source (like a battery) and a complete circuit are necessary for an electromagnet to function.   
this is science homework by the way, please help me with it:)

Electromagnets are only functional when electricity is passing through them. In other words, they only create a magnetic field when there is an electric current flowing.

A power source like a battery is therefore necessary to supply the electric current necessary for the magnet to work however, it will not work unless the circuit is closed as electricity cannot not flow in an open circuit because current needs to be able to flow from one end of a power source to the other end.