Tech a says that a relay is a one-way electrical check valve used in alternators to change ac into dc. tech b says that a relay uses electromagnetism to open or close a switch. who is correct?

The correct answer is from Tech B.

Explanation:

The relay is an electromagnetic device, which is stimulated by a very weak electrical current to open or close a circuit.  It functions as a switch controlled by an electrical circuit in which, by means of a coil and an electromagnet, a sequence of one or several contactors is activated that allow to open or close other electrical circuits independent of the one that controls it.

Have a nice day!

reserve is the answer

force tends to be low and long diustance masses are high