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.

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r = ρ x length / area  

ρ = r x area / length  

ρ is in ohm-meters  

in basic units, it is m³kg/s³a²  

i think this is it.

3. dark matter

empty space could also be a simple vacuum