The north and south poles of a magnetic field produced by an electromagnet will switch when the direction of the changes.

Explanation :

An electromagnet has a coil of wire which is wrapped around an iron core. When an electric current is passed over it, it behaves like a magnet.

For DC current,

When the direction of current is reversed in an electromagnet, its poles gets reversed.

For AC current,

AC or alternating current continuously changes the direction of the current. So, the north poles and south poles keep on changing.

So, the north and south poles of a magnetic field produced by an electromagnet will switch when the direction of the current changes.

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.

b. a lightning bug, plug of grass, and a worm