think “magnet,” we might envision those things we stick
on our fridge door. It may be a bit confusing, then, to discover
that magnetism is closely related to electricity. In fact, there
is a single force—the electromagnetic force—that governs
the behavior of both magnets and electric charges.
We have seen that there is a reciprocal relationship between
electric charges and electric fields: electric charges generate
electric fields and electric fields exert a force on electric charges.
Similarly, there is a reciprocal relationship between a moving electric
charge and a magnetic field: a moving electric charge
creates a magnetic field, and magnetic fields exert a force on moving
Bearing this reciprocal relationship in mind, we can make
sense of electromagnets, the on-off magnets you see,
for instance, lifting and dropping cars at the junkyard. The magnetism
in these electromagnets is generated by a current running through
the magnet that can be turned on and off at will. However, we still
haven’t explained how any of this connects with the permanent
magnets we stick to our fridge door.