So Faraday set up almost exactly this
experiment, except instead of a permanent
magnet here, he used an electromagnet. So
instead of having one coil and a magnet
he had two coils. And one was attached to
a battery and one was attached to an
ammeter. And he found that he didn't get
any current in the second coil when he
just left the first coil sitting there (unsurprisingly). More surprisingly to him
when he connected the battery to the
first coil and left it running, he also
didn't see anything in the second coil,
except for a little flicker when he
first connected it. And then when he
disconnected it, he got a little flicker
again in the opposite direction. And so
he was getting an electric field running
around that loop only when he was
changing the magnetic field that was
going through that loop. And so Faraday's
discovery of this process which was
called 'induction'
so called, because the changing current
in one loop induced a current in the
other loop. And it was James Maxwell and Oliver Heaviside who later went on to
quantify that and really describe
exactly what was going on. And it turns
out that exactly what was going on was
that it wasn't just a magnetic field,
because the magnetic field is different at
different points. So at different points
throughout this entire loop, there's
different magnetic fields. And even at
different points of this coil, there may
be different magnetic fields. And it
turns out that what's important is not
the field at a particular point, but the
whole magnetic flux going through that
loop (and we'll discuss exactly what that
is in a minute) and the changing magnetic
flux causes a force to act on the
charges going around here, and this is an
electric force so we've got an electric field
going around and we know that if we had
an electric field that's a force per unit
charge and if charges go around at a
certain distance, and I get a certain
amount of energy per unit charge, and energy per unit charge is an electric potential. So
you get another potential difference
going around this loop based on the
amount of changing magnetic flux we have.
