We can see a particularly clear and
practical example of Lenz's law if
instead of looking at transformers, we
look at magnetic braking. So what I have
here is a metal disc that's free to spin. Okay so it spins happily now if I were
to put that in a magnetic field it would
be a conductor moving in a magnetic
field and so we would expect from Faraday's law of induction the eddy currents (little
circles of (en-) current) would be
induced inside that disk. And they would
in turn produce their own magnetic
fields and the question is -  in what
direction are they going to do all that?
And the answer is very simple. They have
to oppose the motion, don't they? And so
if I just put this magnet around that
disk,
it stops the disk rather rapidly. This is
a nice strong magnet and indeed it
didn't involve any touching. If I show
you directly, you can see the disk
spinning. There it is spinning nice and
fast. As I bring the magnets in, the
magnetic field induced a current that
opposed the motion and so what we ended
up with was a stopped disc with no
touching. And that's magnetic
braking. It's a very handy tool.
And again this is just a restatement of
the conservation of energy.
If magnetic forces that were induced
helped the motion - helped accelerate the
motion in that direction - then of course
you'd be getting kinetic energy out of
nowhere. We can do another fairly clear
demonstration that what's going on in
magnetic breaking is in fact to do with
currents being induced inside the
material. And the way we do that is we
have magnetic braking where we have
different materials. So what we've got
here is we have three magnets. Three
magnets at the bottom here. And those three magnets are in tubes and there's plenty
of room for them to slide down those
tubes, but those three tubes are made of
different materials. We have clear
plastic, which is not a conductor,
we have aluminium, which is a conductor,
and copper which is a really good
conductor. And so what I'm going to do is
I'm going to take this, (and) I'm going to tip
it upside down and then each of these
three magnets will just slide down the
tube into the other end. Okay, let's see
how long that takes.
Extremely different times you see the plastic went down bang
the aluminium came down a bit slower and
the copper was surprisingly slow. And
indeed if you take these and make them
cold, the resistivity goes down as you
make things colder and so you can find
that their currents really really slow
the motion of the magnets and it gets
slower and slower as you're making them
colder.
