i've got two bits of metal this is an
aluminium cylinder so long and a similar
cylinder of neodymium and these two are
going to pass through
a copper cylinder can you see down it
yeah right all the way through now then
this is the copper
tube with an aluminium cylinder i'm
going to put it in there and no force
will be exerted on this apart from
gravity so doing a quick sum that's
going to fall down in about a third of a
second so
just about caught it oh dear
[Music]
so that's how quickly the aluminium
falls now i'm going to put the neodymium
in neodymium is one of these new magnets
it's incredibly powerful
one
two
three
four
after about a count of four a slow count
of four maybe four seconds it comes out
so this is more than ten times as long
as the aluminium as this magnet falls
down say gets to there
then it's inducing currents which will
be coming around this way
and these currents coming around this
way will induce a magnetic field pushing
in the opposite direction so as this
comes down this imagining magnet that
i've got is going to be pushing in the
other direction holding it up holding it
up stopping it from accelerating and it
gets to a
finite velocity which it travels all the
way down the cylinder i'm going to put
the copper tube inside an aluminium
cylinder i'm going to put the neodymium
magnet in and the question is
what's going to happen as the neodymium
magnet falls through that little
aluminium cylinder is it going to stay
still is it going to be pulled up is it
going to move at all or does it just
stay stationary at first sight you might
think nothing can permeate through this
copper cylinder
nothing can happen at all it might
shield it but this is happening very
very slowly so i'm now going to drop it
and you can see what happens
and you can see as the magnet went past
this
it pushed it down
even though it wasn't touching anything
it pushed it down so i'd do that again
in case that wasn't so good all right
this isn't about magnetic attraction as
you would attract a paperclip this if i
move this very slowly
the aluminium does not pull towards
the neodymium magnet this only works
when the magnetic field that comes out
of here
threads through this
hole
if i put it on the side nothing happens
but if it threads through the hole
then it will move
if i move the magnet down to here
the magnetic field is not threading
through this hole in any appreciable
amount so you put it there and it
doesn't attract at all
if i put it there then the magnetic
field threads through the hole but if i
move it very very slowly inwards
nothing much happens
but if i move it out quickly
without touching the sides the magnetic
field is changing rapidly the part
that's threading through that hole
that's inducing a current in the outside
they're attracting each other
and that's weird because this is without
anything happening a force between these
two which depends upon the speed at
which i pull that away
what do you see in there that's nothing
so why don't we drop something through
it just make sure it's empty right okay
this is not a magnet so this isn't a
magnet lift it up bit by bit
to the point where it's just ready
and then it's
just in the air