 We can see a particularly clear and practical example of lenses 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'd expect from Faraday's law of induction the eddy currents little circles of any of current would be induced inside that Disc and they would in turn Produce their own magnetic fields and the question is in what direction they're 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 disc It stops the disc 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 disc spinning There it is being 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 breaking. It's a very handy tool and again This is just a restatement of the conservation energy if magnetic forces that were induced helped the motion I helped accelerate the motion that direction and of course you'd be getting kinetic energy out of nowhere We can do another fairly clear demonstration than what's going on a magnetic breaking is in fact to do with Currents being induced inside the material and the way we do that is we have Magnetic breaking where we have different materials. So what we've got here is we have three magnets three minutes 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 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 Resistivity goes down as you make things colder and so you can find that the currents Really really slow the motion of the magnets and it gets slower and slower as you make them colder