 We know from Faraday's law of induction the strength of the voltage induced in a loop We know that it's proportional to the rate of change of magnetic flux through that loop And it's also proportional to the number of turns that you apply to that loop But we haven't yet discussed the direction does it go clockwise or anti-clockwise? And there are so many right-hand rules and letter magnetism It would be very confusing if we'd add yet another and there are so many different things You'd have to add into your mind to get this correct, but it turns out There's a really simple way of figuring out using the conservation of energy Using the conservation of energy when you're talking about induction is usually described as following lenses law It was developed by Heinrich Lenz in 1834 before the ideas of energy and conservation energy had really reached their final form Though these days we can see that they are one and the same so Lenz's law states That you can't get something for nothing if I have a coil making a magnetic field And then I have another coil that's going to have a magnetic field induced in it Now if I'm putting current in that first coil It's going to make a magnetic field that reaches right down into the second coil And remember that this is often helped by having a bar of some ferromagnetic material running through the two of them However, the key point is when we change the current We're going to change the magnetic field when we change the magnetic field in the first coil We change the magnetic field in the second coil and that changes the flux flowing through it And that's going to induce some kind of current in the second coil And the question is which way should it go? So if we take an example that this current increases That current increases that's going to increase the strength of the magnetic field Which means it's going to increase the amount of magnetic flux going through the second coil Now if an increasing magnetic flux going through that coil caused a voltage that would increase the magnetic field even more Then you would have a runaway effect. We'd have more flux So we get more of that voltage and so we get more current so we get more Magnetic field and so we get more of that voltage and it will go on to infinity and so that obviously can't be the way it works And so the voltage that's induced has to make a current that resists that magnetic field change So to resist the flux going up. We need some Magnetic field made that way and if I look at that and that wire I see that I need the current to be going out of the page here And that will make a magnetic field that goes up And so I'm going to need the current to be flowing in that direction around this loop And so we're going to have to have a potential going in that direction And so if the magnetic flux is increasing We know that the potential has to be going that way and if the magnetic flux is decreasing then it would go the other way We can see a particularly clear and practical example of Lenders 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 that any 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 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 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 Helped accelerate the motion that direction. Then 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 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 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