Lenz's Law Demonstration - Penn Physics

we've got two magnets here and we're discovering not surprisingly that opposite poles attract the red pole is north and the unmarked pole is south according to Faraday when a conductor is pulled across magnetic field lines an electric current flows in the conductor now we already know that a current in a conductor will in turn create another magnetic field and the important point and this is a really really critical point and it's why lenses law is so important the magnetism that's created in the ring fights the change in magnetism that produced it so in other words when I try to push this magnet through the ring the ring creates a magnetism that pushes back so I don't get something for nothing I don't get an electric current without working let's set up a demonstration we'll take the aluminum ring we started with it's light and it's not magnetic we'll drop the ring between the poles of a very strong horseshoe magnet if there were no magnetism here the ring would fall under the influence of gravity but as it enters the poles we induce a current in the ring because we're increasing the number of magnetic field lines or what we call magnetic flux enclosed within the ring a current starts flowing in the ring the ring becomes an electromagnet the poles of the induced electromagnet oppose the poles of the horseshoe magnet and so the magnetic field of the big magnet creates a net upward force on the ring opposing the force of gravity and slowing the rings to sin as the ring exits the pole region the flux that it encloses starts to decrease inducing a current in the opposite direction but that current also opposes its fall in each case the ring Falls much slower than we expect it to this is because lenses law applies one of the critical tests of a scientific theory is whether we can predict anything if what we've just said about lenses law is true if we can keep current from flowing in the ring the effect disappears so what if we were to take the ring that we just dropped and put a small cut in it so there was not a closed path for current according to that the ring should not be influenced by Lenz's law and it should drop entirely under the influence of gravity here we've got a ring with that cut in it when we drop it through the magnet we note that it falls unimpeded with no surprising delays as we just saw in the previous example another way to have a look at lenses law would be to imagine that instead of one ring as we've just seen we had many rings and those rings were stacked together to form a tube if a magnet were to go down that tube it would induce a current in each ring and each current would fight back against the magnet so we'll take a thin tube and a powerful magnet the tube of course would be not made out of iron or steel so that there were not any magnetic effects on the tube let's drop a pencil through the tube a non-magnetic object to see that under normal circumstances gravity controls the behavior and the pencil falls through as rapidly as we expect but when we drop a magnet through it it takes much longer why is that as the magnet falls through the tube the obvious force acting on it is gravity but along with gravity as it falls through each ring of the tube it induces a current around the ring the current produces a magnetic field that pushes back on the magnet so the magnet is now feeling three forces one downward from gravity one upward as the lower electromagnet repels it and one upward as the upper electromagnet attracts it the two magnetic forces counteract the gravity and so the magnet falls at a constant velocity instead of accelerating like a freely falling body so why does Lenz's law matter well it matters because it tells us we can't get something for nothing that when we make electricity we make it by fighting magnetic fields by pushing so that we still need mechanical energy to make electrical energy energy is conserved even if we switch forms from mechanical to electrical we can see lenses law come into play in many places one of the more interesting places is in the behavior we call maglev or magnetic levitation magnetic levitation generally applies to an object moving over a long distance of a non-magnetic conducting material we can't set up a long track like that that fits in our laboratory but if we take a stationary magnet and a spinning aluminium disc we can get the same effect we have a spinning aluminum disk underneath a strong rare earth button magnet as the disk spins we'll hear the noise of the magnet scraping along the aluminum disk and as the aluminum disc picks up speed the rate at which the magnetism changes for each surface area point of the aluminum gets higher and higher that induces more current and more current produces more magnetism if we move fast enough the induced magnetism pushes back on the button magnet with its own weight and it lifts the magnet off the surface of the aluminum we've now seen three demonstrations of lens's law in all of them we have discovered that if we produce electric energy we've got to do so by putting in mechanical energy that the laws of physics that we've learned in mechanics transfer right over to electricity and magnetism that forces come in pairs that energy is conserved and that these electrical behaviors that are not as intuitive as the mechanical behaviors still follow all the rules we don't get something for nothing in electricity any more than we did in mechanics

Channel: Penn Online Learning

Views: 100,782

Rating: 4.9623637 out of 5

Keywords: Physics (Field Of Study), University Of Pennsylvania (College/University)

Id: k2RzSs4_Ur0

Length: 7min 10sec (430 seconds)

Published: Tue Aug 11 2015