Demystifying Electromagnetic Braking: How It Slows Things Down

[00:00:05]hello everyone today we're going to learn about one of the applications of eddy currents called electromagnetic braking that's of course breaking as in slowing things down not breaking as in smashing something to pieces so eddy currents have a number of useful applications and one of these is as you might have guessed electromagnetic braking well one of the experiments that you can do in school laboratory to demonstrate electromagnetic braking is swinging a metal pendulum in between a magnetic field as we can see a photograph of over here so electronegative breaks use the opposing force created by eddy currents because that's lenses will remember eddy currents create opposing forces and they use these forces to slow down fast-moving objects good use for the force now we can demonstrate electromagnetic braking pretty easily all we need to do is drop a magnet for a copper tube or a tube of any other conductive metal eddy currents in the copper tube will slow down the magnets descent because they of course create magnetic fields that oppose the magnets motion and this means that the magnet will pass through through the tube very slowly far more slowly than if there were no eddy currents so what happens if we have a fast-moving wheel made of metal and we apply a magnetic field perpendicular to it like we can see in this diagram over here well we'll get eddy currents induced in the plane of the disk the part of the disk that is entering the magnetic field will produce eddy currents to oppose the sudden appearance of flux and the parts of the wheel leaving the field will also create eddy currents but these ones will try to regain the flux that they've lost this means that on this side of the disc current will flow in the direction shown to create flux whereas on the other side of the disc can't flow in the other direction to try and produce flux opposing this magnetic field so the force on the wheel due to the eddy currents will oppose the wheels rotation we can see that if there's a current going through here and magnetic field in that direction then the force will be backwards and so all the current flowing through the magnetic field will oppose the direction of the wheel so if the spinning metal disc is a wheel the magnetic field will slow the wheel down that is it'll behave like a break so magnetic fields breeze by electromagnets then we can change its strength very easily all we need to do is change the amount of current that passes through the electromagnet this means that we can have very very smooth breaking indeed because we can control exactly how much the wheel is breaking we have a very very precise control over it the other advantage of course of using electromagnets is that we can produce an alternating magnetic field which will produce more eddy currents due to the changing field than just a constant field now electromagnetic braking is sometimes used in frames and trains so these are the examples you'll see most often electromagnets are placed in a train near the wheels and then switched on when it wants to break that means that we don't have a permanent magnetic fields on the wheels slowing it down all the time so the circular eddy currents induced in the wheel will slow the wheel down now where is the kinetic energy go when it disappears well it turns into the electrical energy of the eddy currents in the wheels and because the wheels have resistance it means that the eddy currents are turned into heat so the turtle effect is that the Train slows down while its wheels heat up same as if we simply put a friction brake on the wheel electromagnetic braking is also used in amusement park rides for example roller coasters we can see that this roller coaster is about to reach a set of magnetic brakes so the magnets can be mounted on either the roller coaster itself or the rails usually roller coasters are very very controllable they're always going to stay on the rails which means that we can mount the magnetic field on the rails themselves instead of on the roller coaster so as the vehicle moves along the rails the eddy currents are induced and those will slow it down once again is kinetic energy is transformed into electrical energy and then heat energy so what happens if we're moving very very slowly so the rate of change of magnetic field is very small the strength of a set of electro magnetic brakes will depend on the speed of the vehicle using them right this means that if the vehicle is moving very fast we'll get a very large rate of change of magnetic field and we'll get a lot of braking force which is good because if something is moving very fast then you want to slow it down a lot and as it slows down the Eddie Collins gets smaller and smaller and smaller now this results in a very smooth break but what happens when we want to come to a complete stop well the answer is we can the electromagnet brakes cause very very small eddy currents which means that there is almost no braking force and the brakes can no longer slow it down but it's not too much of a problem but because by this point is moving very slowly anyway so a different brake usually a mechanical brake like a friction brake or just a rubber tire is used to bring a vehicle to a complete stop so we cannot use electromagnetic brakes to stop completely we can only use them to slow down now that's the end of the theory so I'm going to go into some questions to let you test your knowledge but our question 6 which option is true electromagnetic breaks cannot stop moving objects electromagnetic breaks are not practical permanent magnets are required to construct electromagnetic breaks or electromagnetic breaks are most effective when they are acting on slow-moving objects let's go through these options shall we if we have a very slow moving object then the magnetic field through it will change quite slowly if we have a slow changing magnetic field then we have only a small amount of force and so the brakes are not very effective on the snow on the slow-moving objects if we look at option C permanent magnets are required to construct electromagnetic brakes there's no reason that we can't use electromagnets all you need is a magnetic field and electromagnets can produce that just as well as permanent magnets sometimes they can produce an even stronger field how about be electromagnetic brakes are not practical this is not the correct answer either they used in roller coasters and trains and trams so obviously they have some use the last option that is a electromagnetic brakes cannot stop moving objects now at first brakes they cannot stop an object don't seem very useful but what electromagnetic brakes are very good at is slowing objects down they can produce a very large force on fast-moving objects but only a very small force on slow-moving objects so electromagnetic brakes cannot stop moving objects although this doesn't mean that they're not practical question 7 when a magnet is dropped through a copper tube it passed through the tube more slowly than if it is drop through a plastic tube why is this we've answered a similar question before is it because magnets are attracted to copper because objects fall more slowly through metal tubes because eddy currents in the magnet slow it down or none of the above well let's go through the options first of all magnets are not attracted to copper they attracted to only number are a very small number of methods including iron nickel and cobalt as well as some metal alloys one such metal alloy is what neodymium magnets are made out of so it's not a option B says objects false more slowly through metal tubes and this is not correct either if we drop a rubber ball or something through each tube there will be no change the only objects that fall through more slowly or ones that are generating a magnetic field right so then we come to option C eddy currents in the magnets lower down this is not the correct answer eddy currents are induced in the copper not the magnet the current through the magnet doesn't matter what matters is the current through the copper tube right the movie's name it produces a current in the copper tube and that eddy current is what opposes the magnets motion not a current in the magnet itself so the correct answer is going to have to be none of the above make sense question 8 moving objects have kinetic energy explain how electromagnetic breaks remove this energy now remember due to conservation of energy we can't just destroy the energy we have to convert it into a different form right so we have kinetic energy we have eddy currents and Eddie cards are a way of turning kinetic energy into electrical energy right and that's how we remove the energy of the large object so according to Lenz's law the eddy currents induced by the wheels motion produce a force that opposes their motion and so we turn the kinetic energy into electrical energy this electrical energy quickly dissipates as heat of course because all metals have resistance question 9 explain why brakes that use eddy currents cannot bring a vehicle to a complete stop so how do we go about answering this one we know the strength of the force produced by eddy currents is going to be proportional to the rate of change of magnetic field and the rate of change of magnetic field depends on how quickly the vehicle is moving or how fast the wheels are turning so the strength of the eddy currents depends on the speed of the vehicle more braking forces produced for faster vehicles and in fact if we move down to very very slow speeds there will be almost no braking force which means electromagnetic brakes cannot stop it from moving completely question 10 a metal wheel is spinning clockwise and I going field pointing towards you if placed across the left of half of the wheel only in which directions do eddy currents flow if any all right let's figure this out let's just look at a spoke of the wheel if I can call it that that's about to come out of this magnetic field all right when it comes out of the magnetic field there's going to be less flux so it wants to produce more flux to make up for it it'll create flux pointing towards you just like the field that's leaving so the current will flow in this direction right so we get a current that looks something like this we can also see that the part of the wheel inside the magnetic field will oppose its motion using the right-hand palm rule we are the current going downwards magnetic field going towards you and the direction of the force moving the opposite direction that the wheel is rotating so the eddy current resists both the movement of the wheel and the change of flux so at the top of the wheel this section of wheel leaving the field produces anti-clockwise eddy currents right what are at the bottom of the wheel here we've got another spoke moving to the magnetic field that's going to produce to oppose the motion of the wheel so the force will go this way the magnetic field goes that way and we end up with the current going up as it enters the field so it must go around like this outside the field once again we can see that this is opposing the change of flux through it it doesn't want flux coming that way so it makes flux going that way by producing a clockwise Eddie kind that's straightforward enough isn't it so as the bottom of the wheel as the bottom the wheel enters the magnetic field it opposes this change in magnetic field by producing clockwise eddy currents that means that even though it's the same wheel on the same magnetic field the eddy current is produced on each part of the wheel are in different directions so that's the end of the questions there's the end of the section on electromagnetic brakes which are a way of turning kinetic energy into electrical energy and then heat energy in order to slow down a moving object in the next section we'll be looking at induction cooktops another use of eddy currents