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Eddy currents & their applications (& how to reduce them)

Eddy currents are current loops formed over conductor surfaces due to changing magnetic flux. They are useful in induction heating, levitating, electromagnetic damping, and electromagnetic braking. They can be minimized by adding slots in the conductor surface & laminating. Created by Mahesh Shenoy.

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  • ohnoes default style avatar for user sudhanshu singh
    sir in the last demonstration of lavitating coil i think you have depicted the wrong direction of current in it
    (3 votes)
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  • piceratops tree style avatar for user Samyak Khobragade
    Is Lenz's law applicable to all materials, regardless of whether they are diamagnetic, paramagnetic, or ferromagnetic?

    Suppose a diamagnetic substance is introduced in a decreasing magnetic flux, would it be attracted or repelled?
    (2 votes)
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  • blobby green style avatar for user yomnorikeejrehkum
    Hello! I went through this video and had a small doubt in the induction plate part.

    If there is a current being induced in the vessel, which in turn heats it up.... Why don't I get electrocuted when I try to pick up the vessel while the induction plate is on?

    The vessel is a conductor right? Doesn't that mean that I'm giving the currents an easier way to reach the ground the same way I do when I touch an open wire or something?

    I tried looking it up on quora and I got the following answer but I couldn't really understand it (my bad):-

    "Eddy currents in utensils heat the vessel, and make cooking possible. Eddy currents are limited to vessel base and there is not much voltage involved. Further, our body does not come into picture when induced current is concerned. Whole system is insulated including the top surface."

    Could you please help me out?
    (1 vote)
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    • blobby green style avatar for user anoushka1828
      Not an expert but i would think that frequency of the current induced is either very high, higher than the frequency that cause humans pain or very low(you can checkout the frequency to pain chart on google), moreover the emf cant be induced over larger distances and its a similar reason why wireless phone chargers only work at very close distances(1-3 mm in my experience)
      (2 votes)
  • blobby green style avatar for user mastersgrevolt
    In the last experiment(the spinning aluminum plate stopped by magnet), the total flux through the plate never changed right? How did it induce current then
    (1 vote)
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  • marcimus purple style avatar for user Astrophysikerin
    For the pendulum experiment, what if the plate used were a circle and not a rectangle?
    (1 vote)
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  • eggleston blue style avatar for user Aashi
    When air acts an insulator, the resistance increases tremendously, due to which the current decreases. Shouldn't the heat loss also be increased as the resistance is increased?
    (1 vote)
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    • sneak peak blue style avatar for user tharunathome
      Iam not an expert, but I think if the resistance increases, current decreases. and when current decreases power loss also decreases. The voltage is constant right, so due to I= V/R, since resistance tremondously increases, current descreases at a far greater value and since P=I squared R, power loss descreases. Lets day voltage is 20, current is 10 and resistance is 2. If resistance increases twice, that is 4 ohms, current descreases by 1/2, that is 5 amps (since V=IR). If P was equal to just IR then there will be no change, but actually P is equal to I 'squared' times R. So previosly when resistance was 2, power loss would be (10)squared x 2 = 200 joules. Now, power loss would be (5)squared x 4 = 100 joules. See, power loss has actually descreased, because even though resistance doubles (2 to 4 ohms), current goues down by 4, that is *10 squared) 100 to (5 squared) 25 Considering the equation p=isquared times r. I dont know if its correct, but i hope this helps you to get a start in thinking
      (1 vote)

Video transcript

i have an induction cooker at home on which i'm boiling water and i thought the way it worked was like an iron box the base would just get hot and that heat would get transferred to the water a vessel and start cooking whatever's inside but then i read that that's not how it works the base of the induction cooker does not get hot at all and then i tested it out i took the induction cooker i switched it on and then i put my hand on top of it for a few minutes and nothing it wasn't hot at all it was right it doesn't produce any heat but then on that same thing i keep this vessel of water and within minutes it starts boiling so the question is how does an induction cooker cook things and heat things up without itself getting hot well that's what we're going to find out and it turns out it's due to the magic of i mean science of eddy currents so what are these eddy currents to answer that question let's look inside the induction cooker if you could open up then you might see a lot of things but the most important one is a coil at the center and we know that when you pass current through the coil it produces a magnetic field and that's exactly what happens when you turn on the switch there's a current that starts passing through the coil and that current starts generating magnetic fields but what's important is that the current is fluctuating it's changing its direction continuously it's called alternating current which we'll talk about in separate videos and it changes its strength as a result of that the magnetic field that degenerate also keeps fluctuating in direction and in strength so imagine this is how the magnetic field is generated it's continuously fluctuating and that's all that our induction cooker does it does not produce any heat all it does it produces a fluctuating magnetic field but how does that boil over water we'll get to that eventually but for now instead of thinking about keeping a vessel on top of this let's keep a rectangular loop of wire what would happen well because the magnetic field is fluctuating we have changing magnetic flux through a coil and from faraday's law we know that that produces an emf and as a result there will be an induced current in this coil now the direction of the current will all depend upon whether the magnetic field is increasing decreasing what the direction is in terms of lens law something all we have talked about before but don't worry too much about the direction as of now what's important is there is an induced current due to the changing magnetic flux but now comes the question what if instead of having a coil like this what if it kept a rectangular plate now what would happen well again the flux is changing so there is an induced emf but earlier there was only one path for the current to flow but now the entire thing is a conductor the whole thing is a playground for the electrons which means your currents will be formed in loops everywhere on the surface of this conductor and so you'll have loops loops of current formed on the surface and these loops of current which are formed on the surface of a conductor due to changing magnetic flux is what we call eddy currents and i thought that these are called eddy currents because maybe some guy named eddie discovered it because that's how most of the stuff works in physics but they're called eddies because they're swirling so if you consider a whirlpool of water something that's swirling they're usually called eddies or vertices and since this is very similar to that we call them eddy currents but how does that produce any heat well of course current produces heat we've learned that before when current flows through a wire the wire heats up now there is current everywhere on the surface of the conductor which means this whole metallic plate heats up so if we go back to our vessel of water due to the fluctuating magnetic fields eddy currents are set up on the base of this vessel and that's how the vessel gets heated up directly and then that heat is transferred to the water making it boil well then why doesn't that same thing happen to our hands well that's because our hands are not all that great conductor and therefore the eddy currents formed are incredibly microscopic and they hardly have any effect so in short these induction cookers generate fluctuating magnetic fields and when there is a conductor nearby it experiences of changing magnetic flux that induces currents on the surface which we call the eddy currents which heat it up and that's how you can have induction heating incredible isn't it but wait wait before we start parting doesn't it mean that whenever you have conductors and changing magnetic fields you will have eddy currents and as a result you will have heating even if you don't want them yeah wouldn't that be a problem what if you don't want that happen how do you reduce eddy currents when you don't want them again going back to our metallic plate how do we reduce these eddy currents when we don't need them well we can reduce current by increasing the resistance of our metallic conductor we know more resistance means less current how do i increase the resistance of this conductor well one of the cool ways of doing that is by introducing slots like this so kind of making it like a comb in doing so think of what we are doing we are introducing air in between and air is an excellent insulator which means the overall resistance of this now has increased tremendously and as a result of that the eddy currents now have reduced tremendously which means less heat loss and this also means now there is less area for the current so you have tinier eddy currents and again because of this will have less heating effect a practical application of this is seen in transformers now the details about the working of transformer is something that we'll tackle in a separate video but the important thing is transformers also have changing magnetic fluxes and they have this you can see this giant conductor in between which means there will be any currents formed over here and we don't want the heating effect to happen over here so how do we reduce it well the whole idea over here is instead of having one giant block of metal where eddy currents can keep dancing wherever they want we instead have thin slices of these metals laminated and then glued together in doing so we are introducing air like just like over here and as a result increasing the resistance thereby reducing the eddy currents and because these things are all laminated there is no electrical contact over here which means you can't have a lot of eddy currents over here so in doing so we dramatically decrease eddy currents and as a result we dramatically decrease heating losses so now we not only know how to use eddy currents to heat up things but we also know how to reduce them when we don't want them but wait there's more check out this demo in this case again we have an electromagnet which is going to generate fluctuating magnetic field due to the alternating current and he's press that so it's switched on and then he's going to put a metallic and aluminium ring on top of this and see what happens here we go and voila notice the ring just stays there it's levitating in air beautiful isn't it why is it doing that and see what happens when let's go of it it falls down so why is that doing that can you pause the video and think a little bit about this as to why the ring was levitating all right let's go back to our drawing board here's that picture of levitation one more time why is the ring staying in the air like that well again this electromagnet is generating a magnetic field there's a current running and it generates a magnetic field but importantly it's generating a fluctuating magnetic field just like with the induction cooker and as a result of that we have a conductor placed on top of that and just like how we saw there's going to be eddy currents so there'll be currents running on the surface of that aluminum coil but what does that cause levitation well remember this current loop itself starts behaving like a tiny magnet so this aluminum ring now behaves like a magnet and now from lenz's law we've seen that the current induced will be in such a way that it tends to oppose the cause for it so when you apply lenses law it just turns out that the magnetic field that it generates opposes this and as a result of that this starts repelling this magnet and for that reason once that repulsion is stronger than gravity it starts staying in the air magnetic levitation and one of its applications can be seen in these amazing trains called as maglev trains some of these trains you use the same principle eddy currents are used to levitate the entire train in the air and as a result of that they're not touching the tracks and so friction is dramatically reduced and as a result these strains can achieve a high very high speed maglev trains magnetic liberation trains amazing isn't it but wait there's more check out this demonstration we have a pendulum with an aluminium plate attached to it and we have a couple of electromagnets now once the power is turned down this starts producing a steady magnetic field there are no fluctuating magnetic fields now see what happens and an important thing aluminum is not a magnetic material doesn't get attracted to magnets okay so there goes the pendulum it's swinging just like usual and now it turns on and see what happens what did you see can you explain what you saw again pause the video and think about what might be going on so we saw that the moment the magnetic field was turned on it immediately stopped why did the pendulum stop what made it stop is it any currents how how does that work let's get back to our drawing board so let's say this is that constant magnetic field generated by those electromagnets and here is that moving aluminum piece pendulum why would there be what's going on over here so even though there the magnetic field is not changing notice as the piece enters into the field from its perspective the magnetic flux associated with it starts increasing and as a result of that there is an emf induced this is called motional emf and now this motion emf starts producing eddy currents and the same thing is going to happen when it exits as it exits the magnetic flux is decreasing and that will produce motional emf and eddy currents but why does it slow down let's look at it carefully and use lenses law as it enters into the magnetic field since the flux increases the current induced will be in such a way as to try and decrease the flux in other words the current area currents over here will tend to oppose the magnetic field and as a result it'll end up repelling the magnetic field which means as the plate enters into the magnetic field it gets it repels these magnets and so what happens if you repel as you're entering it'll slow it down but what happens when it exits well now as it's exiting the magnetic flux is reducing so the opposite happens now the induced current tries to increase the magnetic flux as a result it tries to produce magnetic field in the same direction and therefore which means it gets attracted by the magnet or gets attracted towards the magnet and this means now when it's exiting it gets attracted back to the magnet which again slows it down and therefore every time this thing enters into the into the field and exits the field it gets slowed down and as a result it eventually stops and now you can answer this question what would it what do you think would have happened if we had slots in these aluminium in the aluminium plate and re redid the experiment what change would you predict this time okay hopefully you made your prediction let's see if your prediction matches the experiment so now you can see there are slots cut into it and we're going to repeat the experiment there goes the swing now it's going to turn on the magnet now what do you see the magnet is turned on hard to believe it's turned on right it's slowing down but it's taking much longer i'm not gonna tell you what's happening i'm pretty sure you can answer that question yourself now and one final demo which will make a lot of sense now here it is again an aluminium disk this time attached to a motor so when you turn it on it spins that's not the demo but the question is what happens now when you bring a magnet close to it can you predict what's going to happen very similar things well let's see if your prediction matches it slows down does it make sense well again as the pieces of aluminum go through into the magnetic field and exit the magnetic field they experience repulsion and attraction that slows them down and as a result the whole disk slows down which means you can use any currents to to break to slow down things we call them electromagnetic breaking what could be an application of this we can slow down spinning wheels which means we can slow down wheels of a train using electromagnetic braking and eddy currents wow in short eddy currents are currents formed on the surface of a conductor whenever magnetic flux through them changes if you want to reduce them you just increase the resistance by adding some slots or using lamination and they can be used to heat things up or maybe levitate things or slow things down in other words they are pretty pretty awesome