Induced EMF in closed loops around bar magnet

  • #1
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I wonder if there is a way to calculate induced EMF in closed loops around bar magnet, which is traveling with constant velocity v to the right as depicted?
InducedEMF.JPG
 

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  • #3
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The turns are representing single loop of coil around the bar magnet. I would like to know; what amount of electricity is induced in single loop of solenoid when it is approaching toward the center of bar magnet. What I know is that the center of bar magnet is not susceptible to ferromagnetic materials. Preferably I would like to see the plot, what it is look like. The graph below is my artistic view of my problem.
 
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  • #4
Merlin3189
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Won't get much emf if they are shorted turns.
Is that really so?
Isn't the induced emf exactly the same, whatever the resistance of the loop?

I actually get very confused about this, becuse a change of flux in the loop will cause an emf, the emf will generate a current in the loop, the current will create a magnetic field opposing the change of flux, so there will be less change of flux than you first thought, so there is less emf, less current, less magnetic field and now we can have a big change of flux again, etc. ad nauseum. (Becuase I get giddy. Not an expression of distaste or whatever.)

So I just think the emf will be that given by the change of flux from the moving magnet or primary coil or whatever, E=dΦ/dt for a closed loop of any resistance.
 
  • #5
Merlin3189
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I'd be hard pressed to say how the flux & emf vary along the magnet, except to agree with your zero and change of polarity near the centre.
Outside I find your sudden drop to zero emf surprising. I'd have thought the field stretches out to ∞, so you'll always have some tiny emf. I'd want to draw it more like an exponential fall off in emf as you move away from the magnet. Or inverse square maybe.
 
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Thank you for your reply. I was searching further, and asking around. I found out that in theory even magnetic monopoles are possible. I am hobbyist and I would like to create a sort of coil magnet generator. This is why I am asking around on this forum.

The bar magnets, equal in length and flux will be in sequence N-S, S-N, N-S. I have to position the solenoids (length and position) quite precisely, to achieve maximum EMF.

Now, next questions are:
- Will the magnetic zero in the middle of first magnet N-S be shifted toward the left N?
- What will be the ratio between total length of the magnet vs zero point. Single magnet zero point without presence of other magnets is in ratio 1:2
 
  • #7
CWatters
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The bar magnets, equal in length and flux will be in sequence N-S, S-N, N-S.
Why? I suspect that won't have the effect you think it will.
 
  • #8
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Obviously I have to do some experiments.

I will treat magnetic pols as positive and negative electric charge. I will estimate susceptibility of merged bar magnets as described above, to the ferromagnetic materials. Next I will draw Equipotential lines of B field, which are perpendicular to the equipotential lines.

After experimenting, I believe that I will be able to prove / disprove that magnetic zero in the middle of the left magnet is shifted to the left.

If shifted or not, I will be able at least to estimate how to position solenoids. And while writing this I got an idea that solenoids should be narrow and connected to my circuit via Bridge Rectifiers.

Thank you both for replying to my post.
 
  • #9
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I wonder if there is a way to calculate induced EMF in closed loops around bar magnet, which is traveling with constant velocity v to the right as depicted?
View attachment 232537
Yes there is a way to calculate the EMF => Faraday's law of induction.

Then applying that you would first need to plot or estimate your flux lines, then determine the rate of change of flux as the coil moves.

Very important thing to consider is that magnetic fields (and therefore the flux) interact with other magnetic fields, as well as any materials that have a relative permeability other than 1.

So your estimate is about right for a bar magnet in free space. Note if your coils are shorted turns, then the induced EMF will cause current to flow (I=V/R) which will develop its own field opposing the driving change of flux => now requires a force to move (the basis of a hysteresis brake) and changes the shape of the flux around it.

If you have zero volts generated, that just means your rate of change of flux is zero, this is not necessarily a magnetic zero, your coil may just be traveling parallel to the flux...

Then when it comes to permanent magnets, remember they basically act as an H source, then the resulting flux is determined by the reluctance of the magnetic path, then, if the generated flux exceeds some value (depending on the material) it will change its magnetization. So you have to be careful how you shape your magnet and the flux paths to avoid de mag.

What are you trying to achieve with this magnet setup?
 
  • #10
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FWIW, don't forget the classic demo where a bar magnet dropped through a copper or aluminium pipe is slowed by the back-emf.

In extremis, when there's no conductive loss, there's superconducting maglev...
 
  • #11
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What are you trying to achieve with this magnet setup?
For my hobby, I am trying to assemble efficient linear coil magnet generator. I found first idea over here, under the chapter; The simple shake-a-gen: http://www.creative-science.org.uk/gen_notes.html

Then I have posted my question over here. I did not know how the magnetic field is actually distributed along the bar magnet. After posting I have borrowed Oscilloscope from my friend and measured various configuration of cylindrical magnets, which were screwed onto the non-magnetic threaded rod. I slide the configurations trough short solenoid. I was interested especially on SN NS configurations, and especially at what distance between two magnets (SN NS) the best EMF is achieved. In my case the most effective distance is around 8 mm. Now I know how long my solenoids should be in regard with length of my magnets. The experiments were showing that magnetic poles are concentrated very close to the tip of longer (assembled from smaller) cylindrical magnet. In the picture below two longer
(assembled) magnets were 15 mm apart.
image (2).png

Later on I have found applications of Piston Linear Generator, which is generating electricity based on my intentions and finally, I have found a superb research paper on my matter, that can be found in paper titled: Modeling and Design Analysis of a Permanent Magnet Linear Synchronous Generator. https://core.ac.uk/download/pdf/19530183.pdf
 

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