Waveform of Classic Electromagnetic Induction Experiment 1

[shahvir]

Hi guys,

Can someone please provide graphical representation (waveform) of emf induced in coil due to a bar magnet spinning perpendicular to axis of coil as illustrated in attachment.

Thanks,
SB

 

[electricpete]

+ [-] [+] [-] [+] [-] +

...How do we treat the + at the beginning and the + at the end? IF we combined them with the additional adjacent + voltage pulse before and after from additional adjacent pole swings before and after this series of six, THEN they would each contribute a half cycle. BUT since we're not including those adjacent pulses from pole swings outside our series of six, we have to call each of those + voltage pulses at the beginning end of our pattern a quarter cycle. So we have 2.5 cycles in the middle, quarter cycle at each end, it adds up to three cycles like I said.

Here is an alternative (simpler) answer to my question "How do we treat the + at the beginning and the + at the end?":
[ul]
[li]Wrap the end of the series around to the beginning like a circle (since this is a periodic sequence).[/li]
[li]Then the + at the end combines with the + at the beginning to form a [+].[/li]
[li]Now we have three [+] and three [-] for three full cycles.[/li]
[/ul]
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(2B)+(2B)' ?

 

[shahvir]

Kindly check the output voltage waveforms in the attached pdf article. This was the double peaked waveform pattern I was mentioning about. In my opinion, the rotating magnet arrangement would generate such a double peaked output voltage.

 

[shahvir]

In the rotating magnet arrangement is induced emf influenced by flux cutting also or only rate of change of flux linkage?

 

[electricpete]

> Kindly check the output voltage waveforms in the attached pdf article

I didn't look at it closely, but let's attack it another way...

> In my opinion, the rotating magnet arrangement would generate such a double peaked output voltage.

I think it’s useful to look at two extremes of the geometry:
[ul]
[li]If you have a very long magnet whose end passes very close to a very short sensing coil, then clearly you will have a double peaked output voltage. It may resemble swings of the pendulum with 0 voltage between (+ - 0 - + 0 + - 0 - + )[/li]
[li]If you have a very short magnet a long way from the sensing coil, then you have a sinusoidal voltage.[/li]
[/ul]

In between, it is somewhat more difficult to say. If you have all your dimensions and you wanted to calculate the flux from your magnet in a given orientation, you could represent your permanent magnet as an air coil and use Biot Savart law.

Given that no dimensions are provided I assumed prof was interested in the simpler solution.

> In the rotating magnet arrangement is induced emf influenced by flux cutting also or only rate of change of flux linkage?

I view the problem in terms of flux linkage which is a straightforward interpretation of Faraday's law. I don't see any advantage to viewing the problem in terms of flux cutting wires.

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(2B)+(2B)' ?

 

[shahvir]

I view the problem in terms of flux linkage which is a straightforward interpretation of Faraday's law. I don't see any advantage to viewing the problem in terms of flux cutting wires.

So from your above statement I would presume that influence of flux cutting on induced emf may not be significant.
Are there any mathematical expressions/equations for induced emf which can represent the rotating magnet case? Thanks

 

[electricpete]

> So from your above statement I would presume that influence of flux cutting on induced emf may not be significant.
I’d say flux cutting and flux linkage are two different ways of looking at the same phenomenon(I much prefer flux linkage):
Farady's law: Integral E-dot-dL = -d/dt Integral B dot dA
Voltage is left side. Time derivative of flux linkage on the right.
If you have a rectangular coil rotating in a constant magnetic field, you could calculate the flux linkage directly from Faraday by examining flux linkage as a function of angle.... that is a very straightforward application of Faraday's law imo. Alternatively you could compare the rate of flux cutting the top wire and the bottom wire (both of those contribute to the rate of change of flux linkage)... but that is a more complicated and less generalizable way to look at things.... I'd suggest not to look at it that way.

> Are there any mathematical expressions/equations for induced emf which can represent the rotating magnet case?
I would work on getting flux (at measurement location) as a function of magnet rotation angle theta and then take derivative...time derivative of sin(theta(t)) would become cos(theta(t))*w0 (by chain rule) where w0 is constant rotation rate.

As I mentioned if you represent the permanent magnet as an air coil then you can use Biot Savart
B( r) = (mu0/4/pi) * Closed-Loop-Integral {I dl x r / |r|^3} integrated along the source coil wires

If you look at the simplest case I mentioned (very short magnet … represented by a single loop coil … a very long way from the sensing coil) then as theta changes, |r| is roughly constant for all elements of the coil and the only thing that changes is the angle in the cross product, giving a sinusoidal result in terms of theta, leading to sinusoidal voltage.

For the other cases, it’s going to get a lot more complicated very quickly but I'm sure it can be done (from Biot Savart).
... OR you can use a free finite element magnetic solvers.
... OR you can do an experiment. If you don't have a coil and an oscilloscope you might be able to use your phone if it has capability to measure a magnetic field vector (example magnitude along each axis of the phone). I'm not sure if phones have that capability and I take no responsibility for any damage to your phone if you get a powerful magnet too close.


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(2B)+(2B)' ?

 

[shahvir]

https://youtu.be/HI5ERdvroec

Intriguing demo in the link above. If you have access to an oscilloscope, it would be pretty interesting to analyse the output waveforms.

 

[electricpete]

haha, I'm not sure I would trust some random anonymous youtuber named electricpete1

I tried to make that video as a straightforward demo of faraday's law.

To distinguish it from the multitude of other youtuber video's with permanent magnets whizzing by coils to light up LED's, I highlighted in the title there was no relative motion between the magnet and the coil.

Some youtubers apparently took that as an indication that I was somehow talking about free energy and I think my video got linked on some free energy (nutjob) websites and you can see some of the bizarre comments on the video.

I'm not complaining, the extra attention resulted in that video receiving far more views and comments than any other video I've ever made (almost half a million views). Far more than the boring video about force on iron vs conductor which has 2k views. Ironically that "boring" video is the far more interesting one to me although the video doesn't explain what's going on.

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(2B)+(2B)' ?

 

[shahvir]

https://youtu.be/2FvWtEdY4sE

I also pretty much enjoyed your video on moving water with magnets. Wonder if you can use rotating magnetic fields to maybe, spin the water. I don't know if it's technically possible, but would be pretty interesting and entertaining for the YouTube audience and boost views.

Far more than the boring video about force on iron vs conductor which has 2k views.

It wasn't boring, but I guess the absence of a detailed explanation put off viewers. I suggest you remake the video with a detailed explanation of your demo which will definitely attract more views.

 

[shahvir]

https://electricalspace.quora.com/M...ed-so-because-I-have-nev?ch=10&share=856f834a

A peculiar case of electromagnetic induction paradox. Please see above link.

 

[3DDave]

https://www.youtube.com/watch?v=1xFRtdN5IJA

There is no paradox in physics, only those who fail to understand it.

I consider this as a revolutionary insight because in the course of time it will bring a completely new understanding of magnetism.

Yup. As soon as he understands how the magnetic field in each case is changing he will have a new understanding. The rest of us will wait for him to figure that out.

 

[electricpete]

I'm with 3DDave on that. Not a surprising result at all. Draw the lines of flux and think about faraday's law (or if you must, think about flux cutting).


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(2B)+(2B)' ?