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Author Topic: Faraday's homopolar motor revisited  (Read 69446 times)
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img  PhysicsProf
Faraday's homopolar motor revisited « on: 2012-04-08, 19:22:04 »
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Sorry for the lack of postings lately...  My wife and I are in Maryland helping our daughter with the arrival of a wonderful grand-daughter!  she was born last week; some complications, but both doing fine now.  We've been gone from my electronics bench for about a month already...

However, a short vid showing some budding scientists -- grandchildren!   There should be a certain child-like curiosity and joy as we undertake experiments IMO!  Showing that they are interested and setting up their OWN electronics bench LOL.

http://www.youtube.com/watch?v=itXT8H5EXoo&feature=youtu.be

I came across a very good vid, Michael Faraday, on PBS -- and it shows his homopolar motor at the end, the very first motor:

http://www.youtube.com/watch?v=yVDHKKTC4tA

I'm doing some research on Faraday's paradox again... based on the homopolar motor / generator.  Consider the attached diagram and the following vid:
http://www.youtube.com/watch?v=zOdboRYf1hM&feature=fvwrel

First -- is the diagram correct for this vid?  (ie, are the directions of the Lorentz qv X B forces correct?)

Second -- place the whole set-up on a freely-rotating wheel support (vertical axis) -- which way will the battery+magnet spin?

   
img  GibbsHelmholtz
Re: Faraday's homopolar motor revisited « Reply #1 on: 2012-04-08, 20:47:37 »
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It's an interesting motor professor.

I see the magnetic field crossed the vertical side/plane of the loop......is this what makes the motor possible? hm....
   
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Re: Faraday's homopolar motor revisited « Reply #2 on: 2012-04-08, 21:29:12 »
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Quote from: GibbsHelmholtz on 2012-04-08, 20:47:37

It's an interesting motor professor.

I see the magnetic field crossed the vertical side/plane of the loop......is this what makes the motor possible? hm....



In part yes -- there are forces which result in torques on the left (force away from you in the diagram, hence (x), like the feathers of an arrow going AWAY from you ) and on the right (force towards you in the diagram, hence (.) ).  There are also forces on the wires at the bottom where the field is stronger, tending to cause rotation in the same sense -- convention calls this -z rotation (right-hand fingers in direction of the spin, thumb points downward, hence -z axis).

FYI, pls see http://en.wikipedia.org/wiki/Homopolar_motor  where the diagram also comes from ;)

OK -- please pull your right-hand, apply the right-hand-rule (RHR) and check that the forces shown on the diagram are correct!
Direction of the force found from I x B, that is, fingers in direction of current I, rotate rt-hand-fingers to direction of magnetic field line B, then force F is in direction of thumb.  Try it!   (PS -- I find the diagram is correct.)

Note that instead of rotating the WIRE as shown above, we can hold the wire fixed and rotate the magnet -- see attached.  Diagram gives the idea -- note battery "upside down" from photo.  Obviously we can change the orientations of the magnet and the battery.  We can also place the magnetic field EXTERNAL to the system; still works.
   
img  GibbsHelmholtz
Re: Faraday's homopolar motor revisited « Reply #3 on: 2012-04-08, 22:41:55 »
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Actually, I did try the fingers and palm. lol  It does indicates torques and correct direction.

I'm just interested in the diagram because it relates to the experiment we talk about in the other thread.  There are actually field lines crossing the vertical planes, the higher it goes up, the more field lines it captures. 

As for the motor, the concept seems different than a pulse motor.  It uses charges in the conductor as torque while pulse motor use 2 magnetic fields attracting or repulsing.

   
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Re: Faraday's homopolar motor revisited « Reply #4 on: 2012-04-08, 23:19:02 »
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Quote from: GibbsHelmholtz on 2012-04-08, 22:41:55
Actually, I did try the fingers and palm. lol It does indicates torques and correct direction.

I'm just interested in the diagram because it relates to the experiment we talk about in the other thread.  There are actually field lines crossing the vertical planes, the higher it goes up, the more field lines it captures.  

As for the motor, the concept seems different than a pulse motor.  It uses charges in the conductor as torque while pulse motor use 2 magnetic fields attracting or repulsing.



Right - thanks.

Now the question before us:
Consider this set-up:  http://www.youtube.com/watch?v=zOdboRYf1hM&feature=fvwrel

Next, place the whole set-up on a freely-rotating wheel support (vertical axis), wires not touching the support -- which way will the battery+magnet spin?   -z (same as the wire) or +z ??
   
img  GibbsHelmholtz
Re: Faraday's homopolar motor revisited « Reply #5 on: 2012-04-08, 23:37:26 »
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Quote from: PhysicsProf on 2012-04-08, 23:19:02
Right - thanks.

Now the question before us:
Consider this set-up:  http://www.youtube.com/watch?v=zOdboRYf1hM&feature=fvwrel

Next, place the whole set-up on a freely-rotating wheel support (vertical axis), wires not touching the support -- which way will the battery+magnet spin?   -z (same as the wire) or +z ??


I don't know but common sense would be the opposite direction.  It's action reaction. 

   
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Re: Faraday's homopolar motor revisited « Reply #6 on: 2012-04-09, 01:01:06 »
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Quote from: GibbsHelmholtz on 2012-04-08, 23:37:26
I don't know but common sense would be the opposite direction.  It's action reaction.  



A good theoretical way to look at it -- apply Newton's Third Law, action-reaction.   And you're right, this is what theory would predict.
But -- in this field, some are challenging Newton's 3rd law...  
And it's not just a question of the batt+magnet spinning the opposite direction as the wire -- but,

are the torques EQUAL in magnitude (and opposite in direction)?

 So, if we're not willing just to say we already know for SURE the torques are equal based on the Third Law,
HOW do we find out? that's the next question; I have some ideas but seeking input (and hope you're following along the argument).
   
img  GibbsHelmholtz
Re: Faraday's homopolar motor revisited « Reply #7 on: 2012-04-09, 01:24:53 »
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Quote from: PhysicsProf on 2012-04-09, 01:01:06
A good theoretical way to look at it -- apply Newton's Third Law, action-reaction.   And you're right, this is what theory would predict.
But -- in this field, some are challenging Newton's 3rd law...  
And it's not just a question of the batt+magnet spinning the opposite direction as the wire -- but,

are the torques EQUAL in magnitude (and opposite in direction)?

 So, if we're not willing just to say we already know for SURE the torques are equal based on the Third Law,
HOW do we find out? that's the next question; I have some ideas but seeking input (and hope you're following along the argument).



After giving this more thoughts.... I'm heading out to buy battery. lol
   
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Re: Faraday's homopolar motor revisited « Reply #8 on: 2012-04-09, 02:39:17 »
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  If you're saying you are going to do the experiment -- I agree that is the best way to find out.  "Theory may guide, but experiments decide."

Out here, I need a few parts... will see what I can do also on this question.  (And there are other -- somewhat related -- questions of interest as well with this puppy.   The first electric motor, Faraday 1821.  Revisited.)
   
img  BEP
Re: Faraday's homopolar motor revisited « Reply #9 on: 2012-04-09, 04:20:44 »
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PhysicsProf,

Take note that the magnet will not 'feel' a torque when you hold the wires still but the conductor it is attached to will - the screw.

The only time the magnet will feel a torque is when it is also the majority of the conductive loop. N3 doesn't apply to the magnet because the magnetic field is a property of space, not of the magnet.
   
img  GibbsHelmholtz
Re: Faraday's homopolar motor revisited « Reply #10 on: 2012-04-09, 04:32:41 »
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I just read about Faraday paradox.  Indeed it has something to do with action reaction.  Conductor rotate in a magnetic field generate voltage but magnetic field rotates while conductor remains stationary does not generate voltage.  Hm...

I suppose the same thing can be apply on a motor instead of generator.

1.  A conductor move while magnet remains stationary generate voltage.
     A current carrying conductor in a stationary magnetic field produce a force on the conductor.

2.  A conductor and magnet rotate together generate voltage.
    A current carrying conductor produce a force even when the magnet moves with it.

3.  The magnet rotate while conductor stationary generate no voltage.
      The magnet will not experience a force while the conductor carry current.

I will perform some experiments to see if these 3 things matches.



WW!

I just read your post.  So it is true that the magnet would not feel the force!  Wow....
   
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Re: Faraday's homopolar motor revisited « Reply #11 on: 2012-04-09, 04:52:54 »
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Quote from: WaveWatcher on 2012-04-09, 04:20:44
PhysicsProf,

Take note that the magnet will not 'feel' a torque when you hold the wires still but the conductor it is attached to will - the screw.

The only time the magnet will feel a torque is when it is also the majority of the conductive loop. N3 doesn't apply to the magnet because the magnetic field is a property of space, not of the magnet.




Note that the magnet is covered with a conductive surface, so this surface-conductor completes the circuit.  Current in THIS conductor on the magnet-surface generates a torque in the opposite direction from the torque on the wires; but I don't know if the magnitudes of the torques are EQUAL (hence the need for experiments -- thanks Gibbs).

Let me cut to the chase.  Let's set the whole apparatus on a frictionless surface, or suspended from above (free to rotate) -- free to spin about the z-axis one direction or the other.  Let's also AFIX the wires to the surface of the magnet, so there is no relative motion between wire and magnet -- they are constrained to rotate (or remain stationary - the starting condition) TOGETHER.  

So now, what do you fellows predict -- will the system begin to rotate, or not?
I'm trying to see what you would predict, WW, in this case, and Gibbs.  

 I'm going to go with Newton's 3rd = N3 on this; no rotation.  Conservation of angular momentum likewise gives the null result.

  But if experiment shows otherwise -- I will be delighted!  and Faraday's paradox would then acquire new meaning perhaps...
   
img  GibbsHelmholtz
Re: Faraday's homopolar motor revisited « Reply #12 on: 2012-04-09, 05:56:51 »
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I see what you mean professor.  I've designed a setup where we can accomplish it more practically.  Tell me if this satisfy the condition of your experiment.

The picture below shows a pair of magnet sandwiched a conductor in between.  When we supply a current through the conductor, it would either remains stationary or deflects.  If it doesn't deflects, then no momentum is violated.  I think with this setup we can adjust the length to increase torque in case of sensitivity issue.


BTW, I say it will deflects.  :)
   
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Re: Faraday's homopolar motor revisited « Reply #13 on: 2012-04-09, 12:29:56 »
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Quote from: GibbsHelmholtz on 2012-04-09, 05:56:51

I see what you mean professor.  I've designed a setup where we can accomplish it more practically.  Tell me if this satisfy the condition of your experiment.

The picture below shows a pair of magnet sandwiched a conductor in between.  When we supply a current through the conductor, it would either remains stationary or deflects.  If it doesn't deflects, then no momentum is violated.  I think with this setup we can adjust the length to increase torque in case of sensitivity issue.


BTW, I say it will deflects.  :)

Actually, your proposed experiment is not the same -- there is no rotation, is there? 

No, it needs to be a Faraday motor, and it needs to be mounted as a system, FREE to rotate or not, as noted as the crucial condition for the experiment proposed above.
   
img  GibbsHelmholtz
Re: Faraday's homopolar motor revisited « Reply #14 on: 2012-04-09, 12:47:17 »
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Quote from: PhysicsProf on 2012-04-09, 12:29:56
Actually, your proposed experiment is not the same -- there is no rotation, is there? 

No, it needs to be a Faraday motor, and it needs to be mounted as a system, FREE to rotate or not, as noted as the crucial condition for the experiment proposed above.


The conductor and magnet either rotate into or out of the page. 

The point of this experiment is to see if the wire could carry the magnet while using the same magnetic field it carried.  Is this a crucial point?
   
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Re: Faraday's homopolar motor revisited « Reply #15 on: 2012-04-09, 14:36:41 »

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Quote from: GibbsHelmholtz on 2012-04-09, 12:47:17

The conductor and magnet either rotate into or out of the page. 

The point of this experiment is to see if the wire could carry the magnet while using the same magnetic field it carried.  Is this a crucial point?

I don' think there will be any movement there Gibbs. You've removed the relativity factor.


---------------------------
"Some scientists claim that hydrogen, because it is so plentiful, is the basic building block of the universe. I dispute that. I say there is more stupidity than hydrogen, and that is the basic building block of the universe." Frank Zappa
   
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Re: Faraday's homopolar motor revisited « Reply #16 on: 2012-04-09, 14:41:48 »
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Quote from: poynt99 on 2012-04-09, 14:36:41
I don' think there will be any movement there Gibbs. You've removed the relativity factor.


You may be right.  It is just my thought that magnet and conductor rotates together generate voltage, so why current can't rotate magnet and conductor affix.  I'm not too familiar with relativity factor though.

Also as WW stated the magnet would not feel a force because the wire is pushing on space to propelled itself.  So if the wire pushing on the magnet to propel, then there would be no movement, but if it pushes on space, then in theory it could carry the magnet. 



   
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Re: Faraday's homopolar motor revisited « Reply #17 on: 2012-04-09, 14:52:58 »

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Quote from: GibbsHelmholtz on 2012-04-09, 14:41:48

You may be right.  It is just my thought that magnet and conductor rotates together generate voltage, so why current can't rotate magnet and conductor affix.  I'm not too familiar with relativity factor though.

In your experimental setup, there is no relative movement between the B-field and the conductor. In the HG, there IS relative movement between the two.

That's what I mean by relativity factor.  :D

By the way, there is no paradox at all. Conceptually, it's simple.


---------------------------
"Some scientists claim that hydrogen, because it is so plentiful, is the basic building block of the universe. I dispute that. I say there is more stupidity than hydrogen, and that is the basic building block of the universe." Frank Zappa
   
img  GibbsHelmholtz
Re: Faraday's homopolar motor revisited « Reply #18 on: 2012-04-09, 17:22:43 »
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Quote from: poynt99 on 2012-04-09, 14:52:58
In your experimental setup, there is no relative movement between the B-field and the conductor. In the HG, there IS relative movement between the two.

That's what I mean by relativity factor.  :D

By the way, there is no paradox at all. Conceptually, it's simple.

I see.  Then can you explain the third thing in this experiment?  There is no relative movement between the disc and B-field.

The experiment proceeds in three steps:

   1. The magnet is held to prevent it from rotating, while the disc is spun on its axis. The result is that the galvanometer registers a direct current. The apparatus therefore acts as a generator, variously called the Faraday generator, the Faraday disc, or the homopolar (or unipolar) generator.
   2. The disc is held stationary while the magnet is spun on its axis. The result is that the galvanometer registers no current.
   3. The disc and magnet are spun together. The galvanometer registers a current, as it did in step 1.

http://en.wikipedia.org/wiki/Faraday_paradox
   
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Re: Faraday's homopolar motor revisited « Reply #19 on: 2012-04-09, 19:18:39 »
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  This may help explain, Gibbs -- an experiment involving two ring magnets, bottom one free to spin -- but it does not!

http://www.youtube.com/watch?v=IlUY3snoWI8

Note this question with the vid;  any answer to this?

Quote
""Replace the rotating magnet on the drill by a conducting disk that is shorted out from center to perimeter to form a closed circuit. Put a small bulb in this circuit to confirm there's current flow when rotating the drill above the magnet This is basically faraday's homopolar generator. But the thing I want to get at is showing what happens to the bottom magnet that can rotate freely. It should spin in the other direction. According to newton's third law..."question by Broli123"
« Last Edit: 2012-04-09, 21:14:30 by PhysicsProf »
   
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Re: Faraday's homopolar motor revisited « Reply #20 on: 2012-04-09, 19:29:29 »

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Quote from: GibbsHelmholtz on 2012-04-09, 17:22:43
I see.  Then can you explain the third thing in this experiment?  There is no relative movement between the disc and B-field.

The experiment proceeds in three steps:

   1. The magnet is held to prevent it from rotating, while the disc is spun on its axis. The result is that the galvanometer registers a direct current. The apparatus therefore acts as a generator, variously called the Faraday generator, the Faraday disc, or the homopolar (or unipolar) generator.
   2. The disc is held stationary while the magnet is spun on its axis. The result is that the galvanometer registers no current.
   3. The disc and magnet are spun together. The galvanometer registers a current, as it did in step 1.

http://en.wikipedia.org/wiki/Faraday_paradox


Yes, it's quite simple.

When you spin a magnet on its polar axis, there is absolutely no change in its magnetic field. From Shadowitz p.395:

Quote
The lines of B are not defined with respect to the magnets but are defined with respect to a stationary observer, If B is absolutely uniform then turning of the magnets has absolutely no effect on the B at any point: B is independent of time. So there is no transformer induced voltage in this case.

Therefore in case 2., there is no relative motion between the conductor and the magnetic field.

And in case 3. when the disc and magnet are spun together, it is exactly the same scenario as case 1.


---------------------------
"Some scientists claim that hydrogen, because it is so plentiful, is the basic building block of the universe. I dispute that. I say there is more stupidity than hydrogen, and that is the basic building block of the universe." Frank Zappa
   
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Re: Faraday's homopolar motor revisited « Reply #21 on: 2012-04-09, 19:37:57 »

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It's not as complicated as it may seem...
Steve, regarding your question when the magnet is replaced with a shorted disc:

What direction does the guy say the bottom magnet rotates?

To my thinking, it should rotate in the same direction as the rotating disc.


---------------------------
"Some scientists claim that hydrogen, because it is so plentiful, is the basic building block of the universe. I dispute that. I say there is more stupidity than hydrogen, and that is the basic building block of the universe." Frank Zappa
   
img  GibbsHelmholtz
Re: Faraday's homopolar motor revisited « Reply #22 on: 2012-04-09, 20:06:58 »
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Quote from: poynt99 on 2012-04-09, 19:29:29
Yes, it's quite simple.

When you spin a magnet on its polar axis, there is absolutely no change in its magnetic field. From Shadowitz p.395:

Therefore in case 2., there is no relative motion between the conductor and the magnetic field.

And in case 3. when the disc and magnet are spun together, it is exactly the same scenario as case 1.

Poynt,

So in all 3 cases, the B field is stationary? 

   
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Re: Faraday's homopolar motor revisited « Reply #23 on: 2012-04-09, 20:09:12 »

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Quote from: GibbsHelmholtz on 2012-04-09, 20:06:58
Poynt,

So in all 3 cases, the B field is stationary? 

A better way to think of it is that it is unchanging. Yes.


---------------------------
"Some scientists claim that hydrogen, because it is so plentiful, is the basic building block of the universe. I dispute that. I say there is more stupidity than hydrogen, and that is the basic building block of the universe." Frank Zappa
   
img  GibbsHelmholtz
Re: Faraday's homopolar motor revisited « Reply #24 on: 2012-04-09, 20:17:03 »
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So back to the question.  You didn't think there be movement in my setup and the reason is there is no relative movement between the conductor and the B field.  The relative movement between those generate current.  So if we apply a current, then the conductor would move relative to the B field.  And of course, the B field is not affix to the magnet as we have discussed. 

   
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