Author Topic: Magnets!  (Read 13416 times)

Offline bn

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Re: Magnets!
« Reply #15 on: July 04, 2012, 06:45:36 PM »
elephants, eh?
anything for a zoidberg pumpkin.
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Offline snark1994

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Re: Magnets!
« Reply #16 on: July 06, 2012, 11:38:52 AM »
Isn't it the qE term, electric force, which is causing the force, rather than the qvxB?

I thought we could model a magnetic dipole as a pair of opposing charges, if we're sufficiently removed from the dipole, so we would expect an electrical field to exist at the location of a paperclip...

But then something at the back of my head says it has to be a changing magnetic field to generate an electrical field...

Offline G

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Re: Magnets!
« Reply #17 on: July 07, 2012, 12:04:33 AM »
Plus the paper clip is electrically neutral.

Offline Jacob Stump

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Re: Magnets!
« Reply #18 on: July 08, 2012, 04:36:05 PM »
*EDIT: I apologize for these massive images. I could make them smaller, but I've already spent too much time on this...DEAL WITH IT*

Is there perhaps a difference when looking at things macroscopically as opposed to at the individual particle level? 

^ I think that's the problem with this question. You're saying, "The magnetic force (applied by moving through a magnetic field) cannot do work (change the energy state) on a charged particle," but then rewording it to say, "The magnetic force cannot do work on anything."

Macroscopically, a magnetic force CAN do work on an object, DEPENDING on how you draw or illustrate your system. In other words, it's a question of frames of references, and of "reaction forces" vs. "applied forces". Also, the paperclip analogy kind of makes things difficult to understand. Instead of something that weighs effectively NOTHING, lets draw some FBD's with some WEIGHT.

You need to draw FIVE separate FBD's, under THREE separate conditions, to understand the entire situation. I will illustrate this with a handheld magnet and an anvil.



CONDITION 1: Magnet is "out of range" of the anvil.
1) The first FBD is ONLY the magnet and your hand, when you are holding the magnet out of range of the anvil, it is being acted upon by 2 forces, 1) G_mag = gravity force on the magnet, 2) Ry1 = the reaction force of your hand (which balances the force of gravity)


2) The second FBD is ONLY the anvil, when it is not under the influence of the magnet. It is being acted upon by 2 forces, 1) G_anvil = gravity force on the anvil, 2) Ry2 = the reaction force of the surface (which balances the force of gravity)


CONDITION 2: Magnet is just barely "in range" of the anvil, and is applying a force equal to the weight of the anvil.
3) The third FBD is ONLY the anvil, but when the anvil is within the range of the magnetic field. In this case, the force being applied is JUST BARELY enough to lift the anvil. In other words, the anvilis being acted upon by 2 forces, 1) G_anvil = gravity force on the anvil, 2) F_mag = The magnetic force being applied to the anvil, which is exactly equal to G_anvil in this case. Therefore there is no movement of the anvil (because the forces are balanced).


4) The fourth FBD should be drawn of the Magnet and your hand, during the force-balancing of FBD #2. In this case, the magnet is being acted upon by THREE forces. 1) G_mag = gravity force on the magnet, 2) F_mag = the magnetic force pulling the anvil from FBD #2, and 3) Ry3 = the reaction force of your hand (which balances all other forces). NOTE: Ry3 is MUCH GREATER than Ry1, meaning your hand is now pulling MUCH HARDER to counteract the magnetic force.


CONDITION 3: The magnet has been moved closer to the anvil, and has "pulled" the anvil up to contact the magnet

5) The fifth FBD should be drawn of the Magnet, the anvil, and your hand holding the magnet. (EDIT, the FBD is labelled incorrectly. Should be G_mag + G_anvil, not G_mag + F_mag)  In this case, the magnetic force is internal to the system, and not shown. Effectively, the magnet and the anvil "are one object".  The forces involved are 1) G_anvil = weight of the anvil, 2) G_mag = weight of the magnet, 3) Ry3 = the reaction force of your hand (which balances the other forces. So now your hand is carrying the weight of the magnet AND the weight of the anvil. Now ask yourself, while you're straining to hold this freakin ANVIL - did you do any work?


Did the magnetic force do work when the anvil was "picked up"? Or was it your hand, providing the reaction force, that performed the work on the anvil? Think about this for a moment... if your hand could not sustain the additional load from the anvil, the magnet would have PULLED YOUR HAND down to the ground. (Think about picking up a heavy magnet and holding it over a steel beam. You wouldn't be able to resist the force, and your hand would be pulled to the beam)

What is revealed by this is very important: THE MAGNET HAS NOT CREATED THE ADDITIONAL FORCE IN THE SYSTEM. The force applied by your hand has increased, which I have illustrated as a reaction force. HOWEVER, it is actually the other way around - the MAGNETIC FORCE IS A REACTION FORCE ONLY, AND THEREFORE CANNOT DO WORK. BOOM.


The magnetic force in these FBD's is really equivalent to a string or a rope. The rope carries load, but it doesn't create force. Does a rope do work on the object that it is lifting? (Or does the person pulling the rope?)

SO...in conclusion... if you draw your FBD to look at only part of a system (which is totally valid), then you can create a situation where the magnetic force appears to do work. But in reality, the magnetic force itself is only a reaction force, and must be balanced by an additional external force in order to perform work.

Does that make sense?
« Last Edit: July 14, 2012, 03:55:40 PM by Jacob Stump »

Offline Ed Lolington

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Re: Magnets!
« Reply #19 on: July 08, 2012, 06:00:17 PM »
In your description of the 5th FBD the force Ry3 is equal to G_mag + G_anvil, but it is labeled incorrectly (G_mag + F_mag) in the picture. I still agree with your analysis and that's a wonderful explanation!

Offline bn

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Re: Magnets!
« Reply #20 on: July 14, 2012, 07:57:35 AM »
 :P by the way, i haven't forgotten this question, and I'm *SO CLOSE* to having a coherent answer.
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Offline Jacob Stump

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Re: Magnets!
« Reply #21 on: July 14, 2012, 03:56:02 PM »
Doh! Thanks Ed, I made a note of it in the text.

Offline G

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Re: Magnets!
« Reply #22 on: July 23, 2012, 11:38:45 PM »
First of all, Jacob, thank you very much for the detailed response fully decked out with diagrams.  I think we are looking at the situation in two different ways.  If I understand what you are saying, if it were not for me holding the magnet above the anvil, the anvil would not have been lifted. Thus, in the big-picture sense, I'm the ultimate source of the energy to lift the object because the anvil pulls down on the magnet just as hard as the magnet pulls up on the anvil.  While I think I see where you are going with that, it doesn't quite address what I was asking--my apologies if I didn't convey my question clearly enough. 

Let me go through those diagrams and show you where my understanding differs with yours--perhaps that will help clarify my question.

Diagram 1:
Technically, isn't the upward force actually a normal force from my hand onto the magnet?  The reaction force to the force of gravity on the magnet would be the magnet pulling down on the Earth.  Or, are you skipping the intermediary step of the magnet applying a downward force on my hand equal to its weight resulting in my hand pushing up on the magnet?

Diagram 2:
Same deal with the anvil and the ground.

Diagram 3:
Now this is actually where we hit the root of my question, I think--why is it that the magnetic force points upward?  Now, with a horseshoe magnet, you would end up with a magnetic field that is parallel to the ground like in this picture:



In order for the magnetic component of the Lorentz force to point up, there would need to be charged particles in the anvil moving parallel to the ground but not parallel to the magnetic field.  This seems unlikely because if the anvil were stood up on its end, we know that the magnet would still lift it regardless of the new orientation.

In the case of a bar magnet, which also would lift the object if one of the poles was placed close enough to it, the magnetic field would be pointing perpendicular to the ground (into or out of the ground depending on which pole was brought near the object).    How then, could the magnetic force point upward, parallel to the magnetic field?

In either case, what is happening microscopically that, in aggregate, allows for a net upward force and seemingly work being done by the magnetic force?

Diagrams 4, 5, and conclusion:
I am unsure what you mean by the magnetic force is a reaction force and therefore cannot do work.  Again,  is this in the context of Newton's 3rd Law and action-reaction pairs?  You go on to make an analogy to lifting an object with a rope.  I would agree that yes, without the person pulling on the other end of the rope, the object would not move.  However, the person is not directly touching the object.  The person is doing work on the rope-object system, but is it not the force of tension acting directly on the object and in the same direction as its motion that is in fact doing the work on the object itself?

How about we simplify the situation and remove a lot of the complications that I think is adding to my confusion?  Imagine the anvil and a bar magnet of the same mass floating in space far enough away from anything else that the net external force on the system is zero.  When released, the anvil and bar magnet are close enough to attract each other.  The bar magnet exerts a force on the anvil, drawing the anvil near.  What force is that?  If it is the magnetic component of the Lorentz force, how is it doing work when by definition the magnetic component of the Lorentz force cannot do work?

Offline Lynx Cat

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Re: Magnets!
« Reply #23 on: July 24, 2012, 05:55:00 AM »
In order for the magnetic component of the Lorentz force to point up, there would need to be charged particles in the anvil moving parallel to the ground but not parallel to the magnetic field.  This seems unlikely because if the anvil were stood up on its end, we know that the magnet would still lift it regardless of the new orientation.

In the case of a bar magnet, which also would lift the object if one of the poles was placed close enough to it, the magnetic field would be pointing perpendicular to the ground (into or out of the ground depending on which pole was brought near the object).    How then, could the magnetic force point upward, parallel to the magnetic field?

In either case, what is happening microscopically that, in aggregate, allows for a net upward force and seemingly work being done by the magnetic force?

Hmm... As I understand it, what makes ferrous metals magnetic is the "freedom of movement" of the electrons in the atoms' valence band. Correct me if I'm wrong here, I'm mostly a layperson. So, I guess the magnet's magnetic field itself realigns these electrons, making them run little circles perpendicular to the field, which is what creates the upward magnetic force - completely regardless of the anvil's shape or orientation, since it's happening at an atomic level. Am I on the right track?

edit - alternatively:
<a href="http://www.youtube.com/v/OvmvxAcT_Yc" target="_blank" class="new_win">http://www.youtube.com/v/OvmvxAcT_Yc</a>
« Last Edit: July 24, 2012, 05:59:28 AM by Lynx Cat »
Did I do, O CROM, did I as I said Id do? Good! I did.

Offline bn

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Re: Magnets!
« Reply #24 on: July 30, 2012, 04:23:52 PM »


Hmm... As I understand it, what makes ferrous metals magnetic is the "freedom of movement" of the electrons in the atoms' valence band. Correct me if I'm wrong here, I'm mostly a layperson. So, I guess the magnet's magnetic field itself realigns these electrons, making them run little circles perpendicular to the field, which is what creates the upward magnetic force - completely regardless of the anvil's shape or orientation, since it's happening at an atomic level. Am I on the right track?


yeah.
i'm trying to come up with an explanation that involves loops of wire, and electrons flowing around them. i've got most of it worked out...
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