Author Topic: Magnets!  (Read 13371 times)

Offline G

  • talky crow
  • ****
  • Posts: 33
  • Karma: +0/-0
    • View Profile
Magnets!
« on: July 02, 2012, 10:37:42 PM »
Hey Ben, my favorite Paleopal.  I've been teaching non-calculus based physics to premed students for quite a while now and occasionally get asked this question for which I have no good answer (aside from it being beyond the scope of the MCAT).  Clearly the magnetic force cannot do work on an individual particle, as the force is by definition perpendicular to the particle's velocity.  However, when a magnet is gently lowered toward a staple that is resting on the table, at some point the staple will jump to the magnet.  The staple's kinetic energy changed so work must have been done on it--but by what?

Physics boards have been no help.  Half the folks say "of course the magnetic force can't do work" but then can't explain  what precisely does the work in lifting the staple.  The other half say that the magnetic force does do work on the staple but can't explain the contradiction with the formula's definition.

Can you and your Titanium Physicists settle this once and for all?  What does the work when a magnet appears to lift an object?

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Magnets!
« Reply #1 on: July 03, 2012, 02:23:42 AM »
hmm
i'm going to think about it.
i tried answering it just now, but now i need to think about it a little more.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Magnets!
« Reply #2 on: July 03, 2012, 09:08:55 AM »
yeah, i hope i'm not getting myself into trouble for this. It's been a while since i learned my electromagnetism. what's more, i remember these explanations bordering on the philosophical when it comes to the issue of "where is the energy coming from?".

so when we say that magnetic fields can't do work to a particle, we mean that magnetic fields can't change a particle's speed, just as you said.

that isn't to say that a magnetic field doesn't have energy contained within it.
the magnetic field of a system does infact contain energy, and the amount of energy contained within it depends on a bunch of things; but you can make the magnetic field have contain more or less energy.

and so you can make a system where you use EMF to increase the energy in the magnetic field, which then turns into kinetic energy. so a common example of this is an electric motor: say, a permanent magnet on a pivot between two electromagnets. The electromagnets are polarized in a way where the magnetic field of the system has a lot of energy in it. the system tries to lower the amount of energy in the system by realigning the pivoting permanent magnet (when it gets close to the "lowest energy position", the electromagnets switch polarity... and the pivoting magnets ends up spinning).

um.

so in the paper clip case, the paper clip is a ferromagnet. so the metal is composed of little magnetic domains which aren't in any particular orientation. When an external magnetic field is applied, the ones out of alignment will all line up, lowering the amount of energy in the magnetic field of the system. this ends up giving the paperclip an overall magnetic field which is aligned with the magnetic field you are applying to it. The system can even further reduce the energy in the magnetic field by sucking the little magnet (the paper clip) close in to the big magnet.

so the energy in the initial configuration of the magnetic fields is what turns into the kinetic energy of the paperclip as it flies.

so then the question is "where does the energy in the original *messed up* configuration of the magnetic field come from"? I think the answer to that is that you (the person brining the magnet around) end up doing work on the system when you either bring the big magnet close in; or in charging up the big magnet.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Magnets!
« Reply #3 on: July 03, 2012, 09:12:06 AM »
did that... make any sense?   :-\
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline G

  • talky crow
  • ****
  • Posts: 33
  • Karma: +0/-0
    • View Profile
Re: Magnets!
« Reply #4 on: July 03, 2012, 02:54:02 PM »
I think that part makes sense.  Are you saying that there is a sort of potential energy associated with magnetic fields? 

But also, can you break it down to a free-body diagram of the staple?  What has me confused is what force precisely is acting over the distance between the table and the magnet.  You say the staple's kinetic energy has changed--but work must be done to change kinetic energy and ultimately that means some component of a force must be exerted parallel to the staple's motion.  Yet, by F = q(v x B), it seems that it would be impossible for the magnetic force to actually do work.

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Magnets!
« Reply #5 on: July 03, 2012, 03:10:24 PM »
yeah, the magnetic fields have energy trapped in them,
and try to re-arrange themselves to lower the energy inside the magnetic fields. kind of.

but they're not really doing work on the *charges* as much as they are doing work rearranging the charges. blegh. am i explaining this right?

but right, you brought up  f=q vXB . the staple is electrically neutral. so all of the +ve nuclei will feel equal and opposite forces from all the electrons. so that's not really the right equation to describe why the staple gets sucked up. 
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Magnets!
« Reply #6 on: July 03, 2012, 03:14:00 PM »
I'd like to reassure you that i'm racking my brain to remember which of maxwell's equations causes this... I think it might be faraday's law.

there's a story that goes along with it... I... just... can't remember how the story goes.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Magnets!
« Reply #7 on: July 03, 2012, 03:15:09 PM »
well definitely, faraday's law shows how you can relate magnetic flux with doing work on magnetic fields. because you need to increase EMF to increase the flux through an inductor.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline G

  • talky crow
  • ****
  • Posts: 33
  • Karma: +0/-0
    • View Profile
Re: Magnets!
« Reply #8 on: July 03, 2012, 03:24:51 PM »
I appreciate it.  I've found that this is indeed a tough question.  Am I wrong in thinking that ultimately there needs to be a net force pointing upward?  It's been a while since I thought of flux so I'm not surprised that might have something to do with it.  But still, is it incorrect to think that there has to be a force being exerted over a distance on that staple?  If so, what's that force?

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Magnets!
« Reply #9 on: July 03, 2012, 03:32:19 PM »
there is indeed a force.
it's the force that comes from the magnetic field being all smushed up.
as the magnetic field straightens itself out, it does work on the objects involved.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline Weinersmith

  • Administrator
  • Little Puffin
  • *****
  • Posts: 2
  • Karma: +0/-0
    • View Profile
    • Weekly Weinersmith
Re: Magnets!
« Reply #10 on: July 03, 2012, 06:32:53 PM »
FYI-The next Weekly Weinersmith episode is going to be on nanomagnetism. It probably won't answer your question, but it'll be about magnets!

Offline G

  • talky crow
  • ****
  • Posts: 33
  • Karma: +0/-0
    • View Profile
Re: Magnets!
« Reply #11 on: July 03, 2012, 09:08:03 PM »
there is indeed a force.
it's the force that comes from the magnetic field being all smushed up.
as the magnetic field straightens itself out, it does work on the objects involved.

Ben:  Interesting.  So, there's more than one type of magnetic force?  The q(v x B) one can't do work, but this other one can?

Kelly:  Awesome, can't wait.

Offline stephako

  • emo emu
  • ******
  • Posts: 92
  • Karma: +0/-0
    • View Profile
Re: Magnets!
« Reply #12 on: July 04, 2012, 03:23:33 AM »
@G: I guess there are several laws describing the force magnets can produce. The Lorentz-Force contains only the action of an external magnetic field to a charged particle without any magnetic moment.

I would say a soon as you have different magnets interacting you get additional forces (though I can't derive the formulas for them at the moment). I think one of them should be sth. along the lines of m.B, where m is the magnetic moment of the object in the magnetic field.

Edit: As I think about it, the staple you used in the initial though experiment was neutral wasn't it? So the q*(v x B) can't be the responsible force for the movement in any case
« Last Edit: July 04, 2012, 04:09:13 AM by stephako »
Null results, open questions and a bit of my writing: JUnQ

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Magnets!
« Reply #13 on: July 04, 2012, 04:04:57 AM »
Interesting.  So, there's more than one type of magnetic force?  The q(v x B) one can't do work, but this other one can?


no... ...
stupid brain why can't you remember how the answer to this question goes?
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline G

  • talky crow
  • ****
  • Posts: 33
  • Karma: +0/-0
    • View Profile
Re: Magnets!
« Reply #14 on: July 04, 2012, 06:28:10 PM »
Is there perhaps a difference when looking at things macroscopically as opposed to at the individual particle level?  Looks like the magnetic component of the Lorentz Force is incapable of doing work on an individual particle, but that Wikipedia page took me over to a page about railguns and it does indeed seem like down the length of the railgun, the magnetic force is pointing in the same direction of the projectile's motion (but perhaps not the individual charges within the projectile?).

No worries Ben on not having an answer immediately.  I'm perfectly comfortable telling my students "I don't know" when I've reached the bounds of my knowledge.  I've been looking for a precise answer for several years now--so no big deal on having to wait a little longer.  I trust you and your Titanium Physicists will eventually figure something out.  Can your metaphor involve Elephants somehow?  That would be fantastic.