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Magnetic fields do no work? How come

  1. Oct 20, 2009 #1
    According to every textbook I know of, magnetic forces do no work (e.g. David Griffiths Pg. 207). Yet this problem causes this to be hard to believe:

    If I take two magnets, I can set them down on a table (with a little friction). I then slowly push them toward each other, then at some point, the two magnets attract and move toward each other. That is, two magnets appear to exert a force on each other, and this force is exerted over the distance it takes to make contact (even with a little friction). Surly something is doing work to at least counteract the friction, if not cause non-zero mass magnets to accelerate.

    How can you tell me that magnetic forces do no work? These two magnets appear to do work, as far as I can tell.

    This problem has bothered me for a long time. Please explain!

    Thanks.
     
  2. jcsd
  3. Oct 20, 2009 #2

    Nabeshin

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    Haven't read Griffiths at all so I'm not sure exactly what he's talking about, but this statement is usually made with regards to the magnetic force on a charged particle. In this case, it's pretty trivial to show since velocity and force are perpendicular.
     
  4. Oct 20, 2009 #3
    Quote: "Magnetic Forces do no work" in bold, in a box, in the middle of the page.

    But between two magnets, what forces other than magnetic forces exist? What force causes them to move together?

    Thanks
     
  5. Oct 21, 2009 #4

    Pengwuino

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    I believe I remember what Griffiths was talking about. I think he was talking about how an electron or current would gain energy but from where? He should have specifically stated that the energy comes from whatever is maintaining that magnetic field. I know he explained it... wonder where my copy is and if it still hasn't fallen apart.
     
  6. Oct 21, 2009 #5
    A better question: "Can a magnet (e.g. a permanent magnet) do work?"

    If no: how do magnets move together appearing to do work? What's actually doing the work (see first question).

    If yes: how is this possible when "magnetic forces do no work"?
     
  7. Oct 21, 2009 #6
    While you have two stationary magnets, obviously no work is done, one on the other.

    A magnet, in moving, posesses an electric field.
     
  8. Oct 21, 2009 #7
    That's a very good point. But let's say that I bring the magnets to a halt after I feel the force of magnets wanting to move toward each other. At this point, the magnets are not moving, so no electric field exists. Yet when I let go, they will still move toward each other.
     
  9. Oct 21, 2009 #8
    It is late for me, and this is my last post. I think you are questioning the disparity--work is done by the electric fields. But the force developed, when the magnets are stationary, is magnetic. So how is this resolved?

    Wonderful question!

    I'm sorry not to answer at this time.
     
  10. Oct 21, 2009 #9
    Virtually anything capable of doing work involves a carrier of energy. Mostly this involves the electrostatic repulsion between particles (e.g. rocket vs. exhaust, tires vs. road, feet vs. floor, etc.). In other cases you have magnetic fields such as can be found in magnetic levitation trains and linear induction motors in modern roller coaster rides. Carriers of energy do not do the work, they simply deliver the energy.

    Energy ultimately comes from utilizing charge potential, be it nuclear or chemical. Even solar energy is derived this way. There is no other way of adding energy into a system, period. Everything else is simply the hot potato-ing of this energy.
     
    Last edited: Oct 21, 2009
  11. Oct 21, 2009 #10
    An energy transfer is needed to change the separation of two magnets or a magnet and an unmagnetised magnetic material.When attracting magnets are pulled further apart or when repelling magnets are pushed closer together an energy input is needed and there is a change of magnetic field with a resulting increase of potential energy.The stored energy can then do work to push/pull the magnets back in the opposite direction.Like others I think the book was referring to the magnetic force on moving charged particles, in which case the field does no work.
     
  12. Oct 21, 2009 #11
    So is it true that no work is being done on charged particles in the sun's corona? They are not gaining kinetic energy?

    Obviously it can't be the magnetic field. It must be some other force.

    So their trajectory is determined by nuclear fusion? :rofl:

    I don't think so....:uhh:

    Sunspots are not produced by magnetism? Obviously no work was done by the Lorentz force to produce those. They're really a byproduct of a combination of fusion and gravity............. ?

    GIVE ME A BREAK! :mad:
     
    Last edited: Oct 21, 2009
  13. Oct 21, 2009 #12
    Here is an interesting experiment. Use a permanent magnet (PM) and build a solenoid magnet about the same size. Use low resistance wire. Power the solenoid with a constant-current source (you can use two npn transistors and a resistor). Put a voltmeter across the solenoid. Move the PM quickly up to, or away from the solenoid. Do you see a voltage pulse on the voltmeter? Why? What is the approximate volt-seconds? The energy (joules) is the volt-sec times the solenoid current. Where is this energy coming from, or going to? (If the voltage pulse is the same sign as the current, the current regulator is doing work. If the sign is opposite, the current regulator is absorbing energy.)
    Bob S
     
  14. Oct 21, 2009 #13

    Vanadium 50

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    I don't like teaching the meme "magnetic fields do no work." It is true, but it is not useful.

    It's clearly true for a single charged particle: the Lorentz force law has the magnetic force perpendicular to the direction of motion, so the dot product of force and displacement is always zero. It's also clearly true that magnets can do work on each other.

    The solution to this apparent contradiction is that complex objects like magnets are made up of many charges, and these charges exhibit both electric and magnetic forces on each other, and if one does the calculation carefully enough, it can be shown that the work actually comes from these (usually internal) electric forces.

    So what do you gain by thinking about things this way? To my mind, very little: you're trading a relatively simple calculation - say the torque on a magnetic dipole in a magnetic field - for a very complicated one involving internal electric forces. This seems like a poor trade. Note that I am not arguing that "magnetic fields do no work" is not true. I am arguing that it is not useful. It's (relatively recent) overemphasis is not, in my mind, a good thing.

    As far as the solar corona, I am not a solar physicist, but I do want to point out that the sun's magnetic field is far from static, and a changing magnetic field produces an electric field, and electric fields can do work.
     
  15. Oct 22, 2009 #14
    I hope I am summarising the majority of different responses here correctly.
    1.Work can be done on or by magnetic fields.
    2.Work can be done on or by electric fields.
    3.There is no work done by the Lorentz force on charged particles.

    I think that it's point 3. that the text books elliotr is using referred to and it would be instructive to see a relevant quote from the book so that the question can be answered in context.

    kmarinas 86 will you please clarify your post(11) where you seem to imply that as far as stellar events are concerned work is,in fact, done by the Lorentz force.This force is highly instrumental in determining the paths of the particles,it can accelerate the particles by changing their directions but how,for example,can this force,on its own, change their kinetic energies?
     
  16. Oct 22, 2009 #15
    I think the core problem at issue can be illustrated with a superficially different example. Consider two parallel current carrying wires. Each will have an attractive magnetic force on the other, and so the two wires will come closer together. Since the magnetic force is in the same direction as the direction of motion, work is being done by the magnetic force.

    So how is this possible?
     
  17. Oct 22, 2009 #16

    diazona

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    The magnetic force in that case just acts on the electrons that are moving through each wire. It tries to bend the trajectories of the electrons in one wire toward the other wire, and vice versa. No work is involved in that part. But the electrons attract the positively charged atomic nuclei in the wire by the electric force, and that force does the work of moving the wire.

    Whenever it looks like magnetic forces are doing work, if you think about it closely enough it actually turns out to be the electric force.
     
  18. Oct 22, 2009 #17
    If that is true, then the forces in question should not behave differently in relativistic scenarios. But if they do, we should called it the electromagnetic force instead.
     
  19. Oct 22, 2009 #18
    Not! A magnet held over a paper clip lifts said clip by magnetic force, NOT electric force. The clip is increased in potential energy. The magnetic force did the work, not the electric.

    With 2 wires carrying current, the mag force is responsible for the interacting force between the wires, NOT electric.

    Any peer-reviewed text will elaborate.

    Claude
     
  20. Oct 23, 2009 #19
    I don't know what stimulated this excited response, but it wasn't through careful analysis.

    This problem is better analysed by a replacement analogy where we can deal with things we can better understand. Replaced the permanent magnet with a solenoid and the paper clip with an array of wire loops.

    The electric field, not the magnetic field is responsible for the work done.

    Can you see that the electric field of the permanent magnet is responsible for the work done on the paper clip in the inertial frame of the paper clip? A changing magnetic field produces an electric field. This should be quantified in any text that covers dynamic electromagnetism.
     
    Last edited: Oct 23, 2009
  21. Oct 23, 2009 #20
    Where are the mentors?

    1. 1. is wrong.

    3. 3. is wrong. You need to look up the Lorentz force
     
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