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Work done by Magnetic Forces

  1. May 16, 2007 #1
    Since the magnetic force on a moving charged particle is qv X B, the work done by a static magnetic field is always zero.
    But consider two parallel currents, both going from left to right. The second current will experience a magnetic force of attraction towards the first current. Therefore, it will move towards the first wire and work will be done on it by the magnetic field. But didn't we just say that it's impossible for a magnetic field to do work?

    Any help would be greatly appreciated.
    Thank You in Advance.
     
  2. jcsd
  3. May 16, 2007 #2

    Meir Achuz

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    But you didn't say that. You said no work on a moving CHARGED PARTICLE.
    That statement does not apply to currents.
     
  4. May 16, 2007 #3

    Doc Al

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    The electric field does the work

    The magnetic field itself does no work on the moving charges, it merely deflects them. This deflection induces an electric field in the wires, which acts on the wire's positively charged lattice, and does the work in moving the wires together.
     
  5. May 16, 2007 #4
    The reason the first wire is attracted to the second wire is due to the moving charges inside the wire. When a current flows in the second wire, it creates a magnetic field that goes through the first wire perpenticular to the axis of the first wire, so a charge moving in the first wire with a certain velocity v and charge q will experience a force towards the second wire (if the currents are in the same direction) or away (if the currents are in opposite directions) from the second wire by qv x B. The same goes for the second wire, when a current flows in the first, a resultant force is created on the moving charges in the second wire, so essentially the two wires move together or apart depending on the current direction.
     
  6. May 16, 2007 #5

    Doc Al

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    The question was not why there's a force between the two wires, but what is actually doing the work--since a magnetic field does no work on moving charges. To answer that requires looking below the surface at the induced electric field.
     
  7. May 16, 2007 #6
    Yes it does. The force on a current I in a magnetic field B is given by Il x B. The same argument can be applied.
     
  8. May 16, 2007 #7
    It is my understanding that a magnetic field can only induce an electric field in a loop if there is a changing magnetic flux through the loop. What loop are you considering, and how is the magnetic flux changing through the loop?
     
  9. May 16, 2007 #8

    Doc Al

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    No loop, no changing magnetic field. (Perhaps my use of the term "induced" was a bit sloppy.) There are charge carriers--electrons--moving in the wire. The magnetic field deflects them to one side of the wire, creating a charge separation and thus an electric field.
     
  10. May 17, 2007 #9

    Hootenanny

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  11. May 17, 2007 #10
    Here's my 2 cents:

    The magnetic field of wire 1 deflects the current in wire 2 in the direction of wire 1, simply by the Lorentz force IxB. So now the velcoity of the electrons in the wire is no longer parallel to the orginal direction of the wire, but it acquires a component in the direction of wire 1! The Lorentz force thus acquires now both a component in the direction of wire 1, responsible for the attraction between the two wires, and a component in the direction opposing the current in wire 2.

    Notice two points:

    1) To sustain this situation and to get the two wires moving towards each other the Lorentz force in the direction opposing the current in wire 2 must be overcome. It is this force that pushes the electrons forward, overcoming the opposing Lorentz force, that eventually does the work in moving the two wires together. Notice that this is just the electric force driving the current, generated in a battery for example.

    2)In the entire process the Lorentz force is prependicular to the current, and thus does no work! Griffiths has a nice intuitive analogy: when you push a block up an inclined plane by exerting a horziontal force, it is you who does the work. The normal force no plays the role of the magnetic field in this analogy. It is always perpendicular to the ramp and the motion, so does no work. It has however a component in the horizontal direction that you have to overcome to move the block. It has also a vertical component that is responsible for the vertical movement of the block. But it is actually your horizontal force, merely deflected by the normal force in the vertical direction. The role of the normal force (and the magnetic force in your question) is one of redirecting a horizontal force in the vertical direction.
     
  12. May 17, 2007 #11

    Hootenanny

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    Careful da_willem, the Lorentz force includes the electric force, which definitely does do work in this case. However, under no circumstances does the magnetic force do work; by definition the magnetic force is always perpendicular to motion and hence does no work.
     
  13. May 17, 2007 #12
    Sorry, I always, probably faulty, call only the magnetic part the 'Lorentz force'.
     
  14. May 17, 2007 #13

    Hootenanny

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    No problem, I just wanted to avoid confusion for the OP :smile:
     
  15. May 17, 2007 #14
    I understand your explanation. But consider two positive charges q moving parallel to each other, each with speed v. Due to the magnetic field of the first charge, the second charge is attracted to it. What is the source of the work done on each of the charges, if it is not the magnetic field?
     
  16. May 17, 2007 #15

    Hootenanny

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    Indeed there will be a magnetic force, but there will also be an electric force. Are the charges going to move towards one another or away from one another? Consider the relative strengths of the electric and magnetic fields; http://academic.mu.edu/phys/matthysd/web004/l0220.htm. Can the magnetic force ever be greater than (or even equal to) the electric force? And recall the defintion of work...:wink:
     
    Last edited: May 17, 2007
  17. May 17, 2007 #16
    Parallel current attracts/antiparallel current repels....

    Are you people sure that only Electric field in Lorentz force law expression takes the responsibility?

    I am thinking of the source driving the current.
     
  18. May 17, 2007 #17
    This has been an interesting thread. I found it intuitively difficult to reconsile my experience of interaction between permanent magnets with the assertion that the magnetic field does no work. One question:

    Does the magnetic field in the Stern-Gerlach experiment do work on an electron?
     
  19. May 17, 2007 #18
    Which is an electric field...
     
  20. May 17, 2007 #19
    Hmm, that's a very interesting question. Classically the energy of a magnetic dipole in an magnetic field is [tex]-\vec{m} \cdot \vec{B}[/tex] so in an inhomogeneous magnetic field the force is given by

    [tex] \vec{F}=\nabla (\vec{m} \cdot \vec{B})[/tex]

    The magnetic moment will move and it seems as though it is the magnetic field is responsible. But actually what I think will happen is that the current that produces the magnetic moment will decrease, or if it is sustained the force responsible does the work.

    But now arises a problem, in case of a fundamental magnetic moment, spin, what happens? A particle carrying spin also has an energy -mB in a magnetic field. What force is now responsible for the work?

    I think a that here we should change our language and no longer speak of a force, as there is no such thing in quantum mechanics. But still, what is responsible for the energy of a particle with spin in a magnetic field? I once saw a quantum mechanical derivation of this energy formula, I will look into it some further, it is indeed a very interesting subject...
     
  21. May 17, 2007 #20
    I am being more and more convinced about it.However,I am saying for a current carrying wire.You cannot blame me questioning about what is going to happen if we consider only two point charges...They constitute non-steady current ---nonsteady magnetic field and the force is a bizzare one...

    Come to the problem.It's a fact that magnetic force attracts a wire towards another.Consider a moment when one wire is approaching another (horizontally) with speed u (w.r.t. my inertial frame).Let the current in this wire flow as usual with speed w (vertically).Now,EACH charge is moving as v= sqrt[w^2 +u^2] in my inertial frame.Magnetic force is not along horizontal.It tilts down.

    The horizontal component is responsible for vertical motion of charges and only that...It can never influence any motion in the horizontal direction.Similarly,the downward force component is sustained by the horizontal movement of the wire.You also cannot say that this acts on the vertical motion of charges along the wire...Magnetic force never influence charges moving along its direction...

    Then,who does the work?If you stop and think about it,you can see easily unless the battery would do work on the charges moving up the wire,the "attraction" what we see would not be there.Since the battery is out of the scene,it takes a time to conceive this.But note that if battery does this,you do not have to blame any unjust thing on any force (What about the electric force?That leads to repulsion!!!).All we need is to have a means to redirect the verical force of the battery into horizontal motion of the wire.The role of magnetic force is precisely this-simply to "redirect" the vertical force of the battery to horizontal motion of the wire.And this workdone is F_horizontal*dr.Since there is no apparently visible horizontal force apart from magnetic one,one may first think (wrongly) that magnetic force is doing the work...

    How can I justify the concept of "redirecting"?I found it said by Griffiths in a similar example shown by him.He says that magnetic force redirects the efffort and leads to the physical reality.
     
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