Energy transfer in electromagnetic induction

  1. Consider a magnet moving towards a coil.
    We know that the motion of the magnet will induce a current in the coil and the direction of this induced current is to oppose the motion of the magnet.

    Now does the magnet experience resistance to its motion immediately as soon as it starts moving?

    Since the magnet and coil are physically separated, it would take a time t (which is equal to the time taken by light to travel from the magnet to the coil)
    to induce a current in the coil and an equal amount of time for the effect of this current to travel back to the magnet and oppose its motion.
    Hence the total delay appears to be 2t.

    So does the magnet experience resistance immediately or does it have to wait for time 2t?
     
  2. jcsd
  3. Drakkith

    Staff: Mentor

    I'm with you, as it looks to me like 2t. Anyone else know?
     
  4. davenn

    davenn 3,373
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    for what to travel at the speed of light from the magnet ??
    Nothing is travelling from the magnet
    Why ? ... because the magnetic field already exists around the magnet whether its moving or not
    As soon as the wire encounters the field lines, a current will start to flow in the wire
    bringing the magnet even closer just has the effect of more field lines "moving through" the wire and generating a larger current

    The 2 velocities would be
    1) the velocity that YOU move the magnet towards the coil, then
    2) the velocity of the expanding field around the wire ( over a short distance you would be lucky to measure this)

    I would suspect any delay you measured would be more related to the times it takes for the mangetic field around the wire to grow strong enough to have a measurable interaction with the magnets field

    as said above.... The magnetic field around the wire doesnt just suddenly appear, it progressively grows stronger as the magnet comes closer

    Dave
     
  5. Drakkith

    Staff: Mentor

    Dave, imagine we place the magnet there stationary first. That way the field is not changing until we start to move it again. We can imagine a very sudden *bump* that accelerates it. Does this change in the field due to the movement of the magnet move at c?
     
  6. davenn

    davenn 3,373
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    hey Drakkith :)
    seasons greetings to ya buddy .... have you moved ranch yet ?

    .....the field is still only going to move at the speed/velocity at which the magnet is moved
    bearing in mind that we are not going to be able to move it at c anyway
    But I dont see where the OP was asking about moving the magnet at c, just moving it by hand or mechanically towards the coil
    He/she was assuming the magnets field was suddenly appearing and moving out from the magnet at c
    but of course this isnt the case, the field is present all the time

    Dave
     
  7. "The magnetic field around the wire doesnt just suddenly appear, it progressively grows stronger as the magnet comes closer"

    Initially the magnet is at rest. Now there is a certain amount of magnetic flux linking the coil.
    But since the flux is constant there is no current induced.

    Now suppose that we start to move the magnet at instant t=t1. The flux linking the coil will increase since the magnet is moving closer. The question is when does it start to increase?

    Certainly it cannot be t1 since that would be mean faster than light information travel.
    So there should be a delay and will it not be equal to the time taken for the effect of the changing magnetic field to reach the coil.

    As an example, if the sun were to disappear at this instant of time, we would not notice it for the next eight minutes after which we will plunge into darkness.
     
  8. Andrew Mason

    Andrew Mason 6,792
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    Since the field extends outward from the magnet, the question relates to the ability of the physical magnet, as it begins to accelerate, to instantaneously communicate that acceleration to its field lines at the conductor a distance s away: there must be a delay (Δt≥ c/s) before the conductor feels the change in magnetic field and the resultant induced electric field; and since there is that delay, there must be a similar delay in the magnet feeling the opposing magnetic field from the induced current in the conductor. That seems to be the question.

    Feynman studied the interaction of fields of approaching electrons. The effect should be similar. He discovered that it did not matter whether you assumed that the field travelled in advance, was retarded or was instantaneous. The reaction force was always the same. You may wish to google the "Wheeler-Feynman absorber theory"

    AM
     
  9. Hi Drakkith,

    That is fine. But suppose say the magnet starts moving at instant t=t1.
    So the current would be induced in the coil at t=t1+t and the magnet will start experiencing
    resistance only at t=t1+2t.

    But what about the total energy of the system at time t=t1+t. At this time we have a moving magnet which has not experienced any resistance and has not lost any energy and also the current induced in the coil.

    Should not the whole process act as a way of energy transfer between the motion of magnet and the coil? should not the moving magnet lose energy as soon as the current is induced in the coil.
     
  10. What exactly do you mean by the reaction force in this case? Is it the resistance experienced by the magnet and does it feel the reaction instantaneously as it starts to move?
     
    Last edited: Dec 27, 2012
  11. Andrew Mason

    Andrew Mason 6,792
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    Yes. It appears that it does. But it is complicated.

    Feynman's work led to a very successful theory of quantum electrodynamics and earned him the Nobel prize. Here is a bit of an explanation of the reaction force: http://physics.fullerton.edu/~jimw/general/radreact/index.htm

    AM
     
  12. BruceW

    BruceW 3,529
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    The maximum speed at which information can be transmitted is the speed of light. So if I decide to push the magnet towards the coil, it will take an amount of time at least x/c before a current is induced in the coil. (where x is distance from magnet to the coil).

    Now does this mean that there will necessarily be an amount of time at least 2x/c before the magnet feels the force caused by the moving charges in the coil? No way. Remember that in this situation, the 'choice' we made was to give the magnet a nudge. So if we want to consider the region of spacetime which can be effected causally by this action, then at the same position at which the action happened (i.e. at the magnet), it will be causally linked immediately after the action has happened, so there is no necessary delay between the actions of nudging the magnet, and the magnet feeling a reaction force.

    Edit: I should also specify, this explanation is for the least possible time delay. So the time delay could be greater, for example if I shot someone in the foot, that person could decide to shoot me in the foot (as vengeance), and so the time delay could be quite large.
     
  13. But consider the case when the coil is not present. Surely the moving magnet should not experience resistance then.

    According to the Wheeler-Feynman absorber theory radiation resistance is experienced instantaneously because of the advanced waves travelling back from the absorber. This radiation resistance does not depend on the density of absorbers in the vicinity of the emitting particle.

    Radiation resistance being the same in every direction, a radiating particle cannot detect the presence of absorbers instantaneously by measuring this resistance.

    But in the case of electromagnetic induction the moving magnet will feel resistance only if
    the coil is present. The moving magnet will be able to instantaneously detect the presence of any coil nearby.
     
  14. davenn

    davenn 3,373
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    of course....
    a magnetic field needs to be generated in "something" so that it will oppose the field of your magnets field so you/it feels/measures a resistance

    Dave
     
  15. davenn

    davenn 3,373
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    instantly because the field is already in contact with the coil

    D
     
  16. BruceW

    BruceW 3,529
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    eh? Surely, if I nudge the magnet, there is a time delay before a current is induced in the coil.
     
  17. davenn

    davenn 3,373
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    and where would the delay be coming from ??

    the field is already around the coil, the moment the field moves a current is induced

    D
     
  18. Andrew Mason

    Andrew Mason 6,792
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    The time delay would occur because of the distance between the magnet and the conductor. The effects from the sudden acceleration of the magnet cannot instantaneously propagate out to the position of the conductor. Special relativity says it cannot propagate faster than c.

    AM
     
  19. Jano L.

    Jano L. 1,211
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    Davenn, the magnetic field is not supposed to be rigidly connected to the magnet. If we move the magnet suddenly, the magnetic and electric lines will get deformed in the vicinity of the magnet, but the lines in greater distance will not change instantaneously. The deformation of those lines will propagate at the speed of light in all directions from the magnet.
     
  20. davenn

    davenn 3,373
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    uh huh, I didnt say they were fixed, but yeah i can see how it could be taken that way
    I like your answer better :)

    BUT ... do they prop at the speed of light or just at the speed of the motion of the magnet ?
    cant you qualify/clarify that

    thanks Jano
    am always willing to learn ;)

    Dave
     
  21. Hi Andrew,
    So will this not amount to a violation of causality. It appears that the moving magnet has knowledge about the events of the future.

    We can also assume that there is a switch in the coil which will allow us to turn on/off the current flow.
    Now consider the magnet and stationary coil separated by a distance x. Initially the switch is turned off so that no current can be induced in it.

    Now the magnet starts accelerating at instant t=t1. Now it will take a time for the effect of this changing magnetic field to reach the coil.
    It would reach the coil at instant t1+(x/c). But if we turn on the switch before this effect reaches the coil, there should be a current induced in it and according to the Wheeler-Feynman absorber theory the moving magnet should have experienced resistance at t=t1.

    Hence the magnet appears to know at t=t1 whether the switch would be on or off at t= (t1+x/c)
     
    Last edited: Dec 28, 2012
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