Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thought experiment on magnetic field propagation

  1. Jan 10, 2015 #1
    Some 30 years ago I learnt in the elementary school, that if I move a permanent magnet in a coil which ends are somehow electrically closed, than I need to apply force on the magnet, because the generated current creates a magnetic field which is against the magnet's field. However if the propagation of magnetic field is the same as the speed of light, than this counter effect is not instant.
    What happens if the generated magnetic field arrives back to the magnet when the movement is the opposite? It still has to push when I pull the magnet, but I cannot find the reason why it happens if the change in the magnetic field is not instant. Could someone explain why this PM cannot exist?
     
  2. jcsd
  3. Jan 10, 2015 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Your accelerations of the magnet would be so fast, you would basically create an antenna and emit a lot of electromagnetic radiation.
    The counter-effect can exist, but it would be weaker than the amount of energy you need to keep the magnet oscillating. The coil can just reduce your emission of radiation a bit (so the amount of energy you have to put in goes down a bit, but stays positive), if the frequency and orientation is right.
     
  4. Jan 10, 2015 #3
    Thanks for your answer. In a thought experiment one can create a coil with a huge diameter like a couple thousand miles or kilometers. In this case the frequency can be low. What's the explanation of the magnet not moving back and forth of its own after pushing it for the first time?
     
  5. Jan 10, 2015 #4

    Dale

    Staff: Mentor

    Changing the size of the apparatus doesn't change the answer. It just changes the wavelengths of the emitted radiation that would "fit" inside the loop, and lowers the frequency of any resonance with the magnet.
     
  6. Jan 11, 2015 #5
    Yes I know that, but I still don't understand the situation. I thought that working with a "human delay" can help to enlighten the problem better, or I will get an answer that I understand. I know mostly what the first answer is trying to say, but something is still missing for me.
     
  7. Jan 11, 2015 #6

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    You need power to have the magnet oscillating. Without coil, all this power is radiated away as electromagnetic waves. With a coil, a part of this power can be "captured" and transferred back to the magnet, so the oscillation needs less external power. You cannot capture 100% of the power and you cannot "send back" 100% of the captured power, so you will always need some energy input to keep the system running. And even if 100% would be possible, you still would not gain anything.
     
  8. Jan 11, 2015 #7
    So depending on the size of the coil and the duration of the movement the generated magnetic field either helps or slows the movement of the magnet?
    The initial movement can be short then the magnet could oscillate by itself while emit some EM radiation. Magnets can be strong, what would prevent the oscillation to take longer than the initial movement?
     
  9. Jan 11, 2015 #8

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    You'll need very special conditions to "help" the magnet, but yes.

    What do you mean by initial movement? Without a continuous external driving power, the oscillation will get smaller and smaller.
     
  10. Jan 11, 2015 #9
    You got what I meant, it seems.
    I need to think more about this.
    Thanks for your answer.
     
  11. Jan 17, 2015 #10
    I don't know where I am wrong but I think it cannot work that way. Here is my reasoning.
    Let's compare the following two cases.
    Case no 1: I move back and forth a piece of metal.
    Case no 2: I move back and forth a piece of magnet with the same weight (and mass of course) inside a coil which has a lightbulb attached.
    With the right frequency in the second case it is easier to move the magnet than in the first case while I have an illuminated lightbulb and some radiation. It would break the law of conservation of energy, isn't it?
     
  12. Jan 17, 2015 #11

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    You need energy to move the magnet back and forth (more than you need for the non-magnetic piece). This energy is used to generate radiation, and to drive your light bulb. If you replace the light bulb and use a superconducting coil, you'll need less power (but you still need power) because you eliminated one power drain, and the now stronger back-reaction will help more to move the magnet.
     
  13. Jan 18, 2015 #12
    In the case when I continuously push the magnet, the "resistance" of the generated magnetic field needs more energy from me which is converted to electric energy. But in the case above we are talking about a magnetic field which helps to move, so what explains the effect you wrote in your first sentence?

    "You need energy to move the magnet back and forth (more than you need for the non-magnetic piece)."
     
  14. Jan 18, 2015 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You are correct to put 'resistance' in inverted commas because there are two different aspects. Resistance, in electrical terms, gives energy loss, like friction. The other effect is called Reactance (it is analogous to the reaction against an applied force which accelerates an object) . Changing the state of a magnet (rotating or switching current on and off) will always involve some energy loss - radiating EM energy AND there will be induced Electric Fields too. If there is another magnet / object nearby, some energy will go towards causing currents and electrical polarisation of that object and it will produce its own fields. When they are close together, there will be an appreciable interaction and the first magnet will 'feel' that effect of the other. (General term is Mutual Impedance) If the other object is a magnetic substance, then the interaction will be stronger. In the limit, of course, you have the Transformer Effect, where two coils are strongly coupled by a common iron core.
     
  15. Jan 18, 2015 #14

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Compare it with a swing: gravity will help you swing back and forth every time, but you still have to do something to keep it swinging at the same amplitude as you have some friction.
     
  16. Jan 18, 2015 #15
    Thanks for joining to the conversation. I meant resistance as we use in everyday language, but it may not be the correct English word for this. Most of the time I am trying to find the right words as used in physics, because I learnt physics in Hungarian. Thanks for your detailed clarification.
     
  17. Jan 18, 2015 #16

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Just look up the difference between Resistance (Resistors) and Reactance (Capacitors and Inductors). (Friction vs Inertia?) Both factors are at work here - even when the nearby object is a perfect conductor or insulator, because there is always a finite amount of energy radiated out into space.
     
  18. Jan 19, 2015 #17
    Yes, in the case of the swing there is friction due to air and the swing itself. But in our case what is the way a force acting upon my hand when I push and pull the magnet if it is not the force of the fields interacting?
     
  19. Jan 19, 2015 #18
    I mean the fields not acting against my hand...
     
  20. Jan 20, 2015 #19

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    It is a force of the field directly from the magnet, acting on the magnet, resisting acceleration.
     
  21. Jan 21, 2015 #20
    So the magnets own magnetic field acting against the magnet. It might explain what happens during acceleration, but does not explain the constant speed motion that may come after it finished acceleration, but before the counter effect occurs. Or during constant speed of the magnet there is no induced voltage?
    (And for the above effect you wrote we don't even need a coil.)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Thought experiment on magnetic field propagation
  1. Thought Experiment (Replies: 9)

  2. Thought Experiment (Replies: 13)

Loading...