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Why moving charges create magnetic field?

  1. Feb 23, 2016 #1
    (Sorry my poor English) A changing electric field creates a magnetic field and vice-versa. I thought one field were created only when the another field were disturbed and it works finely for electromagnetic waves, and it agrees with Maxwell's equations. I was happy with my conclusion then I found a problem: a moving charge actually creates a magnetic field EVEN with constant speed, it breakes my conclusion apart, because in this case the electric field is not being disturbed and yet we have a magnetic field. How could it be?
     
  2. jcsd
  3. Feb 23, 2016 #2
    A time-varying electric field creates a magnetic field which is time-varying itself.

    What you are saying is valid only for fields that vary with time. But not all the electric and the magnetic fields vary with time. When they are fixed in time, they are called static. For static fields, we can no more talk about "disturbance". Static electric and magnetic fields exist by their own. They are two different entities (mathematically represented by a vector field). The electro-static field can exist regardless of the static magnetic field and vice versa.

    A moving charge with constant speed creates a static magnetic field.
    Maybe other users can add something more interesting about the nature of the electro-static field and the magneto-static field: what exactly are they? To convince that they are two different entities, as a fist suggestion, I could encourage you to observe the different effects that the two types of fields provoke. The electro-static field acts on electric charges, by moving there. The magneto-static field only acts on moving electric charges (even when the field is static, so even when it does not vary with time). If an electric charge is fixed in space (it does not move), even when a static magnetic field is established, the charge will not be subjected to it.
     
  4. Feb 23, 2016 #3

    ZapperZ

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    Actually, this is not correct.

    From Ampere's law, the curl of B is proportional to the time rate of change of E (and current density if there's one). But this curl of B need not have a time varying solution as well. It can easily be a magnetostatic field.

    Zz.
     
  5. Feb 23, 2016 #4
    The origin of electric and magnetic field are charges and currents as can be seen from Jefimenko's equations. On the other hand, besides "because it works", it is hard to answer "why" equations in physics.
     
  6. Feb 23, 2016 #5

    Orodruin

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    No it doesn't. You cannot have a static solution unless the source is static.
     
  7. Feb 23, 2016 #6
    Oh okay. Let me try a different approach. How it's possible that an observer at the same speed of two charged particles doesn't observe a magnetic interaction between them, and an observer in another frame actually observes it?
     
  8. Feb 23, 2016 #7

    ZapperZ

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    Because in one frame, the observer detects a time-varying electric field, while in the other, it is an electrostatic field.

    Zz.
     
  9. Feb 23, 2016 #8
    But how it's possible that at the same time the same two particles are, say, running away each other as seen in one frame, and doesn't do that from the another frame?
    sorry my english.
     
  10. Feb 23, 2016 #9
    The magnetic field is essentially a correction to the electric field due relativity. We assume no relativity, get the electric field, and use the magnetic field to correct for the difference. It works amazingly well.
     
  11. Feb 23, 2016 #10

    Right. The correction for relativity is different in the two cases. So the results for electric and magnetic field are also different.
     
  12. Feb 23, 2016 #11

    Orodruin

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    Put in a different way: In relativity the electric and magnetic fields are really two sides of the same coin. The question is similar to asking why (in two dimensions) you will observe a force in the y-direction after a rotation when the before the rotation the force was purely in the x-direction.
     
  13. Feb 23, 2016 #12

    jtbell

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    Can you give an example where this actually happens?
     
  14. Feb 23, 2016 #13
    Well, let's say there are an observer in a laboratory, and another observer in a street outside. Now, we accelerate the laboratory (LOL) from t0 to t1. From the point of view of the observer in the lab, there was no moviment of charges and at t1 they are in the same place as before. But from point of view of the observer outside the lab, the charges are now distant each other. If this observer move to laboratory, he would disagree with the other observer about what they seen.
     
  15. Feb 23, 2016 #14

    jtbell

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    Why?
     
  16. Feb 23, 2016 #15
    Because when the lab is moving relative to the street, a magnetic force acts on the charges.
     
  17. Feb 23, 2016 #16

    jtbell

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    But an electric force also acts on the charges. The net electromagnetic force is always either attraction for "unlike" charges, or repulsion for "like" charges, regardless of the relative velocity of the observer.
     
  18. Feb 23, 2016 #17
    both observers will "see" the electric force acting on the charges. The thing is that for the observer at rest relative to the charges, there would be a electric force AND a magnetic force, whose action would be put the charges away each other. The another observer who travels at the same speed as the charges, would "see" only the effect of the electric force, therefore he would "see" the charges more closely. Could you understand me?
     
  19. Feb 23, 2016 #18

    Orodruin

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    No, you are wrong. The electric field of a moving charge is not the same as that of a stationary one!
     
  20. Feb 23, 2016 #19

    jtbell

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    Yes, the "stationary" observer "sees" only an electric force, and the "moving" observer "sees" both electric and magnetic forces. The electric forces "seen" by the two observers are not the same, just as the magnetic forces "seen" by the two observers are not the same. Indeed, even the net force is different as "seen" by the two observers! Force transforms under a Lorentz transformation as part of the four-force.

    Nevertheless, it turns out that either all observers "see" a net attractive force, or they all "see" a net repulsive force.
     
  21. Feb 23, 2016 #20
    but if the electric field varies, then the magnetic field would also vary, right? Then how could the magnetic force be constant?
    But why the displacement of the charges would be the same as measured for each observer? When solving problems, we should add the magnetic and electric forces, like q.E + q.V.B. Each charge would have a greater acceleration [q(E+VB)/m] as measured by the "moving" observer than a acceleration q.E/m as measured by the" stationary" observer.
    I remember when solving electrodynamics problems that a charge immersed on a electric field experiences the same F = q.E force as they experiences when at rest. Is there a equation for calculating the electric field of a moving charge? Why should we not consider the change in the electric field in that cases?

     
    Last edited: Feb 23, 2016
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