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magnetic field and a charge in room |
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| Feb23-12, 09:28 AM | #1 |
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magnetic field and a charge in room
Imagine i am in a room in which there is a strong and homogeneous magnetic field ( strong B field ). and in the middle of the room the is a positively charged particle. the room is in space so there is no gravity to accelerate the particle downward, it just floats. since the particle is not moving the is no force on it and it is stationary. but now i start to move with little speed perpendicular to B lines. the charge experiences a force since it has relative velocity. How can that be? How can it experience a force just because im moving. and if i had a friend there who did not move with me he would say particle doesnt move and i would say it moves. how is that possible after all i am not moving at relativistic speed, only a few centimeters per second. how can we not agree with each other ? ( note: i purposely made magnetic field strong so that even little speed creates enormous force on particle, or imagine particle has huge charge so that again we have enormous force even at very little speed )
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| Feb23-12, 03:42 PM | #2 |
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If you are moving, the B-field will change from your perspective. The B-field is not an invariant, and changes depending on the motion of the observer. You will see a moving charge in a different B-field and E-field (depending on your motion and the direction of the B field) such that the effects cancel and the charge is not accelerated.
See here: http://en.wikipedia.org/wiki/Relativ...ectromagnetism |
| Feb23-12, 03:50 PM | #3 |
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I think that you need to consider that you defined your static reference frame to be one with a uniform magnetic field in one direction, and zero electric field in all directions. However, once you move relative to this static frame, electric fields are not zero anymore. In your moving reference frame, you need to consider the total Lorentz force F=qE+vXB. I assume that if you do the transforms correctly, you will find zero net force on the charge in any inertial frame of reference.
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| Feb23-12, 03:56 PM | #4 |
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magnetic field and a charge in room
But Maxwell's equations must be valid in all frames of reference isn't that right? Why is it that I can use them safely for one reference frame, but for other frame of reference I have to supplement it with Einstein's special relativity ?
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| Feb23-12, 06:59 PM | #5 |
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| Feb23-12, 10:18 PM | #6 |
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http://hepweb.ucsd.edu/ph110b/110b_notes/node69.html |
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| charge, electromagnetism, magnetic field, magnetism, relativity |
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