Electromagnetic Field

  • Thread starter DB
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  • #26
krab
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DB said:
Why is it that as a particle propagates through an electromagnetic field, the electric and magnetic forces are perpendicular to eachother?
Electric and magnetic forces need not be perpendicular at all. For example, magnetic and electric fields can be perpendicular to each other in such a way that the electric and magnetic forces are parallel. One can make them in opposite directions so that they cancel. Such a device is called a spin rotator, or Wien filter. It is in common use; I've designed some myself.
 
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  • #27
siddharth
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pervect said:
If you place a magnet near a charged capacitor, the arrangement of charge on the capacitor will not be disturbed. The Lorentz force law shows that a magnetic field will affect only moving charges. The charges on the capacitor plates will be nonmoving when the capacitor is in equilibrium.

Now, if i place the magnet near the capicitor and then move with a constant velocity v, then the charges on the capicator will have a velocity relative to me. Therefore they should experience a force and are expected to accelerate and rearrange.
But there will also be Electric field in the new frame. My question is whether the existing electric field due to the charges in the capactior will change so as to produce no net force, or whether there will be an electric field due to the magnet also in the new frame.
 
  • #28
pervect
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siddharth said:
Now, if i place the magnet near the capicitor and then move with a constant velocity v, then the charges on the capicator will have a velocity relative to me. Therefore they should experience a force and are expected to accelerate and rearrange.
But there will also be Electric field in the new frame. My question is whether the existing electric field due to the charges in the capactior will change so as to produce no net force, or whether there will be an electric field due to the magnet also in the new frame.

I'm not quite sure who is moving in your example.

Assuming we have a magnet moving relative to a capacitor:

In the magnet frame, the charges experience the Lorentz force because they are moving through a magnetic field, and there is no electric field.

In the capacitor frame, the charges experience an electrostatic force due to the fact that a moving magnetic field transforms to an electric field.

However, because the charges are stationary in the capacitor frame, the Lorentz force due to the magnetic field is zero - force = v cross B, and v=0, so the force due to the magnetic field is zero.

So both observers agree that there is a force, one however attributes the force as being due to a magnetic field, and the other oberver interprets the force as being due to an electric field.
 

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