Do single moving charges create magnetic fields, why?

  • #1

Main Question or Discussion Point

The reason this is in relativity is ( please correct if wrong )that magnetic fields are effects created by static electricity through length contractions. If a line of electrons move pass a parallel line of positive charges and a charged observer is outside this it would see electrons being contracted meaning the overall charge will become negative due to increased density of electrons. And the observer would be repelled or attracted. Watch (at 1:10) he does a better job of explaining. So without other charges, would a single charge produce magnetic fields when moving. Please ask me to do a better job at expressing my question if you dont understand. Ty
 

Answers and Replies

  • #2
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What he describes there is not a general derivation, but just a specific example to establish the plausibility of the idea that what is a magnetic field in one frame is an electric field in another frame.

A general derivation was done by Einstein in his seminal paper. See section 6 of On the Electrodynamics of Moving Bodies.

The short answer is: yes, a single electron does produce a magnetic field in a frame where it is moving.
 
  • #3
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The reason this is in relativity is ( please correct if wrong )that magnetic fields are effects created by static electricity through length contractions.
They are not. That is one way to motivate the concept but it is not "the cause".
So without other charges, would a single charge produce magnetic fields when moving.
Yes. The electromagnetic field is linear, the field of many electrons is just the sum of the field of each individual electron. If a single electron wouldn't create a magnetic field then many moving electric charges (e.g. in an electric current) wouldn't lead to a magnetic field either.
 
  • #4
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If a line of electrons move pass a parallel line of positive charges and a charged observer is outside this it would see electrons being contracted meaning the overall charge will become negative due to increased density of electrons.

Well now I must point out the fact that a charged observer next to a wire feels the same force from the wire when there is a current in the wire and when there is no current in the wire.

But a magnetic observer would feel a difference in the forces in those two cases.

(When a charged cat runs past a charged observer the observer sees the cat being contracted, meaning that due to decreased distance between the charges on the cat the observer feels the forces from the charges more at the same time ... so the observer feels a larger force for a shorter time)
 
  • #5
PeterDonis
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now I must point out the fact that a charged observer next to a wire feels the same force from the wire when there is a current in the wire and when there is no current in the wire.

But a magnetic observer would feel a difference in the forces in those two cases.
I don't know where you're getting this from. There is no such thing as a "magnetic observer" different from a charged observer; an object with no charge does not feel any magnetic force any more than it feels an electric force.
 
  • #6
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I don't know where you're getting this from. There is no such thing as a "magnetic observer" different from a charged observer; an object with no charge does not feel any magnetic force any more than it feels an electric force.
A magnet with no net charge feels a force when put next to a wire with a current. It feels some torque, and it feels some force too, because the magnetic file around the wire is not uniform.

I admit that "magnetic observer" is kind of strange term. :smile:
 
  • #7
PeterDonis
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A magnet with no net charge feels a force when put next to a wire with a current.
Yes, but if you analyze the details of this force, you will see that it comes from electric and magnetic forces on individual charges inside the magnet. There is no such thing as a magnet with no charges inside it at all. And the thread topic is about "single" charges, not about magnets with lots of charges in them that add up to zero net charge.
 

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