- #1
Per Oni
- 261
- 1
Some days ago there was a discussion on pf whether eath’s magnetic field was rotating. That got me to think again about magnetism and relativity and I found this article: http://en.wikibooks.org/wiki/Special_Relativity/Simultaneity,_time_dilation_and_length_contraction
Including this section:
To me there are several points which need clarifying. In the 1st sentence:” ….he sees the wire contracted relative to Bill……” should be: …he sees the +ve ions contracted relative to Bill….. Second and much more interesting why should there be electrical neutrality? Ok I do know that a neutral wire contains the same amount of +ve and –ve charges but I can easily add or subtract some electrons so that there’s no neutrality. Alternatively I can redistribute some electrons so that locally the neutrality is disturbed.
Further more in Jim’s rest frame he sees the +ve charges moving. To obtain “electrical neutrality” does he see the +ve ions spread out?
To conclude: to me, in the way things are explained here there’s no difference between Jim moving with the electrons looking at Bill or Bill moving with +ve ions looking at Jim. Both have to come to the same conclusion regarding magnetic forces.
Including this section:
If Jim is moving relative to the wire at the same velocity as the negative charges he sees the wire contracted relative to Bill:
Bill should see the space between the charges that are moving along the wire to be contracted by the same amount but the requirement for electrical neutrality means that the moving charges will be spread out to match those in the frame of the fixed charges in the wire.
This means that Jim sees the negative charges spread out so that:
To me there are several points which need clarifying. In the 1st sentence:” ….he sees the wire contracted relative to Bill……” should be: …he sees the +ve ions contracted relative to Bill….. Second and much more interesting why should there be electrical neutrality? Ok I do know that a neutral wire contains the same amount of +ve and –ve charges but I can easily add or subtract some electrons so that there’s no neutrality. Alternatively I can redistribute some electrons so that locally the neutrality is disturbed.
Further more in Jim’s rest frame he sees the +ve charges moving. To obtain “electrical neutrality” does he see the +ve ions spread out?
To conclude: to me, in the way things are explained here there’s no difference between Jim moving with the electrons looking at Bill or Bill moving with +ve ions looking at Jim. Both have to come to the same conclusion regarding magnetic forces.