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Name: Muntaseer
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 8/31/2004


Question:
Suppose a spaceship is moving with a constant velocity with respect to an observer. Also imagine that an electron in the spaceship is at rest with respect to the spaceship. therefore an observer in spaceship suppose to detect no magnetic field around the electron since it is at rest with respect to him and according to postulate of special relativity laws of science are same in all inertial frame. But an observer who notices both the spaceship and the electron are moving with the same velocity with respect to him must detect a magnetic field perpendicular to the motion of the electron since moving charge creates magnetic field.

What should be the answer of this paradox? Is magnetic field is a relative phenomenon?


Replies:
To detect the magnetic field, the electron must be moving relative to the instrument that the observer is using to detect it. Let us assume the detector is a coil of wire. On the spaceship, the electron, observer, and detector are moving at the same velocity, the electron never passes through the detector and no magnetic field is observed. For the stationary observer, as the spaceship and electron pass through his detector (a very big one!) he will see the magnetic field.

Bob Hartwell


The observer at rest to the electron sees only an electric field. However when one moves over to a moving frame of reference, some of this electric field gets transferred into magnetic field. There is a math relation between the electric and magnetic field. This is generally discussed in many books on electromagnetic field theory, the sloshing of energy from one to another, by Lorentz transformation.

Perhaps another way to look is to see that the (electric) field line from a stationary charge just go out radially, out from the center. But as the charge begins to move, the field lines going out, but into the direction of travel seem to get dragged back. This gives rise to an magnetic field in this (moving)frame. Electric and Magnetic are two sides of the same thing in special relativity.

Steve Ross



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