Physics Asked by Nitin Nizhawan on June 17, 2021
I read that force due to electric field on some particle in one reference frame can exhibit itself as force due to magnetic field in some other reference frame and that electric and magnetic fields are two aspects of same underlying electromagnetic field.
My question is what is the mechanism which can explain how an electric field becomes/creates magnetic field in some other reference frame. Is there any such explanation available in relativity theory? I am not looking for mathematics but a physical explanation.
Wikipedia article http://en.wikipedia.org/wiki/Relativistic_electromagnetism
explains something about origin of magnetic forces in a wire as a consequence of lorentz contraction and motion of electrons in the wire
Calculation of the magnitude of the force exerted by a current-carrying wire on a moving charge is equivalent to calculating the magnetic field produced by the wire. Consider again the situation shown in figures. The latter figure, showing the situation in the reference frame of the test charge, is reproduced in the figure. The positive charges in the wire, each with charge q, are at rest in this frame, while the negative charges, each with charge −q, are moving to the left with speed v. The average distance between the negative charges in this frame is length-contracted to:
where is the distance between them in the lab frame. Similarly, the distance between the positive charges is not length-contracted:
Both of these effects give the wire a net negative charge in the test charge frame, so that it exerts an attractive force on the test charge.
But this still does not explain origin of magnetic field in case when there are no positive charges.
There isn't a mechanism. You're trying to find a mechanism for how two abstract objects can exchange identities. Any mechanism involving abstractions must consist of abstractions. So,the only way to explain it is through mathematics.
I feel that the least abstract way to explain it is to look at two stationary charges. They attract via the Coulomb (electrostatic) force. Now run perpenducular to the line joining their centers. Each charge creates a magnetic field as it is moving (moving charge can be thought of in certain cases as current). The magnetic field acts upon the other charge, creating a force. Meanwhile, the electric force has decreased (no longer electtoSTATIC). The net force is the same, but part of it is magnetically caused.
Another way to look at it is to remember that EM fields are set up by EM radiation. An EM wave carries oscillating EM fields with it; see pictures here. A point charge radiates EM waves in all directions. The oscillating E field of one of these waves interferes with the E field from a nearby wave constructively, creating a nearly non-oscillating field, which decreases as distance squared (Comes from the fact that intensity of a point source $propto 1/r^2$), giving us Coulomb's law. The oscillating magnetic fields destructively interfere, so we get no net magnetic field.
Now, if you start moving with respect to the charge, things get interesting. The relativistic doppler effect will act upon the EM waves, altering them (since the speed of light is the same in all frames, we can't apply relative velocities to it). The interferences won't work quite the same, and we'll get a bit of a magnetic field and mainly an electric field. Move faster, and the magnetic field intensity increases, E decreases. Accelerate, and you get complicated stuff. Note that infact em waves are radiated only by an accelerated charge. A sitting charge does not emit em waves. The waves emitted by an accelerated charge produce change in the fields. The easiest way to visualize this is by assuming that the em waves are radiated in all cases.
I think that explains it without too many abstractions..
Correct answer by Manishearth on June 17, 2021
The simplest explanation I know of requires only one test charge and two reference frames with a relative velocity between them.
Frame 1: The charge is at rest. It is the source of a (purely) electric field.
Frame 2: The charge is moving. It is a current, and the source of a magnetic field.
Answered by tmac on June 17, 2021
Fundamentally they are both the interaction between charges. Magnetic fields are the results of moving charges. When charges move, its time slows down (relativity), therefore, the effect (field) of the charge is weakened. Eg. electrons are moving in a wire relative to a stationary proton, the field of the electron is weakened to the proton, therefore, the wire will be positive to the proton. A repulsion force therefore occurs.
Answered by Leo on June 17, 2021
Have a look at this link - it will probably be better than any of our explanation.
Answered by Adriaan Berger on June 17, 2021
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