# All magnetic effects, simply provided by a contracted electric field?

Is the contracted electric field of constantly moving charge is what we call a magnetic field? Does the charge, that moves in a straight line has a north and south poles? Contracted electric field has density advantages in the direction perpendicular the moving, is that a north and south?

So give a straight answer - Is the contracted electric field of constantly moving charge is what we call a magnetic field, or there happens something more, with moving charge, except electric field contraction?

• Where did you get that picture? The electric field of a uniformly moving charge is contracted, yes, but symmetrically back to front. It doesn't "pile up" in front at all. – knzhou Sep 13 '18 at 18:26
• @knzhou, one of the questions. The guys there actually answered that it is wrong. Or not completely wrong - it should be mirrored, I didn't understand. – user205695 Sep 13 '18 at 18:29

A straight answer is “No, the contracted electric field of constantly moving charge is not what we call a magnetic field.”

A very similar question that often is mistaken as the same question would be “is the electric force on constantly moving charge in its own frame what we call a magnetic force in our frame”. The answer to this one is “yes” as described by Purcell: http://physics.weber.edu/schroeder/mrr/MRRtalk.html

The distinction between force and field is important because force implies a massive particle on which the force is acting. Therefore there is always a unique rest frame for that particle that you can transform to. A field has no such well defined rest frame and some fields (eg a plane wave) have no corresponding sources. Additionally, $E^2-B^2$ is an invariant, so in fields where this quantity is negative there is no frame where the magnetic field is 0 despite the fact that there is a frame where the magnetic force is 0.

• "The distinction between force and field" it is something new for me. Electric field is a function that defines the Electric field strength vector in every point of space. Electric field strength vector shows the intensity of force lines. Is it that difference, that You talked about? – user205695 Sep 13 '18 at 18:25
• Honestly I didn't understand, what are You talking about in the last paragraph, maybe after reading the stuff from the link I will understand.. – user205695 Sep 13 '18 at 18:31
• Perhaps, by saying that there is difference between magnetic field and magnetic force, You meant Lorentz force? – user205695 Sep 13 '18 at 18:33
• The electric field is the force (per unit charge) that would be exerted on a test charge located at that point. The same is not true of the magnetic field because the force (per unit charge) also depends on the velocity of the test charge. – Dale Sep 13 '18 at 18:33
• Does the charge, that moves in a straight line has a north and south poles? – user205695 Sep 13 '18 at 18:36