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The classical theory of electric and magnetic fields, both in the static and dynamic case. Also covers general questions about magnets, electric attraction/repulsion etc. Distinct from electrical-engineering.

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According to one of Maxwell's equations, $\vec \nabla \cdot \vec B=0$. As others have suggested, in an $(n+1)$-dimensional spacetime, the electric field is a [polar] vector with dimensionality $n$, w …
answered Nov 21 '17 by robphy
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Consider the worldlines of the electrons (in motion, say with $v=0.25c$ for convenience) and the protons (at rest) in Alice's frame where the wire is neutral. Let's draw this on a spacetime diagram on …
answered Apr 13 '17 by robphy
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If there is a changing magnetic field, then there are electric field lines that are loops, in the sense that they don't begin or terminate at electric charges. So, one can have electric fields without …
answered Apr 28 '18 by robphy
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Elaborating on the @Qmechanic comment... at each event P on the particle worldline one can do the following construction. Draw a future-timelike hyperboloid [which is asymptotic to the light-cone of …
answered Jul 28 '18 by robphy
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Quoting myself from an old PhysicsForums post If I am not mistaken, (someone check my math) you can calculate the principal invariants of a tensor: $$F^a{}_a,\quad F^a{}_{[a} F^b{}_{b]},\quad …
answered Oct 1 '18 by robphy
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If a massive "point" particle has a 4-momentum of the form, $p^\mu=m^\mu{}_\nu v^\nu$, then that particle has preferred directions suggested by the eigenvectors of $m^\mu{}_\nu$. So, this could be rul …
answered Mar 30 by robphy
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The magnetic field is not a [polar] vector, but a pseudovector. In fact, the cross-product of a vector (e.g. Velocity) and a pseudovector (Magnetic Field) is a [polar] vector (e.g. Force). In a more …
answered Oct 16 '17 by robphy