# Tagged Questions

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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### Magnetism and straight conductors

Through two long, parallel straight conductors gårströmmarna 3.0 A and 2.0 A, as illustrated below. The distance between the wires is 4.0 m. Is there a point where the flux density is zero? Where? ...
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### If $T_{kl}=\epsilon_{kil}m_i$, how to show $m_i=0.5\epsilon_{ilk}T_{lk}$?

In a book I am reading (about magnetic dipole), it is given that $T_{kl}=\epsilon_{kil}m_i$. Then, it says since $T_{kl}=-T_{lk}$, it can be shown that $m_i=0.5\epsilon_{ilk}T_{lk}$. I understand that ...
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### How to derive torque from a velocity dependent potential?

In Jackson's Classical Electrodynamics, the rate of change of the angular momentum of the electron in its rest frame in constant external electric field $\textbf{E}$ and magnetic field $\textbf{B}$ is ...
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### Variation of spin in the rest frame of an electron in an external magnetic field

In Jackson's Classical Electrodynamics, the equation of motion for the angular momentum $\textbf{s}$ of an electron in an external magnetic field is given by (eq. 11.101) ...
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### Calculating the B flux density of a flat spiral coil with N turns

Consider the following topology for a flat spiral coil (with an air gap in the middle). It's been a very long time since I have used Maxwell's Equations and now I find myself trying to figure out the ...
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### Magnetic field between 4 wires with the same current

if I have 4 long wires with a current I and all of them have the same distance from each other (looked at from the front you'd have a square) shouldn't the magnetic field vanish in the middle of all 4 ...
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### Rewriting Maxwell's equation in tensor form

Suppose $F_{ij}=\epsilon_{ijk}B_k$, how to prove the following: $\partial_iB_i=0$ becomes $\partial_iF_{jk}+\partial_jF_{ki}+\partial_kF_{ij}=0$ $B_iB_i$ becomes $F_{ij}F_{ij}/2$ I can see that ...
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### Is it possible that propagation of acustic waves leads to emission of radiation?

Question: Consider large cloud of gas. Assume it is electrically neutral (but as always, matter is composed of smaller things which are actually charged). Is it possible that propagation of sound ...
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### Confusion in Maxwell's derivation of Ampere's Force Law - Part II

I am reading Maxwell's "a treatise on electricity and magnetism, Volume 2, page 156" about "Ampere's Force Law". I have some confusion in the following pages: My question is of two parts: 1. ...
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### How do light waves get their size?

An atom or (small) molecule has the size of about 100pm. Elektromagnetic waves range from about 0.1nm up to 1 km. The most common way waves (like light) are caused by 'jumping' electrons to another ...
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### Can the Biot-Savart law be derived from QED?

It's important that a new theory of physics contains the equations and results of previous related ones. Maxwell theory and QED both have explanations for electromagnetic phenomena so I'm wondering if ...
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### Induced current in a rectangular loop

The question is How much heat is generated in the resistor during the loop's entire path into and out of the magnetic field? Give your answer in J. I used Faraday's law and managed to get the ...
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