# 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.

123 views

### Resource for mass-charge interaction

My GR professor mentioned that trajectories of charged particles in GR are not the same as those of non-charged particles (i.e. charged particles don't move on geodesics). As I understood it, the ...
4k views

### Physical difference between Lorenz and Coulomb gauge-fixing conditions

Lorenz and Coulomb gauge-fixing conditions. What is physical difference between these two gauge-fixing conditions? Mathematical expression are clear but how to we choose one of these means what they ...
1k views

### Why does a magnetic field go anticlockwise of the direction of current?

Why never clockwise? How does it 'know' to go anticlockwise?
50 views

### Why doesn't light vibrate in-situ?

Light always moves in a straight geodesic path (shortest distance between 2 points in flat space where gravity is homogeneous) across 3 dimensions of space and 1 dimension of time. It is consists of a ...
147 views

### Electric Motors: why do they draw more current when stalled, and less when moving? [duplicate]

I'm familiar with how an electric motor works, and also familiar with what stall current and free current refers to in an electric motor. I'm also somehow familiar with the notions of energy being ...
197 views

### Electric field energy of two parallel moving charges at relativity speeds

Assume two particles of the same charge $q$ moving in parallel trajectories on the x,y plane. The distance between them is $d$ and it's defined in the y axon. The problem is to calculate the energy of ...
260 views

### Atomic explanation of magnetic field [duplicate]

We all know how an electric current causes magnetic fields. But when a single electron is flowing through the wire, how is this electron creating the magnetic field or how are electrons responsible ...
121 views

### Reducing eddy currents, changes the property of the conductor & circuit?

In an attempt of reducing the eddy currents in a conductor, such as having gaps to break the induced eddy currents, would that by any chance change the conductor's orientation from series to parallel? ...
166 views

I'm running out of places to look (lots of Googling, SE, [articles and books are too specific and never give a good overview]), and yet I am still unsure about how exactly radiation can degrade ...
78 views

### What is the recent most important discovery and study about magnets? [closed]

I believe that magnetism is a very powerful tool to do various things.It can make amazing machines.What is the recent most important piece of study that has been done of magnetism. The study that can ...
165 views

### Does the skin effect, eddy current / hysteresis losses contribute to Johnson noise in an inductor?

Based on my very basic understanding of the Johnson noise, it's not just a DC phenomena, but should change with frequency in a system, where there is a frequency dependent, real component to the ...
56 views

### Lack of symmetry in Poynting vector

I've noticed that there's a lack of symmetry in the definition of the Poynting vector, being defined as: $$S=E\times H$$ meaning, using the electric field cross the magnetizing field instead of the ...
636 views

### The magnetic field of a magnetic monopole

Let us define the magnetic field $$\vec{B} = g\frac{\vec{r}}{r^3}$$ for some constant $g$. How can we show that the divergence of this field correspond to the charge distribution of a single magnetic ...
322 views

### Force between two point charges moving parallel to each other

When we observe two point charges moving parallel to each other we can see two forces acting on each of the charges: the Coulomb force the magnetic force ($\mathbf{F}=q\mathbf{v}×\mathbf{B}$) (...
154 views

### Derivation of magnetic force for a charge in motion relative to a wire (M. Purcell's Electricity and Magnetism)

I have a question regarding the derivation of magnetic force in M. Purcell and David Morin's Electricity and Magnetism. In the first part, they derived the force first by assuming that a charge q is ...
360 views

Ok, so we all know that the strength of a magnetic field can be increased if a wire with current flowing through it is surrounding the magnet. Because the current flowing through the wire is making a ...
60 views

### Antenna direction

I have a router with a wifi antenna that can be turned in any angle. I wonder what difference does the direction of the antenna make to the electromagnetic signals propagation? Where is the signal ...
38 views

### Border conditions on the separation surface (Electromagnetism & Optics)

My teacher taught me that we can consider the following equation: $$E_{1t}=E_{2t}$$ to the descontinuity of the electric field tangent component on the separation surface of two means with ...
48 views

Can you please explain it to me. As far as I know, we have a monochromatic and coherent em source and we getting an interference picture (because we clearly couldn't make the splitted beams travel ...
64 views

### Regarding Ampere's Circuital Law

If I am to show that Ampere's Circuital law holds true for any arbitrary closed loop in a plane normal to the straight wire, with its validity already established for the closed loop being a circle of ...
29 views

### Relation of resistance between two metals and capacity

My schoolbook goves the resistance between two metals in a material with conductivity $\sigma$ as $R=\frac{\epsilon_0}{\sigma C}$, but they don't show how they got this result. Can someone show me how ...
30 views

### Motion decomposition (Relativistic case)

When an electron moves at very close at $c$ (speed of light), is it physical to decompose the motion in two other directions (like what we do in classical case). If so, the motion in each direction ...
58 views

### Electric field from time varying charge density

Inside a cylinder of infinite length in $z$ axis, there is charge density $ρ = cos(βz -ωt)$. I want to find the electric field and as far as i can understand we will get a radial component of $E$. ...
246 views

### Electric potential between two infinite plates

Below is a solved exercise from Griffiths' Electrodynamics. I don't understand why it's directly assumed that the configuration is independent of $z$. Shouldn't there be a contribution from it since ...
106 views

### Why is the introduction of a quantization volume necessary for quantization of the EM field

I have been working through the quantization of the electromagnetic field, and every source I find introduces a quantization volume with periodic boundary conditions in the process, in which we fit ...
57 views