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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106 views

Klein-Gordon, gauge transformation [closed]

It must be really simple, but I cannot get why can we add an $i e \frac{\partial \Lambda}{\partial x}$ in the second row below. The propagation of a charged scalar particle, along the x-axis and in ...
2
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2answers
2k views

What is the difference between Biot-Savart law and Ampere's law?

What is the difference between these laws? Which law is more useful? When to use Ampere's law and when to use Biot-Savart law?
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2answers
153 views

Is a Perfect/Lossless Mirror possible?

In traditional mirrors, some of the input light is absorbed by atoms in the mirrors surface and are 'lost' as heat, degrading the quality of the reflected image. Could this loss be compensated by an ...
4
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1answer
81 views

Toroid moments tensor decomposition

I am currently working on my bachelor's thesis on the anapole / toroidal moment and it seems that I am stuck with a tensor decomposition problem. I have actually never had a course about tensors, so ...
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3answers
2k views

Is the electric field zero inside an ideal conductor carrying a current?

By an ideal conductor, I mean one with zero resistance. Inside an ideal conductor with no current, the electric field is zero, but is the electric field still zero with the ideal conductor carrying a ...
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6answers
2k views

Why do physicists believe that there exist magnetic monopoles?

One thing I've heard stated many times is that "most" or "many" physicists believe that, despite the fact that they have not been observed, there are such things as magnetic monopoles. However, I've ...
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2answers
111 views

Does electric field cause changing magnetic field also?

According to Faraday's law, changing magnetic field causes eletric field (imprecise wording, but generally accurate). My question is, can Faraday's law be interpreted in opposite way - that is, ...
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3answers
89 views

Faraday's Law - recursive?

So we know that the EMF is induced by change of flux. The thing that was always confusing me is the following: we start changing the magnetic field which in turn induces electric field which makes ...
12
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1answer
446 views

Physical Interpretation of EM Field Lagrangian

Using differential forms and their picture interpretations, I wonder if it's possible to give a nice geometric & physical motivation for the form of the Electromagnetic Lagrangian density? The ...
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2answers
93 views

In calculating resistivity of skin do I use the body's surface area?

Most of the resistance of the human body comes from the skin, as the interior of the body contains aqueous solutions that are good electrical conductors. For dry skin, the resistance between a ...
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2answers
77 views

Gauge symmetry for p-forms

It is well known that the Lorentz invariance of the S-matrix implies Gauge redundancy for 1-forms,'photons'. Does this argument go through to p-forms? That is does lorentz invariance of s-matrix of ...
4
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2answers
161 views

What is happening to the electrons, and E & H fields, in an antenna with a standing wave inside?

Diagrams like the one shown below are often shown to explain antenna theory, but I have always had problem with the concept of voltage being a wave, and because of this the diagrams never make any ...
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2answers
87 views

Distance between two light rays of a beam in a material.

Why is it that the distance between two light rays changes as they pass from one material to another? It must have something to do with the change in refractive index, but to me it seems the ...
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1answer
100 views

Behaviour of free electron with constant velocity

Can a free electron with a constant velocity V то move eternally at this rate in the inertial frame? The question arises from the fact that a moving charge creates near its trajectory varying electric ...
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1answer
97 views

Toroidal magnet in Tonomura et al. demonstration of Aharonov-Bohm effect

What does the magnetic field look like in Tonomura's toroidal magnet coated with a superconductor? See this paper to find results showing conclusive proof of the A-B effect and flux quantization. ...
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1answer
152 views

DC current in ideal conductor and skin effect?

I know that the skin depth derived for AC current goes as $$\delta \propto \sqrt{\frac{1}{\omega \sigma}}$$ where $\omega$ is the angular frequency of the field and $\sigma$ is the conductivty. Now: ...
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1answer
30 views

Energy conservation in diamagnetic levitation

How do we account for conservation of energy in diamagnetic levitation when no external energy is required?
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1answer
128 views

Solving non-linear ODE for divalent solution at a 1-D surface boudary

I am trying to solve the following equation for a positively charged plane with charge density $\sigma$ at $z = 0$. $$ \phi''(z)=-\frac{e}{\epsilon \epsilon_0} \big(z_+n_{+} e^{-\beta z_+ ...
3
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1answer
155 views

Magnetic moment and other invariants of Lorentz System

It's a standard fact that the quantity $\mu = v_{\perp}^{2}/B$ is approximately conserved in the system $d_{t}\mathbf{v} = \gamma (\mathbf{v}\times\mathbf{B})$. The paper Magnetic Moment to Second ...
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0answers
21 views

Earnshaw's Theorem for ferromagnets

Why is Earnshaw's Theorem inapplicable for moving ferromagnets? Can I get a mathematical classical proof for this?
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0answers
50 views

E and B field from Time Varying Current

How would I go about calculating the B field and E field from a time varying current charging a capacitor. Theoretically I feel like a solution should exist, but there seems to be a dependence between ...
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2answers
138 views

Reducing Eddy Currents?

A conductor that passes a magnetic field, decreases its velocity due to the opposing magnetic forces created by the Eddy Currents. This "magnetic breaking" effect can be reduced if we change the ...
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0answers
51 views

Optical chirality and its possible hierarchy of generalizations

Optical chirality refers to a constant of motion of the electromagnetic field, which measures in some sense how chiral a light field is. Specifically, the pseudoscalar quantity $$ ...
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0answers
17 views

Relationship between the force exerted by a coil and the stress

I can't remember what the equation is for the relationship between the force exerted by a coil and the stress in terms of the current density, the radius and the length. I was thinking just standard ...
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2answers
103 views

Is light electromagnetic waves or quantumn particle waves? [duplicate]

Is light electromagnetic waves or quantum physical particle waves. Or are they the same? Note: My question is specifically how electromagnetism plays into the quantum physics and the double slit ...
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2answers
109 views

Electron Electric Field Mass?

I am confused of whether or not the expected electromagnetic field generated by the point-like electric charge of the electron distributed smoothly across space as a probability distribution creates ...
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0answers
45 views

Electric field due to charge in one direction

I am supposed to calculate the electric field due to the charge density $\rho(x) = Ax e^{-\lambda |x|},$ where the density is supposed to be homogenous in $y,z$ direction. The problem is, that the ...
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1answer
60 views

Magnetization force on domains after alignment?

When a domain is aligned to an exterior uniform magnetic field. There are no forces acting on it? Magnetization force = 0, and all other force?
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0answers
65 views

Does the Fringe effect occur in capacitors in the interdigitated form?

I am currently doing my undergrad dissertation on graphene supercapacitors. I have read that the fringe effect is a well established phenomenon on parallel plate capacitors, but does it also occur in ...
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2answers
115 views

Will the way you make a coil affect magnetic induction?

We had an experiment about magnetic induction. We discoved that as the number of coils you make the induced voltage and current increases. But I'm wondering if I made the coil messy or made the ...
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4answers
231 views

Why doesn't the magnetic field of a coil “cancel out”?

In a coil, we can see that the current moves right, then left, then right, then left, and so on as it travels down the coil. According to the right-hand grip rule, isn't the magnetic field going in ...
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1answer
284 views

In which direction does the current flow in a wire moving north to south along the Earth?

I stumbled upon this question and I can't seem to figure it out. I think you have to use the right hand rule, but it doesn't get me the right answer. The question is something along the lines of: ...
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2answers
138 views

When to use which representation for an electric field

In class we covered three types of possibilities to evaluate the electric field for static problems. Unfortunately, most physics textbooks cover these ways without addressing the question of ...
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5answers
419 views

How would charge distribute if electrons were balls?

In a conductor, any excess charge will distribute itself evenly over the surface of the conductor. Because of quantum mechanics, this is possible with small charges (i.e. 1e). But if electrons were ...
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1answer
106 views

Confusion with curl of Lorentz magnetic force

Since the magnetic force is a no work force, $dW=\vec F\cdot d\vec r=0$ for $\vec F(\vec r)=q(\vec v(\vec r) \times \vec B(\vec r))$, therefore $\oint \vec F \cdot d\vec r=0$ by Stoke's theorem. ...
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1answer
2k views

Equation describing magnetic hysteresis

So when you're looking at B-H curves for ferromagnetic substances, you often see these magnetic hysteresis curves, which occur, I gather, largely because of domain formation which has some reversible ...
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4answers
296 views

Explanation of Lorentz-Force

In high-school level books (for example the german standard text: "Dorn-Bader") I have often seen an explanation of the Lorentz force as on the following picture: The textbooks consider the ...
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3answers
2k views

Do multiple permanent magnets aggregated together approach the same strength as a single magnet of the same size?

Here's an applied physics question. ;) If I buy some cube or sphere magnets like these, can I aggregate them together to create a stronger magnet (almost as strong as a single magnet)?
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0answers
67 views

Deriving the relativistic Larmor equation

I have derived the Larmor equation as $$P = \frac{q^2}{6\pi \epsilon_0 c^3} |\ddot{r}|^2.$$ How do I make this relativistic? Apparently, I have to consider the acceleration parallel and ...
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0answers
52 views

Is a magnetic field just a moving charge? [duplicate]

I am trying to understand electromagnetism. I would like to pare myself down to the minimum number of necessary concepts. From my physics education (high school only), I understand that a moving ...
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0answers
39 views

Laminations and current capacity,

When a conductor that's purpose is to carry current ,is laminated to reduce eddy currents, does it affect the current capacity for that same conductor? Will it reduce the current capacity?
1
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5answers
613 views

Relativistic charge density in a closed loop

When charges of conductance are at rest, there is an average distance between them. The relativistic origin of magnetic field says that distances between electrons shrink when they are set into a ...
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0answers
31 views

How do the single photon energy and em-signal energy correlate? [duplicate]

If the photon (as a quantum of the electromagnetic field) has no defined(?) amplitude, how does (or where from?) the electromagnetic wave's amplitude appear? The formulation of the question is not ...
6
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4answers
1k views

Relativity and Current in Wire

If an observer is stationary relative to a current-carrying wire in which electrons are moving, why does the observer measure the density of moving electrons to be the same as the density of electrons ...
2
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3answers
79 views

Is dimensional analysis always sufficient to establish equivalence of quantities?

In dealing with the Biot-Savart law, it was argued that $$ q\frac{d\vec{s}}{dt}\equiv Id\vec{s} $$ using the fact that the units are equal. Does this kind of argument always work? It seems too ...
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1answer
257 views

Why are magnetic lines of force invisible?

We can very well feel the magnetic field around a magnet, but we can't see it. Why is that so? Also, can we cut a portion of the field and use it?
2
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3answers
370 views

Is an electron attracted to one of the magnetic poles in this scenario?

Do magnets attract electrons? I don't think so, but maybe in certain cases they can be? I guess it would depend on what direction the velocity and magnetic field is in so that the force acting on the ...
4
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2answers
111 views

Why can't an excess of electrons or holes by themselves cause current flow?

I am a beginner in electrical engineering. Often times (most cases actually), the underlying physics aren't really explained to us and we are just left to assume that it works "because it works." This ...
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1answer
71 views

How might eddy currents and terminal velocity be broken down?

This is a branch from this question. With regards eddy currents and terminal velocity I have located a homework question but would appreciate more detail (and translation of the symbols described ...
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0answers
11 views

Long wave EM Attenuation in liquid

I want to transmit a long wave (electromagnetic wave) through a crude oil line using the pipe as the ground. I am just wondering what the attenuation will be and how best to set up this test. The ...