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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When the load on a motor, say DC motor, is fixed, what is the relationship between torque and the rotational speed of it?

I have been working on a report about factors which can influence the speed of a DC motor. From research, it said that ,for a given voltage, torque and speed in a DC motor are said to be inversely ...
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1k views
+100

What was Feynman's “much better way of presenting the electrodynamics” — which did **not** appear in the Feynman lectures?

Does anyone know what Feynman was referring to in this interview which appears at the beginning of The Feynman Tips on Physics? Note that he is referring to something that did not appear in the ...
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16 views

Magnetic field of three infinite co-planar wires

I'm reviewing test questions, but getting a little confused. Please see link for problem description. a) I thought one of the locations of 0 magnetic field would be right where the middle wire is, ...
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1answer
30 views

How is intensity defined for quantized EM fields?

Classically intensity is defined as $$ I \equiv \frac{1}{2} c \epsilon_0 E^2, $$ but when you perform a second quantization this definition becomes a bit ambiguous since the $E^2$ could be ...
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2answers
40 views

Finding out the direction of $E$ field

In the Figure we can see a conducting loop of radius $r$ situated in a uniform magnetic field that is perpendicular to the plane of the loop. If we change the magnetic field, then according to the ...
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8 views

magnetic intensity of cylinder sheet of charge having surface charge density K

Here is the problem I have by using $$a_z\times R = (0,0,1) \times (\rho-1.2,0,-z) = (\rho -1.2)a_\phi $$ $$H=\int_{-\infty}^{\infty}\int_{0}^{2\pi}\frac{2.4(\rho-1.2)d\phi dz}{4\pi ...
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2answers
2k views

The electric field of a conductive sphere containing a charge - grounded vs not grounded

Let's suppose we have a sphere but unlike theoretical ones it'll have some thickness say $\Delta r$ and inner radius $R$. What I was wondering about is how will it behave if we place some charge $q$ ...
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1answer
57 views

Magnetic field tangential surface integral

When deriving: $\vec{E}=-\frac{\partial\vec{A}}{\partial t}-\nabla\phi$ from Helmholtz decomposition of $\vec{E}$, It seems necessary to show: $$\iiint \nabla' \times \frac{\vec{B}(\vec{r')}}{|\vec{r} ...
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2answers
192 views

Drift Speed and Current in Two Different Inertial Frames

We have a long, cylindrical wire carrying a constant current I in an inertial frame. At a distance of R from the center of the wire, the magnitude of magnetic field is $μI/2πR$. What is the magnitude ...
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25 views

What causes the Shubnikov-de Haas Oscillations?

If I have a 2DEG with a voltage in the $x$-direction and a $B$-Field in the $z$-direction (so I also get a hall-voltage in the $y$-direction (classicaly)). But if I do this stuff at low temperatures I ...
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58 views

What unit system puts a bunch of powers of ten in Maxwell's equations?

In what unit system can we get the Maxwell equation in the following form (section 1.2 from Lewin's book Advanced Theory of Waveguide) $$\nabla\times \vec E = -10^8\mu \frac{\partial \vec H} ...
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1answer
17 views

Low-altitude EMP

Can a low-altitude EMP exist from a detonated nuclear weapon or is the higher altitude required for propagation of the relevant fields necessary to induce destructive currents?
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1answer
20 views

Electric Fields in a Neutral Object

A neutral object has no net charge since its positive and negative charges cancel each other. However, I’m wondering how, in terms of fields, they do cancel each other. Since fields are vectors, won’t ...
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0answers
61 views

Electrodynamics boundary conditions with complex $\epsilon$ and $\mu$

I wonder if the usual derivation for boundary conditions at an interface given in EMT textbooks hold for complex permittivity and/or permeability? Do the fields carry phase information themselves(i.e. ...
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1answer
2k views

How can a magnet have a greater magnetic field strength than a bigger, heavier and more powerful magnet made of the exact same material?

Here are some informations I found on the magnetic field strength (also called "magnetic flux density" or "magnetic induction", commonly denoted as B and expressed in tesla or in gauss) of an average ...
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1answer
149 views

What is the intepretation of the electromagnetic tensor?

Let $A$ be the four-potential, then we know that we can form the electromagnetic tensor as $F=dA$. This is usually done as a way to have a better writing of Maxwell's equations. So, to simplify the ...
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2answers
76 views

Total time derivative of magnetic vector potential $A$

I am looking at this document, which tries to establish the Lagrangian of the Lorentz force. Everything is fine, but I don't see why: $$\frac{dA_i}{dt}=\frac{\partial A_i}{\partial t}+\frac{\partial ...
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1answer
76 views

Why can I put my hand through sand but not a table?

I've read in books that one can't put one's hand through a table because the table offers a "Normal Reaction" to the hand. And it is also stated that this force is electromagnetic in nature. But what ...
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3answers
439 views

Polarization vector of dielectric electrostatics

I know two laws for Polarization vector of a dielectric material $$\vec P = N q \vec{\Delta L}$$ Where $N$ is the number of dipoles per meter cube, $\vec{\Delta L}$ is the vector that represents the ...
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2answers
51 views

Rotate a small wheel using magnets

I know this question already asked so much times. I tried to rotate a wheel by placing few magnets on it. And I used some powerful magnet and repel it using opposite poles in the magnets placed in a ...
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6answers
3k 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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3answers
63 views

Is all of the energy dissipated in a magnetic core in the form of heat and can any of it be recovered?

In an AC motor where you use a laminated magnetic core to shield the induction process from eddy currents, the field lines that run through it result in core loss through heat. Is all of the energy ...
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2answers
1k views

Is it possible to generalize the Maxwell equations to higher dimensions?

The usual Maxwell equations are for 3 spatial dimensions, right? Is it possible to generalize them to 2 spatial dimensions or 4 spatial dimensions?
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3answers
2k views

Maxwell in multiple dimensions: What happens to curl?

I read this answer a while ago, and while thinking about $\nabla$, I realized something. Since the cross product can be written as a determinant, in higher dimensions we require extra vector inputs. ...
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1answer
111 views

Why do electron and hole mobilities decrease with temperature?

From page 35 of "Microelectronics" by Millman Grabel Mobility $\mu$ decreases with temperature because more carriers are present and these carriers are more energetic at higher temperatures. ...
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4answers
207 views

Method of calculating eddy currents of a conductor, is this correct?

I have a conductor with volume $V$, passing a magnetic field($B$) with velocity($v$): I'm trying to calculate the Eddy currents to figure out the magnitude of the drag force($F_d$) generated the ...
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4answers
192 views

A question about canonical momentum and arbitrariness for potential in magnetism

The following question confuses me: There exists magnetic field $B_z =- \beta x$ where $x > 0$, and a particle is incident from origin point $(0,0)$ with pisitive charge $q$, mass $m$, and ...
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1answer
39 views

What is the strength of the magnetic field required to penetrate an average human body?

Introduction Suppose you are an experimental nanobot researcher trial-ling a new form of medication that involves activation and control of nanobots within the cells of the interior of the human body ...
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1answer
88 views

Electromagnetic cooling

Heat means stored kinetic energy in matter. Microwave owens and induction heaters works on increasing kinetic energy on matter. Is it possible to make inverse of this heaters like cooler. Can we ...
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1answer
46 views

Is the sea of electrons in a metal at rest when the metal is in electrostatic equilibrium?

It is said that charges inside a metal in electrostatic equilibrium are at rest, and are stationary. However, don’t the electrons have their own thermal vibrations and whatnot? Aren’t they always ...
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4answers
455 views

Does radiation force depend on group velocity or on phase velocity?

What is the radiation force $F$ due to a beam of photons of power $P$ undergoing perfect reflection? Is it $$a) F = 2 P / c$$ or $$b) F = 2 P / v_g$$ where $v_g$ is the group velocity ? Note that ...
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5answers
649 views

Can you depict the electron's electromagnetic field?

I've had a number of discussions with individuals about electromagnetism. A recurring issue concerns the distinction between field and force, and seems to be associated with the lack of unification in ...
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1answer
103 views

Pulling a bar through a magnetic field

This is a question on magnetic flux and induced voltage. I believe that this problem's answer key is incorrect, and I want to determine whether my opinion is sound. I also want to explore one ...
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0answers
25 views

Current density in covariant formulation [on hold]

I know that I have messed it up quite frankly, but don't know where- Why? $$qu_\alpha=J_\alpha$$ in covariant formulation.
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1answer
61 views

Why doesn't the two poles of a u-shaped magnet attract each other?

Why doesn't the two poles of a u-shaped magnet attract each other? Since opposite poles attract each other why cant the north and south pole of a u-shaped magnet collapse by attracting each other?
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49 views

Is this idea practically possible? [duplicate]

I have to make a working model for my science exhibition. I have planned to make a wind turbine with some innovations. My model of wind turbine will consist of magnet it at the tip of every fan blade. ...
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0answers
59 views

What empirical evidence is there for displacement current?

I wonder what more or less direct measurements of the displacement current exist. I know that the existence of em waves demonstrates its existence, though somewhat indirectly. I also know that there ...
2
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4answers
209 views

Is Gauss' law valid for time-dependent electric fields?

The Maxwell's equation $\boldsymbol{\nabla}\cdot \textbf{E}(\textbf{r})=\frac{\rho(\textbf{r})}{\epsilon_0}$ is derived from the Gauss law in electrostatics (which is in turn derived from Coulomb's ...
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1answer
34 views

Coupling an electric charge to a gauge field. How is it done in this setup?

In page 9 of Tachikawa's N=2 susy dynamics for pedestrians it says that an electric particle with charge $n$ in the first quantised setup (in what sense first quantised?), Wick rotated to Euclidean ...
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2answers
668 views

How do magnets work?

I've read a classbook on the field theory (including EM): it perfectly describes quantitive patterns in EM-theory, but I have no luck understanding how and why it works. I mean, magnetic substances ...
2
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1answer
22 views

Why does normal incidence eliminate the photocarrier effect radiation in this experiment?

Here's a paper in which they use GaAs crystals at a few orientations to produce THz radiation, by means of Optical Rectification (OR). That's actually not what this question is about, though. They ...
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0answers
25 views

Electromagnaetizma [on hold]

The figure shows a schematic diagram of a simple mass spectrometer, consisting of a velocity selector and a particle detector and being used to separate singly ionized atoms (q = +e = 1.602e-19 C) of ...
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1answer
100 views

Superconducting electromagnets?

Is it true that superconducting electromagnets don't need any power? So... energy to created a magnetic field via a superconductor would be zero? Since resistance is zero, does that also mean that ...
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2answers
65 views

What is induced current? [on hold]

Can someone please provide me with a detailed, elementary explanation about the principle of induced current in electromagnetism.
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1answer
125 views

How can Drift Velocity of an electron causes change?

Assume we have a conductor that has current flowing and its placed in a magnetic field so that it experiences the Lorentz force, and it gains Kinetic energy due to that force. Would drift velocity ...
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1answer
791 views

Electric field from conductive to dielectric media

I am interested in the main difference between transitions from electric fields from Conductive to Conductive/ Dielectric to Dielectric and Dielectric/Conductive media. What are the boundary ...
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2answers
645 views

Why are magnetic field lines perpendicular to the surface of a ferromagnetic material?

It is known that magnetic field lines become nearly perpendicular to the surface of a ferromagnetic material. The quantitative proof which uses the boundary condition requires that magnetization at ...
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1answer
221 views

Electromagnetic inertia due to advanced radiation?

The scalar potential $\phi$ and vector potential $A$ at a distance $r$ from a charge $q$ are given approximately by $$\phi = \frac{q}{r}$$ $$\mathbf{A} = \frac{q\mathbf v}{r}$$ where the constants ...
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1answer
26 views

Already integrated formula of magnetic field for a finite line?

I was looking for a formula to simulate a magnetic field due to a straight finite line. The closest to what I wanted to find was in these lecture notes (formula 9.11.2 page 9-50), \begin{align} ...
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2answers
141 views

Why particles don't lose mass when they radiate

Charged particles radiate when accelerated: in the rest frame of the particle moving with acceleration $\textbf{a}$ the amount $dW$ of radiated energy over time $dt$ is $$ dW \propto \textbf{a}^2 ...