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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Using magnetic field readings for gesture recognition “along a line”

I don't think the title is quite right, I'll try to explain my question as better as possible. I am thinking about using a magnet as an input device for my smartphone. I have been reading some ...
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0answers
17 views

Help understanding Diamagnetism in external field

Lorentz force can not do work. Therefore it can not change the velocity of electron. Then how do you explain diamagnetism in terms of speed of a pair of electrons in the orbit?
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12 views

magnetic field of bar magnet

two bar magnet each of magnetic moment M are hang through mid point of each. such that both are perpendicular to each other . find the magnetic field at the line which is angle bisector of the two ...
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1answer
43 views

What is the difference between the magnetic field of a permanent magnet, and that of an electromagnet?

What is the difference between the magnetic field of a permanent magnet, and that of an electromagnet? In the diagram below, you can see that the magnetic field of a wire which carries a DC ...
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2answers
61 views

Does voltage produce a physical field?

Does voltage produce an electric field, like how a current produces a magnetic field?
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1answer
58 views

Asymmetry of relativistically treated EM force between atoms

There are two neutral atoms set separated at a long distance $R$ and let's consider them phenomenologically through Bohr model. Let's also assume that the nuclei (charged $+q$) of the atoms are fixed ...
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2answers
29 views

Why does the electric polarisation (polarisation density) have this direction?

It can be inferred that the amount polarisation $ P $ is dependent upon and proportional to the magnitude of the electric field $ E $ applied: \begin{equation} P \propto E. \end{equation} This ...
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1answer
108 views

Why will a 'Non-Electric Faraday Motor' which just uses permanent-magnets, not work?

Regarding Faraday's Electric Motor (which is in a pool of mercury, or salt-water) Why will a 'Non-Electric Faraday Motor,' which just uses permanent-magnets, not work? Can someone explain this ...
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0answers
32 views

Why the rotational speed of a simple single-coil DC motor will increase as the number of turns in the coil increases?

I am working on a report about a simple DC motor. I have known that the speed of a DC motor is directly proportional to the applied voltage. However, based on the data collected, why the speed of the ...
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1answer
42 views

Which makes an electromagnet stronger? AC or DC? [on hold]

I need a clear answer and a good explanation about this. Please tell me what happens with an AC or DC current. I have already read a lot of books but I did not get this at all. So please tell me: ...
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0answers
15 views

How does magnetic field depends upon Linear momentum? [on hold]

has magnetic field depends on momentum? if it depends on it then how it works and what will be result about that?
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1answer
33 views

is copper a non-magnetic?

I'm working on an optics project that I'm using Fresnel Equation for Intensity. but I've got a question, is copper a non-magnetic ? You know because in Fresnel Equation, for non-magnetic materials ...
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1answer
39 views

What happens if I have a square conducting wire being permeated by a magnetic field and the field suddenly disappears?

Suppose there is a square conducting wire in a magnetic field. The two vertical branches will have the same emf, and the two horizontal branches will have 0 emf, resulting in a net emf of zero and ...
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0answers
12 views

How to get image current in magnetostatics? (magnetic slab)

I want to know the image current and its location which satisfies the boundary condition at the interface. This problem was originated from the problem 6-33 in Fields and wave electromagnetics, D. ...
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1answer
32 views

Can electromagnets repel?

So I saw a video here https://www.youtube.com/watch?v=wzXRFp0DDrU (See the full video first) I don't understand that how is the electromagnet repelling things in the second half of the video? Also ...
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0answers
10 views

Show that the electric dipole term vanishes for a particular current

I'm interested to show that given a ring with radius a where there's a current $I_0 \cos \omega t$ ($I_0$ is a constant) there is no radiation due to the electric dipole term (appearing in the ...
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0answers
21 views

Magnetic field at focus of parabola [on hold]

A parabola in $x$-$y$ plane carries current $I$, find the magnetic field at the focus of the parabola. Assume focus is at the origin and the focal length of parabola is $d$.
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0answers
18 views

Books of dynamo theory

I wanted recommendations of dynamo theory pdfs to make a report about magnetic fields of planets and stars with all the math behind it, like vector calculus, Lagrange functions, partial differential ...
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4answers
59 views

Why does a current-carrying conductor behave like a magnet?

I have heard that a current-carrying conductor behaves like a magnet, the reason why the magnetic needle of a compass kept near the circuit, deflects. Why is it so?
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1answer
33 views

Can the magnetic force between two magnets be explained classically via magnetization current?

Usually in Electrodynamics courses the magnetic forces are analysed via wire-wire interaction. I don't remember being shown a classical explanation for magnetic forces between magnets. Since it is a ...
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2answers
43 views

Why is the index of refraction different for different wavelengths? [duplicate]

The index of refraction can be written as $$n=\frac{\lambda_v}{\lambda_m}$$ where $\lambda_v$ is the wavelength in a vacuum and $\lambda_m$ is the wavelength in the medium. I’ve been told that since ...
3
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1answer
48 views

Eddy currents Vs. Inducing EMF in opposing the change?

In the following circuit there is a power supply applying a voltage(+$V$) to a circuit with resistance ($R$), current($I$) is now flowing in the circuit, and there is a movable part like so: The ...
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1answer
52 views

Induced voltage by dropped magnet? [on hold]

I am trying to understand the concepts in this lab: Induced Voltage from a Dropped Magnet We dropped a magnet through a guided string from a controlled height through a coil. An oscilloscope captures ...
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1answer
39 views

Will two opposing magnetic fields cancel out?

When a conductor induces eddy currents that creates a magnetic field opposing the change that created it, would the two fields at some point cancel out? Imagine the change to be so great, it ...
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3answers
40 views

Zero current induced in a loop

Consider the following two cases, Case 1 – A coil moving through a constant magnetic field Case 2 – A conductor moving through a constant magnetic field Now, in Case 1 (a coil moving through a ...
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0answers
6 views

Magnet rotor diameter effect on power output in an axial flux generator

I'm trying to determine which parameters of the magnet rotor have the greatest effect on power output of an axial flux generator. Obviously a very large diameter will have a greater tangential ...
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75 views

What maximum theoretical speed can reach spaceship with EM Drive in space? [closed]

There are several studies (by Institute of Aerospace Engineering, Technische Universität Dresden and by NASA) that are concluding that the EM Drive produces thrust. So I am wondering: if the EM Drive ...
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0answers
47 views

Commutation relation between linear momentum and vector Potential [closed]

Does the linear momentum and vector potential commute? How can we show their commutation relation ? I am actually trying to find the commutation relation between both linear momentum of an electron ...
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2answers
31 views

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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2answers
41 views

Magnetic field of three infinite co-planar wires

I'm reviewing test questions from an old exam, but getting a little confused. a) I thought one of the locations of 0 magnetic field would be right where the middle wire is, since the fields from ...
2
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1answer
43 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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1answer
19 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
48 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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0answers
28 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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1answer
19 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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0answers
64 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
25 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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2answers
107 views

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

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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2answers
61 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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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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1answer
48 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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0answers
25 views

Current density in covariant formulation [closed]

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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0answers
50 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. ...
0
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1answer
64 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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1answer
58 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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1answer
35 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 ...
2
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4answers
219 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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3answers
72 views

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

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 ...
2
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1answer
23 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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1answer
27 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} ...