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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2answers
582 views

How does Earth's interior dynamo work?

I'm interested in getting a basic physical understanding of how Earth's magnetic field is generated. I understand that it's a "dynamo" type of effect, driven by convection currents in the molten outer ...
0
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1answer
807 views

Improvement Of Soft Iron And Steel As Magnets

I am given to know that soft iron is used as temporary electromagnet since it has high permeability i.e. the ability to align its domains corresponding to the electric field around it, however has a ...
2
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1answer
233 views

Can a charged black hole interact via electromagnetism? [duplicate]

Possible Duplicate: Detection of the Electric Charge of a Black Hole Light cannot escape from a black hole. However light is also interpreted as the carrier of the electromagnetic force. So ...
25
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8answers
3k views

Is there an intuitive explanation for why Lorentz force is perpendicular to a particle's velocity and the magnetic field?

The Lorentz force on a charged particle is perpendicular to the particle's velocity and the magnetic field it's moving through. This is obvious from the equation: $$ \mathbf{F} = q\mathbf{v} \times ...
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2answers
28k views

Has any permanent magnet motor been proven to run?

I have read lots of articles about permanent magnet motors, some of which claim the possibility and other which refute it. Is it possible to have a permanent magnet motor that runs on the magnetic ...
42
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7answers
5k views

Cyclist's electrical tingling under power lines

It's been happening to me for years. I finally decided to ask users who are better with "practical physics" when I was told that my experience – that I am going to describe momentarily – prove that I ...
2
votes
1answer
676 views

Work done on charged particle by magnetic field in quantum mechanics

Classically, we know from $\mathbf{F}=q\mathbf{v}\times \mathbf{B}$ that magnetic field does no work on a charged particle. In quantum mechanics, the Hamiltonian of a charged particle in a magnetic ...
2
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1answer
127 views

Crushing a magnetic field

What would happen if you crushed a magnetic field to an ever decreasing size? Thanks. EDIT: How small could the field possibly go? Is there a limit on how small it could get? Is there a maximum ...
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1answer
323 views

Gravimagnetic monopole and General relativity

Review and hystorical background: Gravitomagnetism (GM), refers to a set of formal analogies between Maxwell's field equations and an approximation, valid under certain conditions, to the Einstein ...
2
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1answer
395 views

Strength of Magnetic Field Around a Superconductor

I recently learned that the strength of a Magnetic field around a conductor is proportional to the current flowing in it. So if we have a Mercury wire at absolute zero and pass a current through it ...
2
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1answer
114 views

What is the reason behind the shape of the absorption curve of electron paramagnetic resonance

In our EPR experiment, the signal looks like the "first derivative" part of the above picture. Why is this? What does the "first derivative" mean and why is it the quantity our instruments detect in ...
4
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3answers
2k views

Coulomb's Law: why is $k = \dfrac{1}{4\pi\epsilon_0}$ [duplicate]

This was supposed to be a long question but something went wrong and everything I typed was lost. Here goes. Why is $k = \dfrac{1}{4\pi\epsilon_0}$ in Coulomb's law? Is this an experimental fact? ...
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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 ...
2
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2answers
338 views

A Question From Jackson Electrodynamics

I have a question regarding Jackson's Classical Electrodynamics. Consider the equation $$\varphi \left ( x\right )=\tfrac{1}{4\pi\epsilon _{0}} \int_V \frac{\varrho ( x )}{R}d^{3}x+\tfrac{1}{4\pi} ...
7
votes
2answers
2k views

Coulomb potential in 2D

I know that the Coulomb potential is logarithmic is two dimensions, and that (see for instance this paper: http://pil.phys.uniroma1.it/~satlongrange/abstracts/samaj.pdf) a length scale naturally ...
8
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3answers
844 views

Trouble with the Lorentz law of force: Incompatibility with special relativity and momentum conservation?

In Physical Review Letters, there was a paper recently published: Masud Mansuripur, Trouble with the Lorentz Law of Force: Incompatibility with Special Relativity and Momentum Conservation, Phys. ...
1
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1answer
311 views

Impervious nature of solid matter due to quantum degeneracy pressure

On Wikipedia the following statement is made without reference: Freeman Dyson showed that the imperviousness of solid matter is due to quantum degeneracy pressure rather than electrostatic ...
0
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1answer
174 views

Is a low-current electrical arc harmful to humans?

I've heard that electrical flux non-destructive particle testing machines are considered safe because they use less than 2 amps. I have seen an arc created between two objects do considerable damage, ...
1
vote
2answers
642 views

Could a solar flare cause the Earth's magnetic poles to reverse?

With all the hype of the impending "2012 Mayan doomsday" I was thinking it might be interesting to see what principles of physics prevent the theories of doomsday from occurring. One overarching ...
3
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2answers
239 views

When work is performed solely by magnetism, is there an equivalent loss of energy from the magnetic field?

When two magnets are placed within appropriate proximity and released, the attractive force will perform work and bring them together. Work is performed overcoming friction. Can we measure a ...
4
votes
1answer
487 views

Dirac's quantization rule

I first recall the Dirac's quantization rule, derived under the hypothesis that there would exit somewhere a magnetic charge: $\frac{gq}{4\pi} = \frac{n\hbar}{2} $ with $n$ natural. I am wondering ...
4
votes
4answers
866 views

How to interpret the continuity conditions in the PDEs (for example, Maxwell equations) originated in physics?

I am currently working on PDEs in physics, mostly Maxwell equations. I am a mathematics graduate student, and this question has been haunting me for years. In PDE theory, or more specifically the ...
15
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4answers
14k views

Phase shift of 180 degrees on reflection from optically denser medium

Can anyone please provide an intuitive explanation of why phase shift of 180 degrees occurs in the Electric Field of a EM wave,when reflected from an optically denser medium? I tried searching for it ...
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2answers
7k views

How do I relate the direction of a compass needle to the direction of a current?

A compass needle is placed above the wire points with its N-pole toward the east. In what direction is the current flowing? If a compass is put underneath the wire, in which direction will ...
2
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2answers
236 views

Interaction speed between electric charges and magnetic materials

Einstein said that the speed of a matter in universe cannot exceed the speed of light. Is it correct for electric force transmission speed from one electric charge to other one? What is ...
1
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1answer
914 views

Magnetic Permeability & Reluctance on old exam question

This is a past exam question from one of our lectures, and we have an issue with (i), I believe I need to use the equation $\rho=\frac{RA}{l}$, but I am not sure - could someone enlighten me on the ...
3
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1answer
416 views

What is the time correlation function in the Green-Kubo formulation of ionic current?

I am reading a paper, and I came across the Green-Kubo formulation, where the conductivity $\sigma$ of charged particles is related to the time correlation function of the $z$-component of the ...
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3answers
2k views

Electricity & Magnetism - Is an electric field infinite?

The inverse square law for an electric field is: $$ E = \frac{Q}{4\pi\varepsilon_{0}r^2} $$ Here: $$\frac{Q}{\varepsilon_{0}}$$ is the source strength of the charge. It is the point charge divided ...
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2answers
405 views

Why do you get electric field of a light wave?

Why do you get electric field of a light wave in following form: $E(x,t)=A cos(kx-\omega t- \theta)$?( look at: https://public.me.com/ricktrebino -> OpticsI-02-Waves-Fields.ppt, p. 18)
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1answer
740 views

Potential difference between conductors

Reading some books there is something that i don't understand. Is the capacitors topics when they name a potential difference between conductors. As i know, Potential difference is the difference in ...
1
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1answer
204 views

Toroid with variable current and Ampere's Law

Say you have a current $I(t)$ (notice the time dependence) flowing through a Toroid with $N$ total loops and all the usual approximations: $(b-a) \ll r,\; B=0$ outside. You are asked to calculate ...
0
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2answers
197 views

Is it true that capacitors closer to a power source charges quicker and gets priority?

Let's say I hook two capacitor in a parallel circuit with a voltage source. Would the capacitor closer to the battery be charged up first? Because when the voltage excites and draws electrons from the ...
1
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1answer
55 views

Magnetized nail lifting another nail

I have two nails (made of Fe). A and B. A can not lift B. If I rub A on the magnets north pole, then it can lift B. Then if I rub it again but on the south pole, it can not lift B. why is that? ...
2
votes
1answer
173 views

Earth's magnetic field, could the poles be reversing?

Lately, I've been 'monitoring', if you will, the earths magnetic field, i've come across some strange looking activity periodically. I exchanged some emails with a guy from NASA today, Joseph Gurman, ...
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2answers
2k views

A closed surface, no charge enclosed, yet flux not 0?

! The book says it is $E_0\pi r^2$ because the flux through the circle is equal to the curved part of the paraboloid. I don't understand this, shouldn't the total flux be 0 for the whole surface? ...
1
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1answer
92 views

Perpendicular Fields and Equations of Motion [closed]

I have the following problem to solve: A particle of mass $m$ and charge $e$ moves in the laboratory in crossed, static, uniform, electric and magnetic fields. $\mathbf{E}$ is parallel to the ...
3
votes
1answer
554 views

Are perpetual magnetic motors a scam? [duplicate]

I read articles and watched videos and more videos where people say they've built perpetual magnetic motors. I do not see any proof that this can work and also I do not see proof that it is wrong. ...
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0answers
299 views

Equations governing mutual inductance and transformers

What are equations for voltage ratio (primary/secondary) of... the action of a transformer with a turn ratio (turns primary/turns secondary) of X/Y and a toroidal core of cross-sectional radius R ...
2
votes
0answers
470 views

Are there any non magnetic materials that attract to each other as if they were magnetic?

Are there any non magnetic materials that attract to each other as if they were magnetic? This is an argument I am having with a friend. Thanks,
2
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3answers
517 views

Gauge invariance and form of the vacuum polarization tensor

In quantum field theory or condensed matter physics, the fermionic one-loop diagram gives rise to the polarization tensor $$ Π^{µν} = Tr[ γ^µ G γ^ν G ] $$ If we couple the electrons to an ...
1
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1answer
292 views

Relationship between gauss and decibels

In my ongoing effort to understand the world around me, I want to wrap my head around the relationships between two units of measure. Specifically gauss and decibels. The quandary comes from my ...
1
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0answers
132 views

An electron is subjected to an electromagnetic field using the canonical equations solve

So I was given the following vector field: $\vec{A}(t)=\{A_{0x}cos(\omega t + \phi_x), A_{0y}cos(\omega t + \phi_y), A_{0z}cos(\omega t + \phi_z)\}$ Where the amplitudes $A_{0i}$ and phase shifts ...
9
votes
5answers
989 views

Superposition of electromagnetic waves

The superposition of two waves is given by $$\sin(\omega_1 t)+\sin(\omega_2 t)=2\cos\left(\frac{\omega_1-\omega_2}{2}t\right)\sin\left(\frac{\omega_1+\omega_2}{2}t\right).$$ For sound waves, this ...
3
votes
2answers
87 views

A charged particle moves in a plane subject to the oscillatory potential

A charged particle moves in a plane subject to the oscillatory potential: $U(r)=\frac{m\omega^2 r^2}{2}$ There is also a constant EM-field described by: $\vec{A}=\frac{1}{2}[\vec{B}\times\vec{r}]$ ...
2
votes
1answer
213 views

No attraction radially in an cylinder of spherical magnets

I have a set of small magnetic spheres the size of ball bearings. When many of them are built into a cylinder such that they are hexagonally packed, there is no magnetic attraction radially (between ...
2
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4answers
4k views

What is magnetic conductivity?

I searched google for the meaning of magnetic conductivity but couldn't figure out what is it? electric conductivity is usually means that there is the electric field parallel to the interface is ...
7
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2answers
1k views

The equivalent electric field of a magnetic field

I know that Lorentz force for a charge $q$, with velocity $\vec{v}$ in magnetic field $\vec{B}$ is given by $$\vec{F} =q \vec{v} \times \vec{B}$$ but there will exist a frame of reference where ...
2
votes
2answers
3k views

Can a camera flash light near high voltage wires create medium to pass electricity?

I found a story of a boy who took a picture with flash light near 40,000 voltage lines. When it flashed, current came to his camera (as a spark may be) and passed through his body making him dead. Is ...
0
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2answers
992 views

What's the physical meaning of the Fourier transform of magnetic flux density?

I have here below the distribution of the magnetic flux density $B$ across a 1 pole pitch in the airgap of a synchronous machine. The horizontal axis represents the distance along the arc length ...
4
votes
1answer
137 views

Loopless voltage measurement

I think we are all very well familiarized with the classical voltmeter. Classical voltmeter has two conducting wires that bring two potentials into the box. In the box we have well controlled ...