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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498 views

Doubt in problem regarding resistance a of cylinder

I've been working on some exercises and I'm in doubt if my procedure with this one is correct. We have a hollow cylinder with internal radius $r_a$, external radius $r_b$, resistivity $\rho$ and ...
2
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1answer
9k views

What is the difference between a Rodin coil and a Rodin starship?

I've seen various designs for Rodin coil and a 'Rodin starship'? Are these just regular electromagnets? Or something different? How do they differ from regular electromagnets?
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3answers
378 views

Negative Mass and gravitation

Since Newtonian gravity is analogous to electrostatics shouldn't there be something called negative mass? Also, a moving charge generates electric field, but why doesn't a moving mass generate some ...
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2answers
156 views

Free spin (Curie) Paramagnetism

I'm working through a derivation for Curie paramagnetism and hope someone could help clarify a couple of steps. The way that makes sense to me (although now I have seen the wikipedia derivation below ...
5
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2answers
253 views

Why isn't data lost when sent over large distances?

I was thinking about how information is sent, for example through the atmosphere. There are plenty of obstacles, as well diffraction, etc. Still, no information is lost. How is information sent to ...
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0answers
215 views

Easy question about magnetism?

I have to build a simple electric motor by attaching a magnet to a battery, extending the terminals of the battery (with stiff wires so they could act as supports), and placing a coil of wire on top ...
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3answers
4k views

What is the cause of the normal force? [duplicate]

I've been wondering, what causes the normal force to exist? In class the teacher never actually explains it, he just says "It has to be there because something has to counter gravity." While I ...
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3answers
250 views

Energy conserved… or not? Confused!

I am confused. Could someone kindly explain what's going on in this question? A particle of mass $m$ and charge $e$ moves in the $x,y-$ plane. There is a constant magnetic field $B$ that points in ...
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2answers
243 views

Ferromagnetism with mobile spins

How can electron spins in Iron at room temperature have ferromagnetic order even though they are travelling at very high speeds? One could argue that spin and motion are completely uncorrelated and ...
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2answers
4k views

Derivation of the Biot-Savart Law [duplicate]

Can someone provide a derivation of the Biot-Savart law for electromagnetic induction? To be clear, $$ d\vec{B}~=~\frac{\mu_0}{4\pi}\frac{I d\vec{\ell}\times \vec{r}}{r^3}. $$ Is there a simple way ...
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1answer
199 views

Capacitance of a conducting disk

I'm reading this(PDF) derivation of the capacitance of a thin conducting disk. The surface charge density of such a disk can be shown to be: $\sigma(r) = \frac{Q}{4\pi a\sqrt{a^2 - r^2}}$ (in ...
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1answer
107 views

Trying to understand EM wave and photon

When electrical fields and magnetic fields couple together, it forms electromagnetic waves. And we can "quantized" it and see each "package" of it as photon. So can electrical fields and magnetic ...
2
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1answer
88 views

Falling through the ground [duplicate]

I do not know much about physics but I know that according to Newtons third law of motion when we walk we are pushing the ground down but the ground is pushing us up. What force is making the ground ...
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2answers
3k views

Electric field and electric potential of a point charge in 2D and 1D

in 3D, electric field of a piont charge is inversely proportional to the square of distance while the potential is inversely proportional to distance. We can derive it from Coulomb's law. however, I ...
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0answers
96 views

Is this picture of the electron dipole moment correct?

It's the electron spinning on its axis, and a magnetic moment shoots up. The direction of the North-South is opposite what I thought it should be. Why is the vector arrow pointing from North to ...
3
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1answer
3k views

Charge inside conductor

I know that the E field inside a conductor is zero. What happens if I put a source of charge inside the conductor? Say the conductor was spherical centered on the origin and there exists a charge at ...
4
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1answer
1k views

What are the limits of applicability of Coulomb's Law?

Coulomb's law is formally parallel to Newton's Law of Universal Gravitation, which is known to give way to General Relativity for very large masses. Does Coulomb's Law have any similar limits of ...
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4answers
5k views

Displacement Current - How to think of it?

What is a good way to think of the displacement current? Maxwell imagined it as being movements in the aether, small changed of electric field producing magnetic field. I don't even understand that ...
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2answers
332 views

Do black holes have charges?

Do black holes have charges? If so, how would they be measured? Also, does electricity behave the same way? Black holes affect photons, which are carriers of EM radiation, so do black holes have any ...
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2answers
301 views

Simple ohms law on a battery ? Paradox or conceptual error?

Suppose we have a regular pencil battery which supplies DC voltage $V$. Say we take copper wire and connect the ends of the battery to an $R$ ohms resistance. Then Ohm's law tells use the current in ...
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1answer
137 views

Maxwell's Equations-Relativity

How did Maxwell develop the magnetic field without relativity? Was it purely experimental? I don't see how else he would have developed any understanding for the magnetic field.
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1answer
2k views

Magnetic induction outside of the finite length solenoid

I'm programing simulation software for solenoid and hemholtz coils, where I have to calculate B(x,y,z). I have found dozens of variations of Biot-savart law for determining B on axis. But how about ...
2
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2answers
506 views

Irradiance (or intensity) of an electromagnetic wave

Is the irradiance (or intensity) of an electromagnetic wave given by \begin{equation} I = \left<\|\mathbf{S}\|\right>_{T} \end{equation} or by \begin{equation} I = ...
2
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1answer
499 views

Retrieving Maxwell's equations from the minimum action principle

I'm currently working at the start of Alexei Tsvelik's book Quantum Field Theory in Condensed Matter Physics. I'm kinda stumped on a few essential steps. Starting with the action: $$S = \int dt \int ...
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1answer
214 views

iPhone compass not being affected by current

As you probably know, electrical current and magnetic fields are close friends. In an iPhone there is a compass, however there are also lots of cables and in all of them, or at least in most of them ...
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3answers
3k views

Similarity between the Coulomb force and Newton's gravitational force

Coulomb force and gravitational force has the same governing equation. So they should be same in nature. A moving electric charge creates magnetic field, so a moving mass should create some force ...
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1answer
201 views

Ginzburg-Landau model for superconductivity

Could someone kindly elaborate more on the Simple Interpretation section from this Wikipedia Article? I refer to the part on the natures of $\alpha , \beta$. Why can one assume that ...
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1answer
35 views

A simple electromotor?

I have to build a simple electromotor in the following way: I attach a permanent magnet to a battery, connect some metal supports to the terminals of the battery, and place a coil of wire on the ...
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3answers
3k views

Faraday's law - does the induced current's magnetic field affect the change in flux?

I've had this conceptual problem with Faraday's law and inductance for a while now. Take the example of a simple current loop with increasing area in a constant field (as in this answer). So ...
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1answer
123 views

Magnetic Field and the Speed of Light

Is it just a historical choice that both magnetic field and the Lorentz force equation include the speed of light? I figure that whoever wrote up the equations (in cgs!) could have put both factors ...
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1answer
287 views

Uncertainty-principle and the Maxwell formalism of electromagnetic waves

An electromagnetic wave (like a propagating photon) is known to carry it's electric and magnetic field-vectors perpendicular and each depending on the differential change of the other thus "creating" ...
4
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2answers
166 views

Vector potential

I have difficulty understanding the following vector calculus example. Text can be found here. It is the 5th Q&A -- starting with equation (31.1035).It concerns finding the vector potential of a ...
3
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1answer
157 views

CGS Units for Magnetism

Why does the formula for magnetic field force include the speed of light in the denominator in cgs units? Where does the extra $c$ go in SI units?
4
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2answers
579 views

Physical explanation for why total internal reflection occurs

I have been trying to understand total internal reflection (and have read several posts on this site already). Mathematically, I feel that I understand how the evanescent wave decays exponentially as ...
2
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0answers
32 views

Small charged sphere's motion in earth's magnetosphere?

Suppose that a spherical metal sphere with mass $m=10^{-16}kgr$ radius $R=10μm$ charge $Q=10^{-9}C$ travels with $v=c/3$ and is trapped in the earth's magnetosphere at a distance around $r = 1000km$. ...
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2answers
225 views

EM Fields in a Rotating Frame of Reference

I'm struggling on my approach to the problem of figuring out E and B fields in a non-relativistic way for a rotating frame of reference in the x-y plane around the z-axis. I am attempting to do this ...
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1answer
61 views

Phasor representation of voltage in frequency domain

In a text on application of electromagnetism in transmission line, there introduces a phasor for the voltage (in frequency domain) $$\tilde{V}(x) = V^+e^{-i\beta x} + V^-e^{i\beta x.}$$ Here $V^+$ ...
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3answers
211 views

Microscopic picture of an inductor

I have a good understanding of how inductors behave in electrical circuits, and a somewhat rough-and-ready understanding of how this behaviour arises from Maxwell's equations. However, what I don't ...
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3answers
2k views

Why is visible light used in Optical fibers (instead of other EM waves)?

Why aren't other electromagnetic waves used in optical fibres instead of visible light? Is it because the wavelength of light fits the internal reflection/refractive index of the material used for the ...
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1answer
108 views

EM Waves Energy Loss

Where does the energy go when two photons interfere destructively at a point on a screen in Young's double slit experiment ?
2
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1answer
228 views

Induced Current, Dipole Falling Under Gravity Towards Circular Loop [closed]

Suppose a magnetic dipole $\mathbf{m} = m \hat{z}$ is falling towards a circular loop of radius $b$ under gravity. Assuming the dipole always stays along the $z$-axis of the loop, determine the ...
3
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1answer
574 views

Does a microwave resonantly excite the rotational levels when cooking?

Wikipedia states there is no resonance absorption, but says at the same time that the molecules are oscillating like dipoles, which is kind of the same if you are exciting the rotational levels ? The ...
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0answers
163 views

How can I determine the type of magnet used in a DC motor?

I repair electrical motors in a workshop and sometimes I must repair permanent magnet DC motors and other motors that use permanent magnets. I need to specify type of permanent magnet (Ferrite or ...
3
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1answer
135 views

Uniqueness and existence of polygonal orbits through a spherical shell

Say we have a spherical wire mesh raised to a negative voltage. Then let's say we release a proton from near the surface, and away from the surface, at some angle and speed. Also, imagine that the ...
3
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2answers
165 views

What materials focus EM radiation in the 2.4GHz range

If glass and similar materials refract visible light effectively, what materials would be best for focusing lower frequencies of EM radiation, if any? If not, what other methods exist for focusing ...
0
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1answer
349 views

Eddy current losses in electric steel by harmonics of a magnetic field

I am working on an model of a permanent magnet synchronous machine. Right now I am stuck with calculating the eddy current losses caused by the harmonics of the stator magnetic field in the electrical ...
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2answers
825 views

Can electromagnetic fields be used to deconstruct and reconstruct atoms?

I was thinking one day and came up with a theory after reading about how scientists were studying anti-matter by using electro magnetic fields to separate matter from the anti-matter they made. It ...
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5answers
2k views

How are forces “mediated”?

I hope this is the right word to use. To me, these forces seem kind of fanciful (except for General Relativity and Gravity, which have a geometric interpretation). For example, how do two charged ...
10
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1answer
391 views

How the inverse square law in electrodynamics is related to photon mass?

I have read somewhere that one of the tests of the inverse square law is to assume nonzero mass for photon and then, by finding a maximum limit for it , determine a maximum possible error in ...
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1answer
614 views

Importance of Ampere's Law

Consider a hollow cylinder carrying a current $I$ and a wire outside the cylinder carrying a current $I'$. Let's say the cylinder is symmetrical with even current distribution etc.. so the ...