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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84 views

Does a rotating disk produce a DC current

My problem comes from this thread: Does a rotating disk develop a potential difference between the centre and rim? My issue is that I don't see why the assumption of equilibrium commented by Cag must ...
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1answer
102 views

Electric flux density for a hollow cylinder using Gauss's law [closed]

In the textbook of "Engineering Electromagnetics": I understood it except the part highlighted with yellow(i.e. how did he find the charge distribution of the outer cylinder? What are the ...
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37 views

Rearrangement of oil due to high voltage electric field

I've been working on a problem for some time now and I can't figure out why exactly this happens, and I've been looking for some insight. The phenomenon can be seen in this video. We can see that the ...
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1answer
32 views

magnetism induced current

Hi, I have a question about magnetism. I learned 2 rules and they are confusing me. the right handrule which says: stop the magnetic field lines with the palm of your right hand and point your thumb ...
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29 views

How to calculate the maximum spacing between Hemholtz coils

I would like to design a long, uniform magnetic field using Hemholtz coils but am not sure how far I can separate them before the field lines diverge and become two loops that go around each coil ...
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1answer
118 views

Why is the magnetic field of a spherically symmetric current zero?

We now ask about the magnetic field produced by the currents in this situation. Suppose we draw some loop $\Gamma$ on a sphere of radius $r,$ as shown in Fig. 18–1. There is some current through this ...
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68 views

Light's wavelength and pathlength

I see this 2 words (wavelength and path length) used interchangeably. As interpreted here: 'The total power density arriving at the surface depends on the absorption path length of the light ...
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3answers
149 views

Why is visible light easily blocked by pretty much anything but radio waves are not?

Light (visible part of electromagnetic spectrum) is easily blocked by most materials. Radio waves are not but also X-rays are not. What is so special about this small part of the spectrum since both ...
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1answer
41 views

Permanant Magnet Lines & Currents: [duplicate]

A permanent magnet produces lines of magnet flux that we call a "magnetic field". Those lines come from inside the magnet, come out of the N pole, loop outside the magnet, & return back into the ...
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1answer
48 views

determining electrostatic field using only symmetries

As an exercise, I'm trying to (rigorously) determine as much as possible about the electrostatic field due to a infinite line of charge (along the z-axis) without using Maxwell's equations or any of ...
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1answer
40 views

Electrostatic Shielding

suppose there is a solid conducting body with arbitrarily shaped cavity inside. Now a charge is placed inside the cavity.Now the entire setup is placed in an uniform electric field. Will the ...
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1answer
28 views

Frequency vs amplitude for electromagnetic wave

As we know frequency is closely related to wavelength, that means, if we change the frequency, we are changing wavelength. Let assume for damped oscillation, where amplitude gets lower and therefore ...
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70 views

Magnetic force of a magnetic field around a wire

Suppose we have a magnetic field around a wire because there goes a current through the wire. Suppose there are not any magnets near. Is it ture that there is then also a magnetic force? If so, is ...
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68 views

Does current in a part of a wire create magnetic field at any other part of the same wire?

Let there be a circular-shaped wire - a ring in which there exists a steady current. It creates magnetic field around the wire. This can be evaluated by Biot-Savart Law. As shown in the fig., ...
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25 views

Calculating the deflection of an electron in a given magnetic field [closed]

I have been stuck on this question for some time but I am not able to solve it. It is a question from "Concepts of physics" (2nd volume) by H.C. Verma. An electron is projected horizontally with a ...
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1answer
32 views

Induced EMF, by moving the magnet is the same?

I know that when a wire is moving in a magnetic field, there is an induced EMF due to the change in area, therefore, a change in magnetic flux. However, what if things we're switched? Fixing the ...
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2answers
95 views

Two different formulas

My problem is simple : given a particle of mass $m$, charge $q$ and velocity $\bf{v}$. If $\bf{A}$ denotes the magnetic potential satisfying $\bf{B}= \nabla \times \bf{A}$. I want to etablish the ...
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61 views

Velocity of particle constant under null initial magnetic force

I have got a doubt on the exact application of Newton's first, and third, law. Let us for example suppose that a particle, electrically charged with charge $q$, is moving at the instant $t=0$ with ...
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1answer
80 views

What is the speed of magnetism?

I'm not a physicist (just have a few basic notions) and this is a question out of curiosity, so as I'll probably word it the simplest I can, please take that into account when answering. tl;dr Let's ...
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2answers
166 views

Computing the force of the induced electric field on the moving charges that create the non-steady current & thus the same electric field. [closed]

Previously, I asked a question on whether charges suffer force from the magnetic field they create while constituting current. I got responses from Timaeus as well as Sebastian Riese; the former told ...
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1answer
288 views

Can Newton's 3rd Law be considered as a direct consequence of the coulomb's law of electric interactions? [closed]

Let me explain my thought. Lets consider Coulomb's definition of electric force between two charges as the fundamental law. Under this consideration, forces between charges already follow What Newton'...
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58 views

angular momentum of electromagnetic wave

EM waves that are circularly or elliptically polarized have angular momentum. Is there any experiment that proves/shows this ? like shine a light on something and it rotates ? The correspondence is ...
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1answer
312 views

What is difference between polarization and polarizability and how do we define it? [closed]

The book of physics that I have, uses the word "polarization" sometimes and sometimes uses the term "polarizability" and I am getting confused. And I even checked the dictionary for the term "...
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1answer
55 views

Is a traveling magnetic field identical to the field of a moving magnet?

A traveling magnetic field is created by means of applying out-of-phase currents to a number of coils. Is it identical to that created by a moving magnet, including relativistic effects?
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2answers
48 views

Electropermanent Magnet not working?

I have made an electropermanent magnet based on summary provided on this blog. Here are my details - I have used AlNiCo and Neodymium Magnet together in the core with 10mm diameter and 20mm length. ...
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1answer
293 views

Magnetic field inside of a long rotating cylinder

I have a simple question. I need to find out what is the magnetic field inside a rotating long cylinder with a charged density $\sigma$. The cylinder rotates around its axis with angular speed $\...
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1answer
76 views

Temperature dependence of magnetic domains

Does the size of magnetic domains depend on temperature ? Not able to find any papers on this subject, maybe because there is no such dependence...
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48 views

How to calculate the magnetic flux through a circular area [closed]

A constant magnetic field passes through a circular area (of a solenoid). I'm trying to calculate the flux using $$\phi_B= \int B dA$$ $B=0.2$ Tesla, Diameter of the solenoid (and therefore circle) $...
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2answers
96 views

Magnetic field due to stationary electric dipole

As we know from Maxwell's 3rd equation the magnetic field is given as $$\nabla \times \mathbf{E} = - \frac {\partial \mathbf{B}}{\partial t}$$ Now, if we consider an electric dipole which is ...
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1answer
33 views

Do the two coils in electomagnetic induction repel each other?

I have two coil A and B. Coil A has an ac current going into it and is producting a magnetic field of which is oscilating. Coil B is gaining an ac current from the fluctuating field due to Lenz's law. ...
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1answer
59 views

How does electric potential relate to charge density in a wire?

Lets say we are talking about a wire with resistance connected to a battery. How is the charge density, taking into account bound and free charges, at two points connected to the electric potential ...
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1answer
102 views

Why is electric flux zero if my point charge is outside of a sphere? [duplicate]

I am currently studying Gauss Law and Electric flux , during one of my exercises they ask me to find the electric flux of a sphere with radius 5 cm and my point charge is 10 cm to the left of the ...
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2answers
120 views

Displacment field for linear dielectric

Consider the following statement: In the case of a linear dielectric of arbitrary shape, when no free charges are present, in an external applied electric field $\vec E_0$, then $\vec D=0$. Is ...
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2answers
39 views

Can a coil of wire levitate over another, if so how?

I have been looking at Maxwell's equations (intergral form it's easier to understand): $$ \\\iint_{\partial\Omega}E\cdot dS = \frac{1}{\epsilon_{0}}\iiint\rho\cdot dV \\\iint_{\partial\Omega}B\cdot dS ...
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273 views

Motion in a time-dependent uniform magnetic field

Suppose you have an infinite solenoid generating an uniform magnetic field inside. The field is oriented along the solenoid axis : unit vector $\vec{\bf n}$. The field intensity varies linearly with ...
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1answer
39 views

Torque as if magnetic force were concentred in mean point

From what I see in my textbook and several on line resources (such as this, p. 8-8), I am tempted to think that the magnetic force $I\boldsymbol{\ell}\times\mathbf{B}$ acting on a straight conducting ...
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2answers
163 views

What am I missing about the interaction between electromagnetism and Lorentz length contraction?

I was recently reading this short article: http://galileo.phys.virginia.edu/classes/252/rel_el_mag.html It explains how magnetic and electric fields are related because in one frame of reference where ...
3
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1answer
64 views

What is 'momentum outflow'?

Feynman, in his lecture, on Field momentum, used this term while relating the momentum of matter the field is interacting with & the field's momentum itself. Here is the excerpt: Just as the ...
18
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1answer
543 views

Do light waves precisely follow null geodesic paths in General Relativity?

In special relativity one may show that a plane wave solution of Maxwell's equations (in a vacuum), of the form $A^a=C^a\mathrm{e}^{\mathrm{i}\psi}$ has the following properties: The normal $k:=\...
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1answer
83 views

Why is the electric component of a light field more important than the magnetic component? [duplicate]

This is the common impression. But, is it just a rule of thumb and correct only in some wavelength region or does it hold universally?
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1answer
51 views

Why do radio stations have ranges

If you have been on a long car ride you would know that the signal of a radio station becomes weaker and weaker and then stops. But since radio waves are just a wavelength of the electromagnetic ...
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2answers
64 views

Origin of radio waves

In the same way as the origin of X rays is the excitation of electrons , what is the origin of radio and infrared radiations in this respect ?
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2answers
50 views

Why is thin oil on water colorful but thick oil is not?

I understand that thin oil is color because of constructive/destructive interference, but why isn't the same true for thick oil? Is it because the first ray that reflects off the oil reaches your eye ...
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0answers
54 views

When can I use Helmholtz equation for electraomagnetic wave

The complete Maxwell wave equation for electromagnetic field using the double curl operator "∇×∇×". Only when the transverse condition is hold, this operator can equal to the Laplace operator and form ...
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26 views

Purpose of magnetic charge and current density

I am aware that introducing a magnetic charge density and a magnetic current density makes Maxwell's equations much more symmetric. But in what situations/problems is this beneficial? Could someone ...
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46 views

Deriving Maxwell's equation from the Lagrangian of an electromagnetic field with a charge density $\rho$

I want to derive Maxwell's equation from the Lagrangian of an electromagnetic field with a charge density of $\rho$ The Lagrangian is given by $$L={1\over2}(\epsilon_0E^2-{1\over\mu}B^2)-\rho\phi+\...
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0answers
39 views

Can we prove that EM waves are transverse from Gauss's law of magnetism? [duplicate]

Can we use $ \mathbf B \cdot d\mathbf s=0$ to prove transverse nature of EM waves?
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3answers
315 views

Derivation of transformer equations

I've learnt in high school that in an ideal transformer, $$\frac{V_s}{N_s} = \frac{V_p}{N_p}$$ I looked for derivation for this formula, and in every source I look, the argument goes thus: $$|V_p| = ...
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127 views

How force get transmitted when a magnet attracts iron?

According to particle physics , every fundamental force has its force carrier particle. Photon is a force carrier particle of electromagnetic force but how does force gets transmitted when a iron is ...
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1answer
157 views

Why is there ambiguity of the field energy?

I was reading Field Energy & Field Momentum of Feynman's Lectures on Physics Vol.II. There he deduced energy density $u$ & Poynting's vector as $$u = \frac{\epsilon_0}{2} \mathbf E\cdot\mathbf ...