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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79 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 ...
3
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1answer
75 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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4answers
3k views

Does a magnet contain (and potentially produce) energy?

Very quick question, does a magnet contain energy? The general consensus seems to be, it does not. And this is generally confirmed by the fact that it would break the first law of thermodynamics. ...
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1answer
169 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 ...
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2answers
94 views

How does a charge distribution evolve with time? (in classical electrodynamics)

It is often stated that, in classical electrodynamics, the electric and magnetic fields determine uni vocally the dynamics of a charge distribution distribution (how it evolves in time). I can more or ...
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2answers
286 views

What symmetry is associated with conservation of Lipkin's zilch?

The 'zilch' of an electromagnetic field is the tensor $$ Z^{\mu}_{\ \ \ \nu\rho}=^*\!\!F^{\mu\lambda}F_{\lambda\nu,\rho}-F^{\mu\lambda}\,{}^*\!F_{\lambda\nu,\rho} \tag1 $$ given in terms of the ...
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1answer
49 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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56 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
256 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
492 views

What is the Poynting vector of an electron?

On chapter 27 of "The Feynman Lectures on Physics Volume II: Mainly Electromagnetism and Matter", Richard Feynman says: Finally, in order to really convince you that this theory is obviously nuts, ...
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2answers
43 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
32 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
56 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
45 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
92 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
81 views

Does a correct application of the Lorentz force to find induced emf need resistance?

The emf between two points is defined as the work done per unit charge on the charge when moved (along a given and stated path) between those two points. To find the emf due to a magnetic field the ...
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1answer
98 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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4answers
150 views

Why the electromagnetic waves propagate and do not stand still?

I know that the electromagnetic waves are alternating electric and magnetic fields. But I don't get why they can't stand still and alternate at the same point. I would like a logical and verbal ...
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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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1answer
38 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 ...
3
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1answer
60 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 ...
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2answers
74 views

After a slit, are photons polarized?

It seems clear that a photon stream as well as single photons behind a well designed slit - the right slit width for a given wavelength - are polarized. Means, the electric fields of such photons are ...
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1answer
81 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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2answers
62 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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1answer
48 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
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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51 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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1answer
5k 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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2answers
366 views

Is it possible to make an electromagnet with two like ends?

Is it possible to create an electromagnet with one continuous wire with 2 like poles (i.e. both ends either north or south)?. Visualising it with the right hand screw rule for current carrying ...
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0answers
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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0answers
44 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 ...
5
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1answer
156 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 ...
11
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1answer
298 views

How do higher-order optical chiralities look like?

The optical chirality of the electromagnetic field is a conserved quantity, analogous to the energy density, linear momentum density, and angular momentum density, which describes how chiral the EM ...
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4answers
149 views

The electric field of a ring of charge

I'm studying ahead for my Electricity and Magnetism course for next quarter. I have a hard time approaching this course qualitatively, not quantitatively as a computer science major. I was wondering ...
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2answers
79 views

Voltage and Current in an Electromagnet

I'm having a little bit of trouble with electromagnets. I know that a magnetic field is induced by current flowing through a conductor, but what role does voltage play? Does it have an effect on the ...
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3answers
293 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| = ...
4
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4answers
106 views

About the uniqueness of the displacement current

In the Maxwell-Ampère equation, i.e.: \begin{equation} \nabla\times\vec{B} = \mu_0 \vec{J} + \mu_0\epsilon_0 \frac{\partial \vec{E}}{\partial t} \end{equation} the $\vec{J}_d$ term: $$ \vec{J}_d := ...
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0answers
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240 views

Why do we talk of the “weakness of gravity” rather than “the surprising charge to mass ratio of particles”?

The relative strength of gravity and electromagnetic forces is obvious — stand on a sheet of paper, and even with the whole of Earth pulling, your motion is stopped by the electric fields inside that ...
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0answers
31 views

Does a permeable core increase flux or only concentrate what's already there?

I understand that that introducing a high-permeability core (say iron) into an air-core coil (having some current running through it) increases the flux density $B$ in the core. Does the iron core ...
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2answers
87 views

Torque due to magnetic force on a loop

I read that the torque due to the force exerced by a uniform magnetic field $\mathbf{B}$ on a closed conducting rectangular loop is $$\boldsymbol{\tau}=I\mathbf{S}\times \mathbf{B}$$where $\mathbf{S}$ ...
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0answers
21 views

Angular current in torus

I'm trying to find the magnetic field due to a torus anywhere in space. I'm approximating it as a circular current loop. What I'm having a hard time with is the current in the theta direction, that ...
0
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1answer
31 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
20 views

Effect a ferromagnetic object has on the current, voltage and resistance of the coil accelerating it

I've done some research but I'm unsure of what to search or where to look. I would like to know what effect the motion of a ferromagnetic object has on the voltage, current and resistance of the coil ...
17
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2answers
21k views

How and why does accelerating charges radiate electromagnetic radiation?

Lets consider it case by case: Case 1: Charge particle is at rest. It has electric field around it. No problem. That is its property. Case 2: Charge particle started moving (its accelerating). We ...
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4answers
7k views

Confusion between Electric field and Magnetic field of a charged particle.

Consider a charged particle (electron or proton) at rest. It is surrounded by its own electric field. Now consider an electron moving with certain velocity (less than speed of light), Still is there ...
2
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2answers
537 views

Interactions Between the poles of the same magnet

Suppose a bar magnet like so : [(+m)----------------(-m)] Do the two individual poles interact with each other ? If yes then how do you explain it if no then ...
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6answers
8k views

How to find the poles on a spherical magnet?

To any nonmagnet, the whole sphere is a magnet. To another spherical magnet though, there is a rough area on the surface where it is strongly repelled. Given a spherical magnet, how should the poles ...
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8answers
7k views

What is the electric field generated by a spinning magnet?

Consider a cylinder of permanently magnetized material, with uniform magnetization pointing along the cylindrical symmetry axis (the $z$-direction). The magnet is rotating about its cylindrical ...
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1answer
127 views

Faradays law for a falling magnet

If experimentally it can be proven that the velocity of a falling magnet through a coil is proportional to the emf induced, has it also been proven that dB/dt is proportional to the emf induced and ...