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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Potential energy from opposing magnets repelling each other with a gap of 1 mm

I have two powerful rare earth magnets, that are separated by a distance of 1 mm. I applied energy to bring them closer to each other, hence increasing the potential energy. Now, when one of the ...
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1answer
149 views

If an electron can't go faster than light, how is energy conserved here?

I came across a physics question recently about accelerating electrons with potential differences and it got me wondering. Here is the question that brought the concept to mind. First off, imagine ...
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2answers
723 views

Is Newton's third compatible with retarded Lorentz force?

In Griffiths Introduction to electrodynamics it is said that Newton's third law is not valid in electrodynamics, but, in the example given, the it does not consider the retarded values for the fields ...
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2answers
3k views

Gauge Invariance of the Hamiltonian of the electromagnetic field

The Hamiltonian for an electron of mass $m$ and charge $e$ in an exterior electromagnetic field is $$H=\frac{1}{2m}(p-(e/c)A)^2+e\varphi.$$ The corresponding (via canonical quantization) quantum ...
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1answer
117 views

The force exerted on a magnetic dipole $m$ in magnetic field $B$

How can one prove that: The force exerted on a magnetic dipole $m$ in magnetic field $B$, in addition to $F=\nabla(m\cdot B)$, can be expressed by $$F=(m\times \nabla)\times B.$$
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1answer
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Proving Lorentz invariance of Maxwell equations

I've read somewhere that one does not need to prove Lorentz invariance of the Maxwell equations $F_{\mu\nu,\sigma}+F_{\nu\sigma,\mu}+F_{\sigma\mu,\nu}=0$ because it is "manifestly Lorentz invariant" ...
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1answer
118 views

Will a copper plate spin near a motor?

I'm revising old physics papers and I'm unable to answer the following question. If a horizontal copper plate is suspended at the centre by a thread, over a motor whose axis is aligned vertically, ...
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2answers
794 views

What would give us more heat ? infrared or microwaves?

As we know that our body is made up mostly of water and the frequency of vibration of water molecules matches that of microwaves which is the working principle of microwave ovens. When we come in ...
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4answers
752 views

Why doesn't the magnetic field of a coil “cancel out”?

In a coil, we can see that the current moves right, then left, then right, then left, and so on as it travels down the coil. According to the right-hand grip rule, isn't the magnetic field going in ...
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0answers
90 views

High Frequency radio wave propagation time

What is the maximum time for an HF radio wave to reach its destination regardless of propagation mode? Since electromagnetic waves' speed is close to the light speed in the air, can we say it is ...
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1answer
5k views

What force particle mediates electric fields and magnetic fields?

The force carrier for magnetic fields and electric fields are supposedly photons. I don't get it: 1) Wouldn't that mean that a charged particle (e.g. an electron or even a polarized H2O molecule) ...
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1answer
197 views

Ising model for ferromagnetism is not intuitive

In the Ising model for ferromagnetism a lower energy is assumed when two spin magnetic dipoles are aligned parallel to each other and the energy is higher when they are antiparallel. If I take two ...
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1answer
73 views

Help solve this paradox?

If in a uniform magnetic field a conducting rod moves on a set of joined friction less rails which are perpendicular to the direction of magnetic field, a potential is developed and current flown this ...
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1answer
347 views

Magnetic muscles [closed]

Consider two flexible sheets glued at the ends, with electromagnets attached to both of them, so that depending on the directions and strength of currents passing through them, the system could be ...
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3answers
500 views

What force creates ions out of neutral atoms?

Consider the reaction between Na and Cl to form NaCl. Na loses an electron and "gives" it to Cl because this makes both atoms more stable. But what forces "pulls" the electron from Na to Cl? Both ...
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4answers
382 views

Is the concept of a field necessary to electrodynamics?

I've read (in Griffith's text) that it is "possible, though cumbersome" to dispense with the field concept in electrodynamics entirely and instead use an action-at-a-distance theory. What exactly is ...
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1answer
111 views

Do virtual photons/magnetic fields change speed in a medium?

Light can be slowed down to a walking pace and even stopped in a medium of hot rubidium vapour. If an electromagnet was placed in this medium and turned on would the popagation of the magnetic field ...
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2answers
211 views

Rubbing a rod with silk?

Rubbing a glass rod with silk causes charges to be exchanged and consequently both objects get charged. Why do the objects have to be "rubbed"? I get that one has a stronger pull on the electrons ...
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2answers
350 views

Geodesic for Electromagnetic forces

Considering the fact that electrons tend to take the maximum conductance path to flow from A to B. This is justified by saying that $\vec{E}$ is larger in conductors. But once similarly it was thought ...
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2answers
735 views

Why do waves diffract?

There have already been a lot of questions on this site on diffraction but I still believe this one might be slightly different. In electromagnetic waves, diffraction and any other phenomenon of wave ...
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1answer
581 views

How does temperature affect magnets?

How does an increase/decrease in temperature affect: Strength of the magnet Polarity Other changes? I have made some research and I have found that increasing the temperature turns ferromagnetic ...
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2answers
667 views

Where does the 3rd and the 4th Maxwell's equations lead us in the end?

Take the 3rd and the 4th equation from this table. The first tells us that an electric field can be generated by a magnetic field. The second, says that a magnetic field can be generated from an ...
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5answers
7k views

Why do electromagnetic waves oscillate?

I've been considering this question, and found many people asking the same (or something similar) online, but none of the answers seemed to address the core point or at least I wasn't able to make ...
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2answers
827 views

Electric charge is lorentz invariant

I know that electric charge is lorentz invariant quantity and I can easily think of experiment to check that. Is a though experiment that can prove that also?
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1answer
293 views

Will magnetic field lines change when a magnet gets closer to other magnets?

As many books tells that the magnetic field can be illustrated by a bunch of field lines, which start from one pole and end up at the other. If I have a strong uniform field with field lines along ...
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3answers
239 views

About field gradient

I read the term field gradient in most of the article about magnetic field. I search it online but most of the explanation is about the math. I wonder in physics, what the gradient field really mean? ...
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2answers
177 views

If you “disobey” the constraints of the Kramers-Kronig relations, what happens? Do you get non-physical results?

If you "disobey" the constraints of the Kramers-Kronig relations, what happens? Do you get non-physical results? I am simulating reflection and transmission off/through a slab of material. I specify ...
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1answer
2k views

A basic question: what is accelerating voltage? [closed]

Or would it be acceleration voltage? Acceleration sounds like it makes more sense, but my paper says accelerating. What are possible ways you could go about calculating it?
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1answer
230 views

Earnshaw's theorem and springs

Earnshaw's theorem states that the Laplacian of the potential energy of a small charge moving through a landscape full of static negative and/or positive charges (and gravity) is zero. Thus you can't ...
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3answers
17k views

Reducing Eddy current losses

Eddy currents are setup in any metallic block which is in the vicinity of changing magnetic fluxes. These primarily cause heat losses, and in certain cases causes damping of the relative motion ...
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1answer
973 views

Imaginary part of Poynting vector

When I am studying the total reflection phenomenon, I calculated the Poynting vector of the transmitted wave, which can be written as $S_t=A(k_{x}\hat{x}+i\alpha\hat{z})$ A is some constant. I ...
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0answers
654 views

Complex polarizability of a dielectric sphere in a homohenious electric field

It is well known that complex polarizability of uniform dielectric sphere with radius $r$ and complex permittivity $\hat\epsilon_{in}(\omega)$ placed in a medium with complex permittivity $\hat\...
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1answer
84 views

Objects made up of electrons?

Say you have a neutral rod, and you bring a positively charged rod beside it (call the side the charged rod is brought near side A and the other side side B). The electrons from the side B will start ...
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2answers
6k views

Why do atoms stick together?

The atoms in my table "stick together" to form a rectangle. Why? What makes them stick together? I know about ionic/covalent bonding etc., but consider a sheet of pure iron. Just atoms of one element....
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1answer
82 views

Electromagnetism Permittivity simple question

Just a simple question regarding $\epsilon$, the absolute permittivity. I read that it measures the resistance to of certain medium to 'permit' the formation of an electrical field. Does this mean ...
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3answers
459 views

How to understand holography and hologram

I've spent some time reading wiki etc. What I get now is that apart from the normal light amplitude information, holograms also record the phase information of light. But this is so difficult for me ...
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1answer
99 views

A simple way to calculate the potential electrical output of a magnet. [closed]

I am a software engineer. This I understand very well. I am attempting to build a prototype with arduino circuits. This I know a little less about. Part of my project requires a electro-magnetic ...
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1answer
17k views

Dielectric constant or permittivity of metals

I'm wondering what the dielectric constant or permittivity of metals is --particularly copper. Do metals have an infinite permittivity?
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1answer
56 views

Diffraction from interatomic spacing

In diffraction from a single slit, we learn that the angular width of the central maxima, is given by $2\sin^{-1}\frac \lambda d$. For $d\approx \lambda$, the incoming wavefront should be spread to ...
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2answers
3k views

Does AC current produce EM waves?

Does AC current in simple wires produce electromagnetic waves? AC current entails very rapid changes in polarity and therefore the electrons in the metal will feel rapidly changing forces which should ...
2
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1answer
82 views

Separating the convective and the inductive electric field

My question is basically this, if I am only able to measure the total electric field and the magnetic field at a few discrete points in space and time, is it possible to separate the convective and ...
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2answers
614 views

AC Electromagnets

Could someone help explain the uses of AC electromagnets. Wherever I look it says that DC electromagnets create stronger magnetic fields. I understand why AC electromagnets could be used in ...
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2answers
71 views

Is charge 'localization' implicit in the idea of current?

If it was possible for charge to assume arbitrary densities, like we often see electrostatic exercises, and one could spread charge density uniformly over a ring, then how one would, theoretically, ...
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1answer
692 views

Homopolar motor and Lorentz force

My second grader thought making a homopolar motor for her science experiment would be fun. And, it was. Now I am trying to explain how it works and the Lorentz force. Please help me by giving me a ...
2
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1answer
220 views

Why in some cases $0\alpha$ component of stress-energy tensor don't form 4-vector?

In electrodynamics there is Poynting vector and energy density, which refer to $0\alpha $ components of stress-energy tensor, don't create 4-vector. Analogous situation with mass density and mass ...
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1answer
70 views

How 4-vector nature of the value is connected with it's conservation law?

In electrodynamics Poynting vector and energy flux of field don't create 4-vector. Also they aren't conserved independently from substance (conservation law includes summand connected with current ...
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1answer
119 views

Show that the electric field, $\mathbf{\vec{E}}$ cannot be generated by any static distribution of charges

Show that the electric field $$\mathbf{\vec{E}}=\begin{pmatrix}0 \\ E_0x \\ 0\end{pmatrix}$$ where $E_0$ is a constant, cannot be generated by any static distribution of charges. I understand that ...
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1answer
446 views

Is it possible to extract the index of refraction from reflection/transmission measurements like this?

I'm trying to manipulate some data to see if my analysis method is reliable: I want to use transmission and reflection measurements within a certain wavelength range to get the index of refraction (...
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3answers
2k views

Work done by Lorentz Force in case of motional emf

In the classical example of the slidewire generator where the rod slides on a U-shaped conductor in a magnetic field, we get a charge separation due to the Lorentz force. The way the induced emf is ...
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4answers
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Why are there dust particles on TV screens?

My professor gave us the following reason: The screen is positively charged. When dust particles fly near it, the positive charges in the screen induce a charge in the dust particle, pulling the ...