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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Derivable Concepts in Mechanics and Electromagnetism

In Classical Mechanics, one of the possible foundations is based on three concepts aka mass(equivalent to energy), length and time. This is a foundation because we can model everything ( pressure, ...
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4answers
217 views

The fainter the star, the further away it is?

If a star appears to be very faint, would it imply that it's really far away or that it's luminosity is minimum?
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0answers
197 views

Proof that electric and magnetic fields in a EM wave are perpendicular

Is there a general proof, that for electromagnetic waves the magnetic and electric fields are perpendicular? The only ones I can find only focus on plane waves.
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1answer
105 views

Induced EMF of a rectangle loop?

If a rectangle loop would move inside a magnetic field, would the induced $\epsilon$ be zero? Why would it? Is it due to the induced $\epsilon$ canceling out? Being in series and in opposition? ...
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2answers
369 views

Evidence for electrodynamics in curved spacetime

Field theories in curved spacetime is usually formulated by integrating their Lagrangian over the curved spacetime. For example, for electrodynamics, we have the action $$ S = \int d^4x \left( ...
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2answers
312 views

Why is Terahertz radiation so hard to generate?

This paper (and many others I've read) claim that searching for ways of producing THz radiation is a high-interest research topic. However, something I've just never understood is why it's so hard ...
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1answer
81 views

Is what i think Magnetism is, Magnetism? [closed]

So, i was randomly thinking about magnets one day and decided to try and figure out what Magnetism actually is. This is what i came up with: Magnetism, the magnetic field of an object, is a form of ...
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0answers
59 views

Physical significance of toroidal moments

The electric field of a toroidal dipole moment $\vec t$ is the same as the electric field of a electric dipole moment $\vec p$, except that it is scaled with the factor $ik$. The $i$ makes the ...
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33 views

Mosotti-Clausius formula from the first principles

I would like to understand how to get the Mosotti-Clausius formula for the dielectric susceptibility of a dielectric material with spherically symmetric molecules from the first principles, where by ...
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1answer
95 views

Separating aluminium sheets with magnetic field

all! I would like to know if it would be possible to induce a strong enough magnetic field in sheets of aluminium so they would be separated between each other, like when you use magnets to separate ...
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0answers
48 views

Why does charge build up at the boundary surface of two media?

On a homework problem, we are asked to to use the first two Maxwell equations, $$\nabla\cdot \mathbf{B} = 0$$ $$\nabla \cdot \mathbf{D} = \rho$$ to show that along the boundary surface of two ...
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19 views

The movement of magnetic rings along a current carrying wire

The creation of circular magnetic rings around a direct current is easily illustrated by sprinkling iron filings onto a horizontal sheet, which has the conducting wire passing vertically through its ...
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1answer
77 views

How did Lenz derive his law?

I read about it but am not sure if it actually follows law of conservation of energy.I tried reading some reference books but could not find the derivation of the law.
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1answer
39 views

What does an induced emf do to a loop of varying current?

I know that the magnetic flux through a loop in space is proportional to the current such that $\Phi=LI$. Therefore a change in magnetic flux will produce an emf, so $\mathscr{E}=-L\dfrac{dI}{dt}$. ...
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1answer
57 views

Resistivity dependance on temperature

I'm just starting to study electricity and magnetism. The resistivity $\rho$ of a conductor is defined as $$\rho = \frac {\mathbf E}{\mathbf j},$$ where $\mathbf E$ is the electric field and ...
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1answer
54 views

How is one side of a magnetic material attracted to opposing poles of a magnet?

Magnetic materials are commonly diamagnetic, paramagnetic or ferromagnetic. The following pdf (http://faculty.washington.edu/mrdepies/122/Workbook_122/WB_Solution_Ch32.pdf; see exercise #2) claims ...
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1answer
69 views

How do I properly calculate the curl of the Aharonov-Bohm flux line vector potential?

Given a vector potential describing an infinitely thin line of flux, $$\vec{A} = \frac{\Phi}{2\pi r} \vec{e}_\varphi,$$ How can I calculate the curl so that the magnetic field is given by $$\vec{B} ...
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0answers
57 views

Noether current scale transform of EM

I'm trying to solve a question about scale tranform of free EM. I got the next trnaform rules (these two line where EDITed later) $\delta x = -bx$ $\delta A = bA$ the current I got $D^\mu = ...
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1answer
103 views

What are the electromagnetic fields of a photon?

I'm looking for expressions for the electromagnetic fields (preferably $E$ and $B$) of a typical photon which is localised in space to some extent (i.e. I'm not interested in the infinite plane wave ...
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1answer
174 views

How will SR EM Lagrangian change if we find a magnetic charge?

When we introduce electromagnetic field in Special Relativity, we add a term of $$-\frac e c A_idx^i$$ into Lagrangian. When we then derive equations of motion, we get the magnetic field that is ...
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6answers
533 views

Momentum of stationary electron in a curl free vector potential

The essence of this question is simplicity itself: There is an electron in a curl-free $\vec{A}$ field. The electron is stationary so its m$\vec{v}$ momentum is 0. However, it has "momentum" from ...
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0answers
44 views

Deriving Lorentz force on a point charge moving perpendicular to external magnetic field using Maxwell stress tensor?

I heard that when we use Maxwell stress tensor to derive Lorentz force on a point charge moving perpendicular to external magnetic field, the result is not $qvB$ but $\frac{2}{3}qvB$. This should be ...
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1answer
72 views

How do I set up the Biot-Savart equation for the magnetic field inside a circular loop of current

The way I have derived the magnetic field for inside the current loop of radius $R$ is very different from how I have seen it online. Generally they take the equation for the magnetic field at a point ...
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1answer
43 views

Electromagnetic tensor notation

How do you transform between the electromagnetic tensors $F_{\mu\nu}$ and $F^{\mu\nu}$? $$ F_{\mu \nu}= \begin{pmatrix} 0 & E_x & E_y & E_z \\ -E_x & 0 & -B_z & B_y \\ -E_y ...
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0answers
98 views

What does the net force do with this droplet in magnetic field?

Consider a spherical droplet with radious $R_d$ which is coated with an immiscible ferrofluid with volume equal to $2v_f$. While the droplet is in rest on a surface, if I approach two magnets in ...
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1answer
81 views

What is the charge density in the proof of Earnshaw's theorem?

I am trying to understand the proof for Earnshaw's theorem. Though the theorem states that a collection of point charges cannot be maintained in a stable stationary equilibrium configuration ...
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1answer
95 views

How do you calculate the free space wavelength of an electron? [closed]

The only thing I know about an electron is that its mass is $m_0 = 9.109 * 10^{-31} kg.$ How would you calculate the wave length from here? Ok, using de Broglie's relation we have $p = h/\lambda_e$ ...
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2answers
149 views

Why do we need the curl and divergence on maxwell equations? [closed]

Is there a particular reason to use curl and divergence on the description of electromagnetic fields? Given boundary conditions, if someone knows the curl and divergence of any field, is it always ...
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2answers
112 views

Why can't mutual inductance create perpetual electricity?

Consider the following image. When the battery is connected then current starts flowing through the primary cicuit and magnetic flux changes through the coil, this causes electricity to be induced ...
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1answer
84 views

Energy stored in a coaxial cable before reaching breakout field

Yesterday I had a (multiple choice) exam and the following was one of the questions. We have a coaxial cable (basically two coaxial conducting cylinders) with the inner radius of $a$ (variable) and ...
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4answers
99 views

Where does the energy of the electric cable come from?

I'm stuck on one really simple example, I can't figure out what's happening to energy here... (This is not homework) Let's consider an uncharged electric cable, we'll model it by an infinite ...
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1answer
81 views

Can someone reconcile the Boltzmann transport equation with the Maxwell equations for photons/light?

Having taking courses in both physics and nuclear engineering, I've noticed that the two fields tend to describe photons/light in two different settings. In nuclear engineering, the radiative ...
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0answers
29 views

Will any massless particle/object in the vacuum of space always travel at the speed of light? [duplicate]

The question as stated above, is "Will any massless particle/object in the vacuum of space always travel at the speed of light?"
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1answer
187 views

How to observe Newton's third law violation? [duplicate]

I met a statement, that Lorentz force disobeys the third Newton's law. Is this true? Also I met some setups, where it was stated, that third law is violated. Also I met some explanations about WHY ...
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2answers
113 views

Using Electric Potential to Float an Object

I've been trying to answer the following question but I'm stuck at one step. The question essentially states that a magician is trying to perform a "floating objects" act, for which she has a thin ...
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2answers
229 views

Why is there no induced electric field in the experiment (Faraday's Law)

Below are three circuit diagrams for each of Faraday's experiments that allowed Faraday to come up with Faraday's Law. In Griffiths' Introduction to Electrodynamics Griffiths states (on page 302 of ...
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0answers
39 views

Fresnel equations of S and T waves and Isofrequecy Curves: Feeling confused

First of all sorry for my (probably) bad english. I've been studying propagation of light in anisotropic media from the Born and Wolf and from Landau "electrodynamics in continuous media" and I'm ...
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19 views

a quandry into electromagnetic fields and generating usable power

I have a question, regarding generated electricity via earth's magnetic field. What if you put a coil of flat high conductive wire with short intervals between the loops on a maglev train? Would that ...
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1answer
61 views

Relationship Between Conductivity and Lossiness of a material

I read that a material is loss-less if the conductivity is zero. I have always learned that conductivity is a measure of how easily the material can conduct a current. Does this then mean that the ...
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1answer
49 views

Change in work done by dipole

How would you explain the intuition behind the equation $dW = -\tau d\phi$ for an electric dipole?
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9answers
2k views

“Reality” of EM waves vs. wavefunction of individual photons - why not treat the wave function as equally “Real”?

In thinking how to ask this question (somewhat) succinctly, I keep coming back to a Microwave Oven. A Microwave Oven has a grid of holes over the window specifically designed to be smaller in ...
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0answers
35 views

why do everything tend to be stable? [duplicate]

Whenever I ask'why does a proton attract electron?' People say that 'because they are oppositely charged' But I want to know why do opposite Charges attract each other and tend to become neutralised? ...
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1answer
98 views

Finding Transversal Components from Longitudinal component for Electric and Magnetic Field in a cylindrical coordinates system

Can someone explain why this two equation are equivalent? $\nabla_T$ denotes the transverse two-dimensional nabla operator: $\nabla_T=\hat{x}\frac{\partial}{\partial ...
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0answers
24 views

Why breakdown voltage decreases with increasing FWHM of an asperity?

I am running a simulation for a dielectric breakdown. I am getting breakdown voltage decreases with FWHM, which does not make sense as field enhancement is higher for a small width. Simulations solve ...
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0answers
18 views

How to conserve power at the reflection boundary of perfect conductor?

How to conserve energy during the reflection ? I am taking the abs value of $\sum E_{field}$ along spacial direction and plotting at each time step. Then it is normalized to the input E filed. ...
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2answers
147 views

Do Ampère's and the the Biot-Savart law give different answers for the magnetic field of a circular loop? [duplicate]

I have been trying to calculate the magnetic field due to a circular loop using Ampère's circuital law. However, I am unable to do the same. Whenever I apply the law, while integrating length ...
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1answer
72 views

How can we define the energy stored in a (conservative) force field?

I have come to know from my textbook that energy is stored in the E-field of a capacitor, in the B-field of an inductor and so on. Take the example of an inductor. The derivation bewilders me ...
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16 views

How can two Electromotive forces exist at the same time?

This thought is derived from counter-induced $\epsilon$ in a motor, there is an applied voltage source that creates an $\epsilon$ to allow current flow from the power supply to the motor's windings to ...
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1answer
89 views

Where did earth's electric charge come from?

I get between .7 and 1.3 volts out of a 20 foot tree depending on the weather. I understand this is "telluric" electricity with pretty low amperage. Where is this charge coming from? The sun, right? ...
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1answer
199 views

Why is Bose-Einstein condensation a phase transition?

Bosons may succumb to a Bose-Einstein condensation at a certain critical temperature $T_c$, thus entering the BEC phase. The only thing I know about the BEC is that since we are talking about bosons ...