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.

learn more… | top users | synonyms (6)

0
votes
0answers
23 views

Discrepancy between Sidebottom and Kittel's Solid State Textbooks

There seems to be some discrepancy between Sidebottom and Kittel in the value of the diamagnetic susceptibility of the hydrogen gas. Sidebottom states that it is $-2.2\times 10^{-9}$, while Kittel ...
0
votes
0answers
30 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 ...
0
votes
0answers
10 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 ...
0
votes
1answer
53 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.
1
vote
0answers
23 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 $\mathscr{E}=-L\dfrac{dI}{dt}$. ...
0
votes
1answer
50 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 ...
0
votes
1answer
20 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 ...
0
votes
1answer
48 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} ...
1
vote
0answers
47 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 = ...
1
vote
1answer
86 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 ...
5
votes
1answer
154 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 ...
-1
votes
7answers
394 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 ...
0
votes
0answers
25 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 ...
0
votes
1answer
53 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 ...
0
votes
1answer
35 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 ...
2
votes
0answers
67 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 $2v_f$ (shown dark in Fig. 1). While the droplet is in rest on a surface, if I ...
0
votes
1answer
50 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 ...
0
votes
1answer
45 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 $p = h/\lambda_e$ ...
1
vote
2answers
92 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 ...
0
votes
2answers
64 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 ...
1
vote
1answer
39 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 ...
-1
votes
0answers
25 views

electromagnet field vs resistive losses

The field of an electromagnet depends on current and turns, but if significant energy is lost to resistive heating in the leads to the magnet what is the effect on the field, if any?
1
vote
4answers
85 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 ...
0
votes
1answer
38 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 ...
1
vote
0answers
24 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?"
1
vote
1answer
160 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 ...
1
vote
2answers
60 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 ...
1
vote
1answer
71 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 ...
1
vote
0answers
19 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 ...
0
votes
0answers
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 ...
0
votes
1answer
32 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 ...
0
votes
1answer
23 views

Change in work done by dipole

How would you explain the intuition behind the equation $dW = -\tau d\phi$ for an electric dipole?
13
votes
9answers
1k 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 ...
0
votes
0answers
34 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? ...
0
votes
1answer
58 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 ...
0
votes
0answers
14 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 ...
0
votes
0answers
14 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. ...
0
votes
2answers
59 views

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

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 ...
1
vote
1answer
46 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 ...
0
votes
0answers
15 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 ...
1
vote
1answer
58 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? ...
3
votes
1answer
62 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 ...
-1
votes
1answer
60 views

Did light already experience death of the universe immediately after the Big Bang?

Let's suppose that I am on Point A with a light-beam launcher. My goal is to launch a beam of light to Point B, which is about 8,000 quadrillion light years away. My friend named Jack is at Point B ...
-1
votes
0answers
38 views

Why does light not destructively interfere when coming from an object?

This is something that I have been wondering for a long time. How come when an object is scattering light, the light does not destructively interfere? Should we not be able to find for every light ...
0
votes
0answers
43 views

Time reversed Abraham-Lorentz reaction force

The Abraham-Lorentz radiation reaction force on a charged particle is given by: $$\mathbf{F_{rad}} = \frac{q^2}{6\pi\epsilon_0c^3}\mathbf{\dot{a}}$$ I understand the situation where one fires a ...
0
votes
2answers
61 views

Field Vectors and satisfying Maxwell's equations

If I have an electric field that its direction is parallel to the direction of the wave propagation, it will not satisfy Gauss's law for vacuum. However we can say it satisfies Gauss's law for ...
0
votes
0answers
43 views

Extremely long wavelength electromagnetic radiation

High frequencies are used to eject electrons, because only electrons can really be affected at such a small wavelength scale. But could an electromagnetic wave have such a ridiculously low frequency ...
13
votes
3answers
733 views

How to experimentally reconstruct Maxwell's equations from scratch

What are the minimal experiments would one need to perform in order to reconstruct Maxwell's equations from scratch, assuming even the concepts of $\vec E$ and $\vec B$ are unknown? While I'm not ...
1
vote
1answer
46 views

Magnetic Field Direction — a single one — on a current carrying wire

Take the wire on the right hand side. It experiences a force to the left of the page. To experience a force pointing left-wards, the magnetic field must point into the page. Why does it point into the ...
6
votes
3answers
135 views

Do the integral forms of Maxwell's Equations have limited applicability because of retardation?

In the usual bookwork treatment, it is easy to show that the differential and integral forms of Maxwell's equations are equivalent using Gauss's and Stokes's theorems. I have always thought that ...