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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Question about one of the problems of the Bohr model

This is probably extremely basic physics that I don't know, but I'm still going to ask: Say in hydrogen, according to the Bohr model the electron is "really" orbiting the proton, and as a consequence ...
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1answer
82 views

Electric field of a finite, conducting plate

let's assume a finite, conducting plate of dimension: 10m x 10m x 1m. I want to determine the electric field at the middle of one of the plates 10m x 10m surfaces. Using Gauss's law one finds the ...
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1answer
66 views

Field and Charge densities in two dimensional corners and along edges

In jackson's book, we can derive the equation as following. $\sigma(\rho)=\epsilon_{0}E_{\phi}(\rho, 0)\approx-\frac{\epsilon\pi a_{1}}{\beta}\rho^{(\pi/\beta)-1}---(2.75)$ My question is there are ...
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2answers
233 views

Equivalent formulas for electric current density

I am reading an article that states that the "ratio of current to the area for a given surface is known as current density" and is defined as $J = \frac{I}{A}$ where $I$ is the current and $A$ is ...
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2answers
35 views

Explain the concept of “accumulated momentum” for an electron

I need someone to help me understand this equation: $ \Delta p = qEt$ where $q$ is the charge, $E$ is the electric field and $t$ is in seconds. I thought that we were supposed to write momentum in ...
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0answers
178 views

Problem in Classical electrodynamics Jackson

There are two problems. On page 35, Jackson try to use scalar potential that satisfy the poisson's equation. But, i completely lost the step.    ...
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1answer
103 views

Lagrangian density for the electromagnetic field

I want to know how the Lagrangian density for the electromagnetic field is written in the following form:
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0answers
50 views

Electrodynamics boundary conditions with complex $\epsilon$ and $\mu$

I wonder if the usual derivation for boundary conditions at an interface given in EMT textbooks hold for complex permittivity and/or permeability? Do the fields carry phase information themselves(i.e. ...
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2answers
2k views

Deriving Lagrangian density for electromagnetic field

In considering the (special) relativistic EM field, I understand that assuming a Lagrangian density of the form $$\mathcal{L} =-\frac{1}{4}F_{\mu\nu}F^{\mu\nu} + \frac{1}{c}j_\mu A^\mu$$ and ...
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4k views

Magnetic field inside and outside cylinder with varying current density

I am reading through Introduction to Electrodynamics by David J. Griffiths and came across the following problem: A steady current $I$ flows down a long cylindrical wire of radius $a$. Find the ...
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1answer
69 views

Show that $\mathbf{A}$ is a valid vector potential [closed]

Is $$\mathbf{A} = -\frac{1}{2}\mathbf{r \times B}$$ a valid vector potential in the Coulomb gauge? Here's my work so far. Using the identity for the curl of a cross product I get ...
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1answer
149 views
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2answers
3k views

How strong of magnetic field would noticibly attract a person?

There is Iron in blood. Iron is magnetic. Roughly how strong would a magnet have to be to induce a noticeable attraction? It would be nice to know this for several distances. Also, do electromagnets ...
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26 views

Magnetic field of a living human [duplicate]

Is it true that a human body is like a bar magnet? Say our head is north pole while the legs are south pole, or vice versa? If so, can we confirm it by experiment say with the 360 Pro compass etc?
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1answer
80 views

if light is electromagnetic then can light produce electricity or attract metals

what I mean to say is that if light is electromagnetic in nature then shouldn't it show electric or magnetic properties on matters? Like if light falls on a metal it should produce current due to ...
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0answers
119 views

Would a portable superconducting coil “battery” be impractical due to magnetism?

I'm working on a science fiction story idea, and trying to make sure I get all of the physics right. One of the concepts I'm using is superconducting coils as "batteries," because they can store huge ...
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0answers
45 views

Does Dowell's Method of determining losses include winding losses?

Dowell's Method of determining losses uses the DC resistance of the portion of wire in the equation, however does this give the total resistance of the wire? Or only the resistance due to the ...
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1answer
126 views

Can an analog to the Meissner Effect be proposed for matter and gravitational fields?

In the study of electromagnetic fields and quantum electrodynamics we observe and theorize on the behavior of superconductivity and the Meissner effect. Has an analog of these behaviors been proposed ...
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2answers
989 views

Why are EM waves transverse?

I was reading Griffiths' Introduction to Electrodynamics, specifically the section on plane waves. I can see that if we want a transverse wave traveling in the $z$ direction that we are only going to ...
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0answers
35 views

Calculating Velocity in motional EMF

To calulate motional $\epsilon$, we use the following formula: $\epsilon$ = $-vBL$ What is the proper formula from what is below to calulate the velocity of a conductor moving in a magnetic field? ...
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140 views

Odd experiment results (electromagnetics)

Okay, a bit of background. So I'm working on a linear accelerator (you might know it as a Gauss rifle), and I had worked through the equations I thought I needed (Magnetic force, displacement, etc.) ...
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1answer
679 views

What is the conclusion from Aharonov-Bohm Effect?

What is the conclusion that we can draw from the Aharonov-Bohm effect? Does it simply suggest that the vector potential has measurable effects? Does it mean that it is a real observable in quantum ...
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1answer
129 views

Description of the motion of the COM of a system of charged particles moving in a magnetic field

I recently came across a problem where an electrically neutral particle is at rest in a uniform magnetic field. The particle now splits into two charged particles of equal mass $m$; charge and mass ...
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0answers
25 views

Longitudinal dielectric wave does it exist in charge free region?

can we transmit energy through Longitudinal dielectric wave when the surrounding is charge free. Please give me a good link about L.M.D. I a beginner about this topic.
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1answer
243 views

Why there is a time gap between the news presenter and the field reporter?

This one has been in my mind for years, but I simply can not find any reason for this event. The Situation Imagine yourself sitting in the living room while watching television. It's 9 p.m and the ...
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2answers
139 views

Is this an example of Parity violation? [duplicate]

I always hear about parity violation in high energy physics, but what about examples in classical physics? Say we have a wire carrying current in the $+x$ direction, thus generating a magnetic field ...
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3answers
4k views

What is the answer to Feynman's Disc Paradox?

[This question is Certified Higgs Free!] Richard Feynman in Lectures on Physics Vol. II Sec. 17-4, "A paradox," describes a problem in electromagnetic induction that did not originate with him, but ...
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3answers
244 views

How does an electron undergoing uniform circular motion exactly lose energy?

One of the main reasons for the failure of the Rutherford model of atomic structure, it is famously stated, is that the electron undergoing circular orbit loses energy since due to its centripetal ...
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1answer
41 views

What is the dormant common ground between harmonic mechanical oscilators and electromagnetical ones?

When I learnt electromagnetic oscillators I couldn't help but notice that it has many common stuff with mechanical ones. I know that it had to have sinusoidal equations. I (firstly, without ...
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4answers
645 views

Why does optical pumping of Rubidium require presence of magnetic field?

The optical pumping experiment of Rubidium requires the presence of magnetic field, but I don't understand why. The basic principle of pumping is that the selection rule forbids transition from ...
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0answers
18 views

Mechanism of Current flow in Metals [duplicate]

I do not fully understand the mechanism of current flow in conductors. It cannot be that a current starts only when an electron from one terminal of the electric supply physically reaches the other ...
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1answer
173 views

Vector fields and tensors in E&M

I'm confused by a very basic property of electric fields. The electric field is a vector field. Vectors are tensors. Wikipedia has the following statement in the article about the electromagnetic ...
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4answers
4k views

Can superconducting magnets fly (or repel the earth's core)?

If a superconducting magnet and appropriate power supply had just enough $I\cdot s$ (current $\cdot$ length) so that when it was perpendicular to the earth's magnetic field, the force of the ...
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0answers
74 views

Induced emf in a circular conducting wheel

Consider a conducting wheel with $N \in \mathbb{N}$ spokes which is completely in a homogenous magnetic field $\vec{B}$ perpendicular to the wheel plane. ...
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3answers
711 views

What causes radioactivity? Is it a quantum mechanical effect?

I'm just curious what causes radioactivity. I've been told that in the case of alpha decay, since the nucleus is quantum mechanical, there is a probability that the configuration of protons and ...
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1answer
44 views

Help understanding this differential equation of a varying electromagnetic field [closed]

I'm reading some lecture notes (we're studying blackbody radiation and are approaching Planck's formula) but I'm struggling to understand exactly what's being said. So the lecture notes read as ...
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0answers
117 views

Velocities of waves (such as visible light, and radio)

The speed of light in a vacuum is a constant 299,792,458 m/s. In water, it is much slower. Does this happen to such waves as radio, or X-ray? (Would say, a 3 meter wavelength travel slower in air than ...
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2answers
72 views

$ε_0$ affects electric field intensity, but $μ_0$ doesn't affect magnetic field intensity?

I'll be honest: this question is actually a homework problem. I've spent the past hour going through Google and several textbooks trying to answer the question "Why does $ϵ_0$ affect electric field ...
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1answer
412 views

Derivation of Maxwell stress tensor from EM Lagrangian

From Noether's theorem applied to fields we can get the general expression for the stress-energy-momentum tensor for some fields: $$T^{\mu}_{\;\nu} = \sum_{i} (\frac{\partial \mathcal{L}}{\partial ...
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0answers
104 views

Electrical Resistance of Spherical, Contacting Colloids

So, I work a lot with Magneto-Rheological (MR) fluids - which are essentially electrolyte solutions with suspended, paramagnetic colloids/particles. Once a magnetic field is applied to such a fluid, ...
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2answers
7k views

How do transformers work?

A transformer is basically a primary inductor connected to a voltage $U_P$ which you want to transform. You also have an iron rod and a secondary inductor. So when there is a current $I_P$ the iron ...
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1answer
116 views

Does magnitude of a charge influence magnitude of force that individual charge exerts on another charge [closed]

two point charges, q1 and q2, are placed 0.3m apart on the x-axis, as shown in the figure above. Charge q1 has a value of -3 nano Coulomb and q2 has a value of +4.8 x10^-8 C. The net electric field ...
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0answers
112 views

Electrical field of the coil

Take the case of two coils, first one connected to alternating current supply, and the second one connected to load. First coil creates magnetic field, and electric field is reduced by anti-emf ...
3
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2answers
1k views

Prove a dielectric with infinite dielectric constant behaves as a conductor for static fields

I read the following problem: Prove that a dielectric medium for which $\varepsilon \to \infty$ behaves as a perfect conductor in the presence of static electric fields. So, the easy part is that the ...
0
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1answer
186 views

Voltage across an inductor contradicts Lenz's law?

Imagine a simple circuit consisting of an alternating current source connected to an inductor. Assume they are connected in the following fashion: AC source - terminal A - Inductor - terminal B - AC ...
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0answers
109 views

Electric charges on compact four-manifolds

Textbook wisdom in electromagnetism tells you that there is no total electric charge on a compact manifold. For example, consider space-time of the form $\mathbb{R} \times M_3$ where the first factor ...
2
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1answer
29 views

What happens when a force is applied to a flux pinned superconductor?

When a force is applied to a superconductor to move it in a direction that it resists because it is flux pinned, where does the additional energy used to overcome the flux pinning resistance go? E.g. ...
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1answer
41 views

How can we derive that an accelerated charge particle will radiate EMWs?

All the properties of the electromagnetic entity is said to be fully described by the four golden rules of Electromagnetism : The Maxwell's Equations.How can we derive that a charged particle will ...
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101 views

Conceptual question on helmholtz equation

Given the homogeneous Helmholtz Equation $[\triangledown ^2 + \kappa^2] E= 0$, I understand $\kappa$ represents the wavenumber $\omega_0 / c$. In light of this, what physical meaning can be taken from ...
2
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1answer
89 views

What is $\epsilon_\infty$ in this equation and why can it be neglected in the IR?

I'm reading this paper (warning, PDF) and they mention that the complex permittivity $\epsilon$ and complex conductivity $\sigma$ are related through the equation $$\epsilon - \epsilon_\infty = (4\pi ...