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1answer
39 views

EM radiation explanation [duplicate]

Why is it so that a charged particle when accelerated radiates energy? I tried to think if it might be violating the law of conservation of energy but the energy transferred by the external agent may ...
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2answers
61 views

Electric field 0 everywhere inside Gaussian surface

Gauss's Law shows that the electric field everywhere inside a spherical shell of uniform charge density is $0$. Suppose we have a surface which divides space into two disjoint regions (an interior and ...
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1answer
46 views

Does a magnetic field decrease or increase entropy?

My question is just the title: Does a magnetic field decrease or increase the entropy of a system? For example if we apply a magnetic filed to a substance, is the entropy decreased or increased?
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2answers
117 views

Magnetic field due to a charge having uniform velocity

Faraday's law states that "Any change in electric field induces a magnetic field and vice versa". I don't see exactly where these fields are induced, but I assume that these fields are induced at each ...
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1answer
119 views

What are the boundary conditions for EM waves normally incident on the interface between two dielectric media?

An EM wave, amplitude $E_0$, frequency $\omega_0$, is incident upon a material with relative permittivity (dielectric function) $$\varepsilon \left( z \right) = \left\{ \begin{gathered}{\varepsilon ...
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0answers
24 views

Interaction between charged particles as seen from two inertial frames of references

Let us consider two charged particles travelling at a uniform velocity, V, as seen from a frame of reference A . Now let us consider a frame of reference, B, which is also travelling at a uniform ...
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2answers
230 views

Classical Viewpoint on Electromagnetism

Note: This question may be difficult or impossible to answer within the rules of these forums due to its philosophical nature. I will delete the question if I am violating the rules. Onto the ...
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3answers
97 views

The propagation of electric field

In case of a charged particle which is travelling at a uniform velocity, the electric field due to it at a given point doesn't change instantaneously . The reason for this delay in change of electric ...
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1answer
18 views

potential inside a cylindrical shell in terms of the surface potential?

Given a potential distribution $V(\phi)$ at the surface of an infinite cylindrical shell, is there an easy way to derive the potential inside the cylinder. No charges or currents anywhere.
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1answer
232 views

Frequency of rotating coil

Given a coil initially in the x-y plane, rotating at angular frequency $ \omega $ about the x-axis in a magnetic field in the z-direction. This uniform time varying magnetic field is given by $B_z ...
1
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1answer
265 views

What's the reason behind calling cathode rays tube by the name cathode?

I do believe that maybe due to the accumulation of negative electron on the metal surface so we called it cathode. But the thing is that we have studied that regardless of the polarity, the cathode ...
2
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2answers
96 views

Electromagnetic reaction force?

The classical (retarded) Lienard-Wiechert scalar and vector potentials describe the electromagnetic field due to an arbitrarily moving electric point charge. Thus given the motion of electron $A$ one ...
3
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2answers
146 views

Problem with Maxwell's theory

What exactly is the problem with classical Maxwell theory and the blowing up of energy at $r=0$? Does it have any other problems on the classical level?
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0answers
42 views

What are some open problems in classical electrodynamics? [duplicate]

I am about to finish reading 'Introduction to Electrodynamics' by David Griffiths. Throughout the textbook, Griffiths makes frequent references to current literature (mostly articles from American ...
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2answers
145 views

What is the area in Faraday's law if we have only a piece of metal moving in a magnetic field?

If a piece of metal of length $l$ is moving with a speed $v$ in a region where there is a uniform magnetic field $B$ perpendicular to it, there will be a potential difference across its terminals ...
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1answer
67 views

calculating electrodynamic momentum of a dumbbell (consisting of two point charges) in longitudinal motion

I'm working through a paper on momentum in electrodynamics that requires the integration below and would greatly appreciate any help. I'm pretty sure it evaluates to $2/d$ but I can't quite figure ...
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2answers
89 views

Energy conservation in electrodynamic system?

Consider two charged particles initially at rest in the configuration below. Let us assume the following: Starting at time $t=0$, we apply a constant force $f$ to the the bottom particle so that ...
0
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0answers
20 views

Anyone know of a flow chart or list of common/useful consequences of Maxwell's equations?

I just recently started to appreciate the Maxwell equations. I had never really take the time to study them but I feel like I'm finally more familiar with them. I've noticed that it seems like a lot ...
1
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1answer
46 views

What are some practical things one can do with classical electrodynamics and QED?

Many basic types of physics have ready and obvious everyday applications. For instance, basic electromagnetism vector calculus can give great insights into how something as simple as a bar magnate ...
0
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1answer
64 views

Momentum around an accelerated electron

Assume that an electron is accelerated along the +x-axis. The electron will radiate electromagnetic energy and momentum in every direction. But it seems to me that the EM momentum it radiates in ...
5
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1answer
95 views

Time reversal in classical electrodynamics

It is known that classical electrodynamics is time reversal invariant if one assumes that the transformation laws under such operation are $$\mathbf E(t,\mathbf x)\mapsto\mathbf E(-t,\mathbf x)$$ ...
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2answers
321 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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0answers
28 views

Can a charge moving in an open trajectory qualify as current?

It is sometimes said that a point charge is equivalent to an electric current. If it were a steady current, I should be able to find it from Ampere’s law or Biot-Savart’s law. Even if the current is ...
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2answers
181 views

Faraday's law and “infinite induction”

I may have confused after thinking too much about Faraday's law. If an emf is induced in a circuit due to some changing magnetic field, the induced current will be in a direction such that the ...
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2answers
66 views

Force on a loop with induced current

Consider an infinitely long ideal solenoid with current $I$, radius $a$, turns per unit length $n$. Put a closed conducting loop around it (radius $b > a$), on a common axis through their centers. ...
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1answer
46 views

Induced electric field due to a moving wire

An infinite wire carrying a constant current $I$ in the $\hat{z}$ direction is moving in the $y$ direction at a constant speed $v$. Find the electric field, in the quasistatic approximation, at the ...
2
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1answer
76 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
42 views

Equivalence of integrals in Classical Electrodynamics

I have a technical question about a section from Jackson's Classical Electrodynamics 3rd ed. In chapter 14, Jackson derives an expression for $ \frac{d^2I}{d\omega d\Omega} $, the frequency spectrum ...
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1answer
492 views

Recovering all of Maxwell's equations from the variational principle

Whether you can get the first couple of Maxwell equations from a variational principle? In the second volume of the Landau theoretical physics said that it is impossible.
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0answers
40 views

A question about the chapter 6 of jackson?

This is what explained in answer for part b of exercise of 6.1 of Jackson. This represents a plane traveling in the positive x direction and a plane traveling in the negative x direction, both ...
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0answers
27 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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0answers
36 views

Help understanding electromagnetism integral from exercise in MTW? [closed]

I was skimming through Misner, Thorne and Wheeler's book Gravitation looking for exercises to challenge myself with and came across the following exercise on page 178: Verify that the variational ...
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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 ...
2
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1answer
98 views

Far Field Diffraction of EM waves: what does the zero frequency signify?

If you have a system of independently radiating electrons/point-charges, the far field distribution of the EM waves can be approximated by the fraunhoffer diffraction integral, or simply by the ...
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2answers
72 views

Why does the classical electrodynamics Lagrangian density equation have a “field” term and an “interaction” term?

On Wikipedia's page on classical electrodynamics, they state the Lagrangian density equation as follows \begin{equation} \mathcal{L} = \mathcal{L}_{\text{field}} + \mathcal{L}_{\text{int}} = ...
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1answer
112 views

Motion in a Paul trap: $2n$th harmonic with larger amplitude than $n$th harmonic

Using a Paul trap, we captured the motion of a light charged particle (based on a rotating potential applied by AC current). Our rotational frequency was 50 Hz, and so when used FFT on the data, we ...
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12answers
1k views

Metallic and glass sphere of same size released at a height

This question was in my exam today : A metallic and glass sphere of same size were dropped at same height. Which sphere would hit the ground first and why? I have thought about several things and ...
2
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1answer
115 views

Falling charged objects: energy conservation paradox?

Imagine that we start with two oppositely charged objects on the ground, separated by a distance $d$, with charges $+q$, $-q$ and masses $m$. We raise them both up to a height $h$. In doing so we ...
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2answers
146 views

Drift Speed and Current in Two Different Inertial Frames

We have a long, cylindrical wire carrying a constant current I in an inertial frame. At a distance of R from the center of the wire, the magnitude of magnetic field is $μI/2πR$. What is the magnitude ...
3
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1answer
476 views

Width of a photon. And its length

Everyone is always talking about photon's wavelength. But what about its dimensions? What is length and width of it? And does it even have a point to think about such things? Or those dimensions are ...
1
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1answer
72 views

Particle in Oscillating Field

Ignoring quantum effects, what are the dynamics of an electrically charged particular placed with zero initial momentum in an oscillating electromagnetic field, e.g. light?
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0answers
20 views

Charge movement in a rotating EM field (relativistic)

I have this problem let's see if anyone can help me advance a little. (No need to say that if my approach is ridiculous or too hard and anyone knows a better solution, I would like to know it). I ...
0
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1answer
36 views

Plasma frequency

I have a neutral plasma and I need to solve Maxwell equations given the charge and current densities on the plasma. In order to do it I need to know the electrical permittivity $\varepsilon$, I've ...
6
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1answer
90 views

Is the electromagnetic mass real?

In his Lectures on Physics vol II Ch.28-2 Feynman calculates the field momentum of a moving charged sphere with charge $q$, radius $a$ and velocity $\mathbf{v}$. He finds that the total momentum in ...
0
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1answer
52 views

My model of Conductors in “static” condition

My textbook presents an idealization of a conductor as made up of infinitesimal units of charge and derives results. I was not convinced, so I started thinking of how electric fields are in real ...
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0answers
10 views

Force on a small conductor in an EM wave

What forces act on a small, flat conductor subjected to electromagnetic radiation, if the conductor is much smaller than the wavelength? My guess is that the magnetic field component of the wave ...
1
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1answer
20 views

Non-constant potential distribution

I know that given a static charge distribution, the closed loop integral of the electric field is zero. But what if the charges are moving, i.e, the potential at a point a changing. Is the line ...
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0answers
35 views

Electric and Magnetic Field Created by Moving Electron

From a classical perspective, what are the electric and magnetic fields created by a single electron, initially located at the origin and moving along the $x$ axis with velocity $v \ll c$? I'm ...
2
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2answers
56 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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2answers
123 views

Charge vs Charge Density in classical electrodynamics

What is assumed to be a more fundamental physical quantity in classical electrodynamics. The charge density as a scalar field or the physical entity charge.