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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2
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2answers
29 views

Why is bandwidth, range of frequencies, important when sending wave signals, such as in radio?

So in wired/wireless networking and radio, signals are sent in form of wave. Then the concept of bandwidth comes in, which is the difference between highest frequency and lowest frequency in a signal. ...
1
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1answer
42 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 ...
0
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1answer
69 views

What is the significance of the difference in the eigenvalue equations of Bloch functions for electrons vs photons?

any text on photonic crystals will highlight the almost perfect analogy between electrons in a periodic potential and photons in a periodic dielectric. The analogies are: $$V(\vec r + \vec R) = ...
2
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3answers
95 views

Describing a circular current loop as delta function

It would be really nice to see how Jackson got eqn 5.33 on his example problem for finding the vector potential of a circular current loop $$ ...
0
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1answer
663 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 ...
2
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1answer
52 views

Copper coils for nMRI

Given the lack of financial efficacy in the medical industry, I was wondering if nuclear magnetic resonance imaging would be possible with a copper/silver coil if the use is short 10 second imaging ...
2
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2answers
48 views

Can the magnetic fields of EM radiation be harnessed or measured?

We use the electric component of EM radiation to create the EM radiation and to detect it (antennas and Etc.), but does anyone know of a situation where the magnetic component of EM radiation is used? ...
0
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0answers
42 views

F-mu-nu notation [on hold]

This is a really simple question that I cannot find an answer to, strangely enough. What do these (similar) notations stand for: $F^{\mu\nu}$ $F_{\mu}^{\nu}$ $F^{a\mu\nu}$ etc etc.
0
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1answer
48 views

Magnetic field of a stationary electron

As far as I know, a magnetic field can only be produced by a moving electric charge, or from a particle's spin (this is how a permanent magnet works, all the spins are in the same direction) What is ...
0
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2answers
50 views

Induction on a circuit with swich

I have the following circuit: It is subject to a steady, time-invariant magnetic field which points out of the paper. At $t = 0$, the switch closes. I thought that the magnetic flux would decrease ...
8
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2answers
102 views

Does the geomagnetic field rotate?

The Earth rotates about it's own axis. Do the geomagnetic field lines rotate due to this rotation or not?
19
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4answers
10k views

Why do electrons occupy the space around nuclei, and not collide with them?

We all learn in grade school that electrons are negatively-charged particles that inhabit the space around the nucleus of an atom, that protons are positively-charged and are embedded within the ...
1
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1answer
27 views

Historical analysis of light interference - difference frequencies

It is well-known that light of two different frequencies illuminating a detector will produce an output with a component at the difference frequency. While such considerations are eminently useful ...
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0answers
12 views

Question on envelope-carrier description of traveling wave

I'm doing a research internship in attosecond physics right now, and one of the really important things in the field is the description of a propagating laser pulse as the combination of a slowly (or ...
3
votes
3answers
75 views

In scattering, how does a particle 'know' which direction it is being illuminated from?

In scattering experiments, for example light scattering, the scattering strength from different sized particles is depicted as below. What I can't understand is: how does a particle know which ...
3
votes
2answers
162 views

What happens from the point when we apply potential difference across an inductor?

I am in a serious doubt about it. Consider a battery of emf E and we connect it to an inductor. Initially the switch is open, now we close the switch. My question is: What mechanism happens just ...
2
votes
2answers
455 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 ...
6
votes
3answers
643 views

Aharonov-Bohm Effect and Flux Quantization in superconductors

Why is the magnetic flux not quantized in a standard Aharonov-Bohm (infinite) solenoid setup, whereas in a superconductor setting, flux is quantized?
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2answers
110 views

Can electromagnetic fields be used to shield electromagnetic radiation?

Can electromagnetic fields be used as shielding for electromagnetic radiation?
0
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1answer
51 views

How does an accelerating charge radiate electromagnetic waves? [duplicate]

When a charged particle gets accelerated it emits electromagnetic waves. In reality, when a charged particle gets accelerated the electric field around the charge remains unchanged, but the magnetic ...
11
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3answers
2k views

Why are magnetic fields so much weaker than electric?

In EM radiation, the magnetic field is $ 3*10^8$ times smaller than the electric field, but is it valid to say it's "weaker". These fields have different units, so I don't think you can compare them, ...
14
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2answers
929 views

Why there does not exist any Gravitational Magnetic Field?

We think that the electric field and gravitational field operate similarly with their corresponding charges/masses. With just a difference that the electric field is sometimes attractive and sometimes ...
-1
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2answers
46 views

Find electric field given the magnetic field [duplicate]

Given the magnetic field as a vector, how do I find the electric field?
4
votes
1answer
147 views

Boundary conditions on current carrying wire

I'm trying to simulate by finite elements method Maxwell equations for a current carrying wire. My 3d geometry consists of a cylinder and a box containing it. I will use a mixed formulation and ...
1
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2answers
53 views

Where does wave frequency come from

I am trying to wrap my head around where do oscillations in electromagnetic waves come from. As an example if I would take a string of guitar and ring it, it would produce a certain sound based on the ...
-3
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0answers
31 views

Determine the emf [on hold]

A meter long metal rod of mass m is falling with a constant velocity of 10 m/s. The rod is attached to two conducting rails. Determine the emf if there is a uniform magnetic field directed ...
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votes
0answers
61 views

Find the electric field [on hold]

Let $V=(xyt^3T)i+(x^4tT)j$. Find the curl of the magnetic field and the electric field. I've found the curl. The curl I found to be $-(4x^3t)j+(yt^3)k$ It's sensible to leave out the units T right? ...
-1
votes
0answers
15 views

Obtain the electric field [duplicate]

Let $V=(xyt^3T)j+(x^4tT)k$. Find the curl of the magnetic field and the electric field. The curl I found to be $-(4x^3t)j+(yt^3)k$ It's sensible to leave out the units T right? How do I find the ...
-1
votes
1answer
41 views

Determine the electric field [on hold]

A loop of wire is put in a changing magnetic field. The magnetic flux through the loop is given by $4t(t+2)$. The loop is connected to a parallel plate capacitor that has a plate separation of 15mm. ...
0
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2answers
40 views

Electromagnetic Inductance: Different voltmeter readings from different positions

In the setup below, the voltmeter on the right would read differently than the voltmeter on the left even though they are both connected to point D and point A. This picture is taken from Lecture ...
2
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3answers
42 views

Magnetic moment - magnetic field relation without free currents

I'm trying to understand magnetostatics in the presense of ferromagnetic material. But I'm ending up in a contradiction: Lets take a piece of iron: Assuming that we don't care about the hysteresis ...
4
votes
1answer
144 views

The Aharonov-Bohm effect is purely classical, right?

Every discussion I've ever seen of the Aharonov-Bohm effect makes a big deal of its being a quantum effect with no classical analogue. But as far as I can tell it is present already at the classical ...
2
votes
3answers
81 views

When to use method of images in Electrostatics?

I am a bit confused about when to use the method of images in E&M? For example, in Griffith's Electrodynamics Example 3.2, the problem reads: A point charge $q$ is situated a distance $a$ from ...
1
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1answer
42 views

Calculating equipotential lines and current density in a rectangular conductor

(This isn't homework, I'm trying to make an illustration for an article I'm writing.) Let's say that I have a thin rectangular bar of uniform conductivity, and I have point probes at various places: ...
3
votes
5answers
3k views

Why are AC quantities represented by sine waves always?

Usually we use a sinusoidal wave form to represent a alternating quantity. Why not a cosinusoidal wave or a ramp wave form? In sine wave forms we can indicate the maximum and minimum amplitude and ...
1
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1answer
1k views

The electric field of a conductive sphere containing a charge - grounded vs not grounded

Let's suppose we have a sphere but unlike theoretical ones it'll have some thickness say $\Delta r$ and inner radius $R$. What I was wondering about is how will it behave if we place some charge $q$ ...
1
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1answer
621 views

Magnetic field in a cylinder with an off-axis hole

Take a long cylinder of radius $ a$. It has a long cylindrical hole of radius $b$ parallel to the cylinder axis. The distance between the two axes is $d$. If the cylinder has a uniform current density ...
0
votes
1answer
113 views

Symmetries of a Uniform Magnetic Field

Simple question. A system with a uniform electric field everywhere in space has translational invariance in the directions perpendicular to the electric field but no translational invariance parallel ...
1
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2answers
50 views

Moving coil contradiction of Faraday Induction

In Faraday's Induction Experiment, the e.m.f. induced in the induction coil becomes zero when the relative velocity of the coil and the magnet becomes zero. But one can also argue from a stationary ...
0
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2answers
100 views

How can I apply Conservation of Energy in electromagnetism?

How can I apply C.O.E to a system that applies magnetic & electric fields at the same time to do work, and convert energy from one form to another? Let assume we have a conductor that moves ...
2
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0answers
15 views

Equation describing the motion of an object attracted to magnet [on hold]

Imagine there is a strong magnet at (0, 0). There is a horizontal rail on y=1 on which a ferromagnetic object rides. The object ...
1
vote
1answer
46 views

Why can we allow the speed of light being infinite in case of Surface Plasmons?

I have a problem with understanding of these sentences: We have indicated in the opening paragraph of the Introduction that surface plasmon polaritons are solutions of Maxwell’s equations in ...
2
votes
1answer
45 views

Why do we take the value of the constant in Coulomb's law as $\frac{1}{4\pi\varepsilon_0}$? [duplicate]

Why do we take the value of the constant in Coulomb's law as $\frac{1}{4\pi\varepsilon_0}$?
0
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1answer
57 views

How are contact forces electromagnetic forces?

How are contact forces electromagnetic? What are they: Coulombic forces between polarized molecules in contact? Or they are the magnetic forces due to the motion of subatomic particles? I also would ...
2
votes
1answer
64 views

If light is an electric and (magnetic field), how can it be absorbed?

I was wondering how light or any electromagnetic radiation can be "absorbed" if it consists of electric and magnetic fields. For example if there is a charge at point A, and the light reaches point ...
0
votes
0answers
8 views

Does the skin effect and the proximity effect cancel each other out?

I'm interested in how the Skin Effect and the Proximity Effect interact with each other. From what I can understand: The Skin Effect is when AC current 'collects' on the skin of conductors due to ...
0
votes
1answer
127 views

Surely force on shell can't be balanced by field momentum?

Imagine a particle with charge $q$ at rest at the origin. It is surrounded by a concentric spherical insulating shell, also at rest, with charge $Q$ and radius $R$. At time $t=0$ I apply a constant ...
0
votes
0answers
14 views

Molecular field meaning in Liquid Crystal Theory

Given the Frank-De Gennes free energy $F = \int f(\boldsymbol{p},\nabla\boldsymbol{p}, ...)\ d\boldsymbol{x},$ for liquid crystals (see De Gennes-Prost, p. 107, formula 3.21), the vector ...
2
votes
1answer
43 views

Relationship between Polarizability and Conductivity

I've seen in the literature the relation: $\sigma (q,\omega) = \frac{i e^2 \omega}{q^2}\chi(q,\omega)$ where $\sigma$ is the conductivity and $\chi$ the polarizability. However my attempt to derive ...
0
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2answers
45 views

How to feel or sense magnetic fields from earth?

I know that earth has magnetic fields/forces, but why we don't feel them? So if I hold a magnet and earth's magnetic field is positive and my magnet is from the positive side, then why the magnet does ...