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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1answer
87 views

All possible electromagnetic Lorentz invariants that can be built into the electromagnetic Lagrangian?

Given the electromagnetic Lagrangian density $$ \mathcal{L}~=~-\frac{1}{4}F_{\mu\nu}F^{\mu\nu}~=~\frac{1}{2}(E^2-B^2) $$ is a Lorentz invariant, how many other electromagnetic invariants exists that ...
2
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0answers
67 views

Can the two electromagnetic field tensors be combined into a more general tensor?

Given the electromagnetic field tensor $$\begin{align} F_{\mu\nu} = \begin{pmatrix} 0 & -E_{x} & -E_{y} & -E_{z} \\ E_{x} & 0 & B_{z} & -B_{y} \\ E_{y} & -B_{z} & 0 ...
7
votes
5answers
380 views

Is there a fourth component to the electric field and magnetic field?

The Question If the three vector electric and magnetic fields come from the four component four-potential, then is there a fourth component to the electric and magnetic field? Related Question I ...
1
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0answers
60 views

What is $T_{\mu\nu}T_{\mu\nu}$ for the electromagnetic stress-energy tensor?

Given the electromagnetic stress-energy tensor components \begin{align} T_{\mu\nu} = \begin{pmatrix} u_{00} && s_{0 \nu} \\ s_{\mu 0} && \sigma_{\mu\nu} \end{pmatrix}_{\mu\nu} ...
0
votes
1answer
50 views

How does this particle behave in the magnetic field?

I have the above question and I have though about it every way and can't seem to find out. So what I have concluded (ignore the answers on the img) Is that the force is on the z axis because the ...
1
vote
1answer
135 views

How does current flow perpendicular to the wire?

This answer gives a great explanation of how surface charge builds up to force the current to move perpendicular to the wire: http://physics.stackexchange.com/a/102936/41086 However, it fails to ...
1
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0answers
24 views

Drift velocity of electrons in a superconducting loop

Do electrons travel at the Fermi velocity in a superconducting loop? For metals the Fermi velocity seems to be around $10^6$ m/s. So would electrons (in a Cooper pair) travel around the loop at this ...
0
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0answers
37 views

Why plane em waves are basic?

Why are plane EM waves required in the derivation for the speed of EM waves in term of absolute permeability and permittivity? I can't understand how to calculate flux for electric field passing ...
1
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2answers
44 views

Visualising scalar and vector fields

As I have been studying electromagnetic theory, I have always been lead to confusion when trying to visualise the fields. Fortunately, the electric and magnetic fields are vector fields and also along ...
4
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2answers
133 views

What is potential energy truly?

I have a problematic question for which I have been unable to attain a satisfactory answer. What is potential energy truly? - I have read about how potential energy can be seen as the "highering" ...
1
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1answer
56 views

The question about quantization of free EM field

Let's have the free EM field theory with Coulomb gauge: $$ \partial^{2}A_{\mu} = 0, \quad A_{0} = 0, \quad (\nabla \cdot \mathbf A ) = 0. $$ One of the ways of quantizing the field is the following. ...
2
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1answer
48 views

What are “parity considerations” in deciding the form of the Hamiltonian?

In "introductory Quantum Optics", by Gerry and Knight, the Jeynes model is considered. In this model of electron-EM field interaction the electron is approximated by a two state system ($\lvert ...
2
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1answer
50 views

What's an ideal wire?

I'm not talking about an ideal wire in a circuit (a wire with infinite conductance). I'm talking about an ideal wire in the case of the magnetic field of an infinite current carrying wire. What ...
2
votes
0answers
25 views

How to calculate the force acting on a magnet due to a solenoid

Can I assume both magnet and solenoid as magnetic dipole and use coulombs law to find the force acting the magnet?
1
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3answers
143 views

Why do both plates of a capacitor have the same charge?

How do we know that both plates of a capacitor have the same charge? You could argue conservation of charge, but I don't see how conservation of charge implies the charge on both plates is the same. ...
0
votes
0answers
31 views

Field outside a solenoid?

Say you have a solenoid with diameter $d$ and length $L$. How can I show that as $d/L$ approaches 0, the field outside the solenoid is zero? I'm looking for a formal proof involving integration. ...
0
votes
0answers
34 views

Resistance of a cylinder contacted by two smaller circular faces

Suppose we have a solid homogeneous cylinder with radius $a$, heigth $h$ and conductivity $\sigma$. The top and the bottom face is contacted with a smaller circular face with radius $b$. How can I ...
1
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0answers
19 views

Electric Field: distributed uniformly in one infinity tape of length [closed]

One charge density surface is distributed uniformly in one infinity tape of length with $2a$ width from distance $d$. Determine the Electric Field in the point perpendicular from the distance $d$ ...
1
vote
1answer
75 views

Step in a proof that $\textrm{div} \ \mathbf{B} = 0$ from Biot-Savart's law

Notation: The magnetic field $\mathbf{B}$ generated by a point charge $e$ moving with velocity $\mathbf{v}$ is given by Biot-Savart's law $$\mathbf{B} = \frac{\mu_0 e\ \mathbf{v} \wedge ...
1
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5answers
251 views

Field inside a wire?

This answer gives a great explanation of why the field inside a wire connected to a battery must be equal at all points: Why doesn't the electric field inside a wire in a circuit fall off with ...
4
votes
1answer
76 views

Poynting theorem and entering power

I refer to the time-domain version of the Poyinting theorem in electro-magnetism: $- \displaystyle \oint_S (\mathbf{E} \times \mathbf{H}) \cdot d\mathbf{S} - \int_V \mathbf{E} \cdot \mathbf{J}_i \ dV ...
1
vote
1answer
63 views

What is the place of an electromagnetic field in the electromagnetic spectrum? [closed]

What place should I give an electromagnetic field (produced by a current conducting coil) in the electromagnetic spectrum? What will its wavelength and frequency be?
0
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3answers
197 views

Prove EM Waves Are Transverse In Nature

Why we say that EM waves are transverse in nature? I have seen some proofs regarding my question but they all calculate flux through imaginary cube. Here is My REAL problem that I can't here imagine ...
4
votes
2answers
203 views

Reason why $F^{\mu\nu}F_{\mu\nu}$ and $\tilde{F}_{\mu\nu}F^{\mu\nu}$ are Lorentz invariant

I'm trying to think of an intuitive reasoning for why $F^{\mu\nu}F_{\mu\nu}$ and $\tilde{F}_{\mu\nu}F^{\mu\nu}$ are Lorentz invariant. By this I mean that I don't simply want to show that they remain ...
1
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0answers
40 views

Space time curvature due to electric charge or magnetic charges [duplicate]

since we know that gravitational force is nothing but a curvature in space-time. I have a similar analogous for the electric or magnetic charges. Similarity is that both electromagnetic and ...
3
votes
3answers
78 views

Current density?

The current $i$ can be defined as: $$i = \int \vec{J} \dot{}d\vec{A} $$ where $\vec{J}$ is the current density and $d\vec{A}$ is the area vector. Is it possible for: $$i = \int \vec{J} ...
1
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0answers
35 views

Superconducting wire in a Magnetic Field?

A superconducting wire($SC$) is moved rapidly in a magnetic field( $1$ $Tesla$), what would happen to the wire? Are there any forces induced of attraction or repulsion? In a typical conductor, we ...
1
vote
2answers
120 views

Integration constants in Maxwell's equations (ambiguousness?)

In classical electrodynamics, if the electric field (or magnetic field, either of the two) is fully known (for simplicity: in a vacuum with $\rho = 0, \vec{j} = 0$), is it possible to unambiguously ...
2
votes
1answer
49 views

Why the electric potential of earth is zero?

For a localized charge distribution the potential is set to zero far away from the charge distribution (at infinity) Now, when grounding a conductor, i.e. connecting it to Earth, it is said that we ...
0
votes
0answers
24 views

phase difference between incident plane wave incident on a dipole and radiation fields from dipole

i have an incident plane wave and a dipole, consider that plane wave incident on dipole. at this moment what happen for dipole ? we know that after incident of plane wave on dipole, the radiation have ...
1
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4answers
48 views

difference between dielectric constant and dielectric function

I'm confused by the definition of dielectric constant. We all know that dielectric is function of wavelength, because materials respond in different ways depending on the energy of the light. But what ...
4
votes
1answer
246 views

Why doesn't the electric field inside a wire in a circuit fall off with distance from the battery?

We studied electric fields due to point charges. The magnitude of these fields decreases with the square of the distance from the point charge. It seems to me that we could treat the positive ...
0
votes
2answers
63 views

Two masses connected to a spring separated are charged to q Coulombs

I am working through Yale's Physics 201, and on the first problem set there is a question that is puzzling me. From my reasoning, I think the answer they provided is incorrect hence why I am here. The ...
6
votes
4answers
432 views

What is the current of a capacitor when the derivative of voltage is undefined?

This is from the textbook I am reading: I know this equation for capacitors: $$i=C\cdot \frac { dv }{ dt }$$ Here is my question: how can diagram (a) be allowed if the derivative of the voltage ...
0
votes
1answer
53 views

Current vs Voltage Drive for Loudspeakers [closed]

Please see this article: here I don't know enough EMFT to comment on this but I am working on other tasks for a wireless speaker system prototype called "Busker's Friend". Completing my Schaum's ...
1
vote
1answer
55 views

Understanding fields and their correlation to forces

I seem to be confused between the concept of a "force", and a field. Now let's assume there is a magnetic field of $1$ $\mathrm{Tesla}$, what does that mean in relation to force? Finally, if field is ...
0
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0answers
23 views

Would electron degenerate matter be a good x-ray reflector?

I do not know much about x-ray physics or degenerate matter, but I have the intuitive feeling that the high electron density and what must be some crazy band structures in electron degenerate matter ...
0
votes
2answers
85 views

How can you use magnets to rotate a shaft, which in turn powers a generator? [duplicate]

I recently was given the task to create power using magnets, I would think that if I were to put some same poled magnets together, and they were facing the same poled magnets, they would repel and ...
2
votes
1answer
67 views

What does the electric field caused by a charged cylinder look like?

I am talking about a cylinder with a large enough radius:length ration that it cannot just be treated as a line. I would think that the electric field lines come out the curved and flat surfaces ...
1
vote
1answer
35 views

Saturation of an iron core?

Lets assume we have a 1 kg iron core, and a 100 kg iron core. Now saturation is defined as how much that core can absorb the magnetic field, since they are different sizes, don't they saturate at ...
-1
votes
1answer
49 views

How do magnetic objects exhibit attraction/repulsion across empty space?

Magnets will attract or repel over a distance before physically touching each other. What makes this effect possible? My best guess is that the forces generated by the angular momentum of the ...
7
votes
2answers
140 views

Why don't black holes form from forces other than gravity?

Gravity is the weakest of the fundamental forces, so what is so special about gravity that it can form an inescapably strong field while a force like the EM force cannot? It seems to me that if there ...
1
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0answers
22 views

Conduction and propagation

What is the difference between conduction of electric wave in conductor and propagation of electromagnetic wave in dielectric? Why propagation term is used for dielectric and conduction for ...
4
votes
6answers
746 views

A paradox to Lenz's law

I have read that in simple words, Lenz's law states that: The direction of current induced in a conductor is in such a fashion, that it opposes its cause. This validates law of conservation of ...
0
votes
1answer
60 views

Force from solenoid

I'd like to approximate the force from a solenoid, or at the very least find a formula which is proportional to the force so that I can experimentally find the constant for my particular case. ...
1
vote
1answer
106 views

How can electrons move along the conductive wire? ( seems to be a paradox )

Tangential components of the electric field across an interface between two media, with no impressed magnetic current densities along the boundary of the interface, are continuous. So: $ n \times (E_2 ...
0
votes
1answer
27 views

Why does surface charge not move?

If you have a wire with current flowing through it, and the current flowing the wrong way (not parallel to the wire) surface charge will buildup, generating a field to force the current to flow the ...
1
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1answer
100 views

Is it possible to detect fake Tungsten aka Wolfram gold bars with a strong magnet?

Tungsten aka Wolfram is paramagnetic so it is weakly attracted to magnets. A guy devised the following to test for Tungsten in gold bars: http://www.youtube.com/watch?v=foELQ7T8_90 But he is using ...
4
votes
1answer
437 views

How can KVL & KCL be derived from Maxwell equations?

How can KVL (Kirchhoff's Voltage Law) & KCL (Kirchhoff's Current law) be derived from Maxwell equations in lumped circuits?(Lumped network : if $d$is the largest dimension of the network and ...
5
votes
1answer
67 views

Is it possible to see light intensity fluctuate?

Solutions to Maxwell's equations shows that the $\vec{E}$ and $\vec{H}$ component are of the same phase, which means they go to maximum and zero together, therefore the intensity of the ...