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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Adding Magnetic Fields?

Ok, so we all know that the strength of a magnetic field can be increased if a wire with current flowing through it is surrounding the magnet. Because the current flowing through the wire is making a ...
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93 views

Why do electron and hole mobilities decrease with temperature?

From page 35 of "Microelectronics" by Millman Grabel Mobility $\mu$ decreases with temperature because more carriers are present and these carriers are more energetic at higher temperatures. ...
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25 views

A magnet floating due to Meissner effect

If a magnet is floating due to the Meissner effect, can the magnet rotate freely under influence of external field in spite of the presence of north and south pole in it ?
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1answer
58 views

What is the metric of a constant electromagnetic (pure electric or pure magnetic) field?

For example, imagine a magnetic field $B_x$ directing in $\hat{x}$ direction filling all the space. What is its associated metric field? I can construct the electromagnetic stress-energy tensor for ...
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1answer
99 views

Superconducting electromagnets?

Is it true that superconducting electromagnets don't need any power? So... energy to created a magnetic field via a superconductor would be zero? Since resistance is zero, does that also mean that ...
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2answers
90 views

Is it possible to make an electromagnet w 2 like ends?

HELP! Is it possible to create an electromagnet with one continuous wire with 2 like poles (i.e. both ends either north or south)?. Visualising it with the right hand screw rule for current ...
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3answers
39 views

Antenna direction

I have a router with a wifi antenna that can be turned in any angle. I wonder what difference does the direction of the antenna make to the electromagnetic signals propagation? Where is the signal ...
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1answer
31 views

Border conditions on the separation surface (Electromagnetism & Optics)

My teacher taught me that we can consider the following equation: $$E_{1t}=E_{2t}$$ to the descontinuity of the electric field tangent component on the separation surface of two means with ...
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29 views

About Michelson-Morley experiment

Can you please explain it to me. As far as I know, we have a monochromatic and coherent em source and we getting an interference picture (because we clearly couldn't make the splitted beams travel ...
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39 views

Regarding Ampere's Circuital Law

If I am to show that Ampere's Circuital law holds true for any arbitrary closed loop in a plane normal to the straight wire, with its validity already established for the closed loop being a circle of ...
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4answers
5k views

What is the difference between gravitation and magnetism?

If you compress a large mass, on the order of a star or the Earth, into a very small space, you get a black hole. Even for very large masses, it is possible in principle for it to occupy a very small ...
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1answer
17 views

Relation of resistance between two metals and capacity

My schoolbook goves the resistance between two metals in a material with conductivity $\sigma$ as $R=\frac{\epsilon_0}{\sigma C}$, but they don't show how they got this result. Can someone show me how ...
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116 views

How can Drift Velocity of an electron causes change?

Assume we have a conductor that has current flowing and its placed in a magnetic field so that it experiences the Lorentz force, and it gains Kinetic energy due to that force. Would drift velocity ...
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19 views

Motion decomposition (Relativistic case)

When an electron moves at very close at $c$ (speed of light), is it physical to decompose the motion in two other directions (like what we do in classical case). If so, the motion in each direction ...
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37 views

Electric field from time varying charge density

Inside a cylinder of infinite length in $z$ axis, there is charge density $ ρ = cos(βz -ωt)$. I want to find the electric field and as far as i can understand we will get a radial component of $E$. ...
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50 views

$E$-field from changing current in straight wire

A very long insulated wire oriented along the $z$ axis of a cylindrical coordinate system is carrying current that is defined by the following function: $$I(t) =Io + k⋅t$$ (current increases linearly ...
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2answers
47 views

Electric potential between two infinite plates

Below is a solved exercise from Griffiths' Electrodynamics. I don't understand why it's directly assumed that the configuration is independent of $z$. Shouldn't there be a contribution from it since ...
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2answers
52 views

Why is the introduction of a quantization volume necessary for quantization of the EM field

I have been working through the quantization of the electromagnetic field, and every source I find introduces a quantization volume with periodic boundary conditions in the process, in which we fit ...
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2answers
48 views

Total angular momentum of electron in a magnetic field

In this question: Electron in the proximity of a magnetic monopole It is stated that for an electron in the magnetic field of a monopole, $ \vec{B}(\vec{r}) = \frac{g}{r^3}\vec{r} $ , that the ...
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0answers
50 views

question about Blandford-Znajek original paper

I'm trying to follow the original Blandford-Znajek paper (found here), and I can't find how equations 2.1, 2.2 and 2.3 are obtained: 2.1 refers to "the charge density required to ensure that $E \cdot ...
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17 views

Ratio E/B and power in rectangular cavity

I would like to know if the relation $E=B*c$ still holds in a rectangular cavity Let's consider this rectangular cavity where the wave is travelling along the z direction ...
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29 views

Reactor for electrical energy design base assumption inefficient?

Im no physicist or engineer but as I observe our methods to obtain electrical energy I noticed a pattern where we cause a exothermic reaction, the resulting energy heats water, that turns into vapor, ...
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6answers
882 views

What was Feynman's “much better way of presenting the electrodynamics” — which did **not** appear in the Feynman lectures?

Does anyone know what Feynman was referring to in this interview which appears at the beginning of The Feynman Tips on Physics? Note that he is referring to something that did not appear in the ...
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5answers
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How do you determine power flow direction in a transmission line?

Below is a picture of a typical transmission line(about 200 kV). Is there a simple physics experiment which can be performed safely near the line, to determine the power flow direction. Or in other ...
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2answers
958 views

Local explanation of the Aharonov-Bohm effect in terms of force fields

Here is an interesting paper for the Physics SE community: On the role of potentials in the Aharonov-Bohm effect. Lev Vaidman. Phys. Rev. A 86 no. 4, 040101 (R) (2012). arXiv:1110.6169 ...
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1answer
319 views

Force on a coil inside of a voice coil [closed]

as a homework exercise we were given the following question: Given the following voice coil: With depth = 20 mm Permanent magnets with B = 1.23 T, Ur = 1.1 The coils has 20 turns, I = 1 Ampere. a) ...
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4k views

Conversion of Moving coil galvanometer to ammeter

A galvanometer can be converted into an ammeter by connecting a low resistance (called shunt resistance) in parallel to the galvanometer. Firstly, why do we need to connect the resistance? If a ...
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12 views

Coordinate Transformation across different media

I have an arrangement that is as follows : (1) inner cylinder(radius r1) filled with water (2) outer cylinder(concentric, radius r2>r1) made of glass I have a sensor S1 at radius r(r1) at angle beta. ...
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23 views

How is the electromagnetic tensor expanded?

The electromagnetic tensor is given by $F_{\mu \nu} = \partial_{\mu}A_{\nu} - \partial_{\nu}A_{\mu}$, and it appears in the Lagrangian as $L = -\frac{1}{4}F_{\mu\nu}^2 - A_{\mu}J_{\mu}$. The text I'm ...
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1answer
779 views

Electric field from conductive to dielectric media

I am interested in the main difference between transitions from electric fields from Conductive to Conductive/ Dielectric to Dielectric and Dielectric/Conductive media. What are the boundary ...
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1answer
16 views

$H$ field from corresponding $E$ field (antenna)

The $E$ field at $kr >> 1$ due to a short dipole oriented in the $z$-direction and located at the origin is: $$E_0 = \frac{10^3e^{-jkr}}{r}\sin\theta\space\space(V/m)$$ Can I just solve for the ...
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1answer
331 views

Question about superconductivity

A long cylinder of radius $R$ is made from two different material. Its radius $r<r_0$ $(r_0<R)$ part is a material with superconducting transition temperature $T_1$, and its $r_0<r<R$ ...
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1answer
25 views

what is the meaning of spatial function of Ex?

I came across this while i was studying Resonant cavities. Consider the component of $E_x $ of $ E $. Within the cavity $$ E_x = E_x(x,y,z) $$ Why should we take $E_x $ in all three directions?
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2answers
609 views

Vector Potential for Magnetic field when the field is not in simply-connected region

According to Poincare's Lemma, if $U\subset \mathbb{R}^n$ is a star-shaped set and if $\omega$ is a $k$-form defined in $U$ that is closed, then $\omega$ is exact, meaning that there's some ...
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0answers
49 views

Spectroscopy from a classical light wave or photon only?

In chemistry we mostly regard light/electromagnetic radiation as a beam of particles or photons. This is a very useful model to explain molecular excitations and ionisations from quantum interactions. ...
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1answer
40 views

Apparent discrepancy between Lagrange field equation and Maxwell equation [closed]

I am deriving Maxwell's equations from a Lagrange field equation and have come across something I can not figure out no matter how hard I try. The problem is in the signs. If we take the Lagrange ...
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1answer
2k views

What is the meaning of this “let there be light” joke?

Someone across the restaurant is wearing this shirt, and I certainly don't get it. Update Related: What does this quote about the four dimensional divergence of an antisymmetric tensor mean?
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1answer
62 views

Phase on Aharonov-Bohm effect doubts

How I show that $$\Lambda(\textbf{x}')=\frac{q}{\hbar}\int \mathbf{A} \cdot d\mathbf{x'}$$ on $$ \tilde{\psi}(\textbf{x}',t)=e^{[\frac{iq\Lambda(\textbf{x}')}{\hbar c}]}\psi(\textbf{x}',t)$$ for ...
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2answers
239 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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8 views

EM wave - phase change - amplitude [duplicate]

A plane electromagnetic wave has the shape: $\vec{E}(\vec{r},t)=E_0\cdot cos(\vec{k}\vec{r}-\omega t)\cdot \vec{e}_y$ $\vec{B}(\vec{r},t)=(B_1\cdot cos(\vec{k}\vec{r}-\omega t)+B_2\cdot ...
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0answers
22 views

Conductors - force needed to maintain a constant velocity

There runs a current $I_0$ through a straight and long conductor. Two more conductors are placed parallel to it in a plane with distances a and b respectively. The resistance of the connection is ...
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2answers
90 views

How come the length of a wire does not affects on the circuit current?

Today we started learning about the electromagnetic induction. Out teacher gave us the following explanation: Suppose we have a conduction frame inside a magnetic field $\vec{B}$ going towards the ...
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2answers
1k views

Meaning of boundary condition for steady current density?

Although I understand the derivation of boundary condition in case of steady electric current but I did not understand, that the electric field which is in direction of $J$ current density that is ...
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3answers
171 views

Why the induced field is ignored in Faraday's law?

Suppose we have a conducting ring in a constant magnetic field $\vec{B}$. Suppose that the ring is being deformed. We know from Faraday's law that such an action will induce a current in the loop. ...
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2answers
2k views

Magnetic field lines

When I searched the net about magnetic field lines, Wikipedia told something about contour lines and that magnetic materials placed along a magnetic field has some specific loci, which i did not ...
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2answers
65 views

Calculating the resistance of a 3D shape between two points

If I have an arbitrarily shaped object made of a uniform material of some specified resistivity, how would I go about calculating the resistance between two measurement points with known contact ...
2
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1answer
258 views

Ampere's circuital law for finite current carrying wire

When I was studying about Ampere's circuital law. Then there comes a question in my mind that "whether this law is applicable for finite current carrying wire or not"
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1answer
51 views

Predicting the correct graph for the E.M.F induced

To be honest, this is a homework question. A screenshot of this question is given below. NOTE: The correct word for the one that I encircle with blue colour should be "rotates" My argument towards ...
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2answers
652 views

How do magnets work?

I've read a classbook on the field theory (including EM): it perfectly describes quantitive patterns in EM-theory, but I have no luck understanding how and why it works. I mean, magnetic substances ...
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1answer
129 views

Dielectric boundary

I am trying to determine why electric field may be confined to a certain region if there is a large difference in the permitivity for example if electric field flows through water and then reaches a ...