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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63 views

Gravity vs. Electromagnetism Scenario

Imagine a two dimensional world where there are only two electrons. They are set right beside each other. Of course, immediately they will start to separate, being repelled. My question is, as they ...
4
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2answers
553 views

Why are magnetic field lines perpendicular to the surface of a ferromagnetic material?

It is known that magnetic field lines become nearly perpendicular to the surface of a ferromagnetic material. The quantitative proof which uses the boundary condition requires that magnetization at ...
2
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0answers
19 views

Magnetic monopoles and special relativity

I was thinking about magnetism as a product of special relativity and the result of this approach to the magnetic monopoles. So if magnetism is a product of electricity(like electricity from another ...
1
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0answers
27 views

Bar magnet dropped through coil

If you have drop a bar magnet through a coil so that it goes all the way through I was told the graph of emf induced in the coil vs time looks something like this: (emf induced is on y axis, time ...
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2answers
43 views

Conducting rod moving through magnetic field

If a conducting rod moves through a magnetic field which way do its electrons move? In my revision guide it shows the following picture (more or less, but the following is my drawing of it -- I ...
2
votes
1answer
20 views

Faraday and Lenz's laws

in my revision guide it gives the equation for Faraday's law as $$\text{induced emf} = N\frac{\Delta \phi}{\Delta t}$$ while the one for Lenz's law is given $$\text{induced ...
2
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0answers
12 views

Calculation of the potential of 2 spherical perfect conductors with the image method

I am searching for a way to calculate the potential on the surface of two perfect conductors that are spheres. I am not sure my method is correct. Here is a diagram of what I am studying : They ...
1
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1answer
29 views

Is it possible to pass electric current through magnet without affecting it?

Suppose we have a row of $5$ electromagnets, with a copper wire connecting them. Is it possible to create magnetic field in the $4^{th}$ magnet without affecting the $1^{st}$,$2^{nd}$,$3^{rd}$ and ...
1
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1answer
28 views

Interacting magnetic fields

Is there a reason why two magnetic fields perpendicular to each other do not interact? If they are parallel or at a non-90 degree angle they interact. Is it because magnetic field lines can be viwed ...
0
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0answers
17 views

What is space charge and how to calculate it?

I want to clarify the meaning of space charge. What I know is that the space charge is the total charge in a small region in space. I really confuse this in the ion beam context. Many text book says ...
4
votes
3answers
314 views

How are the Lorentz force, Maxwell's third law and Faraday's law of induction clasically related?

Faraday's law of induction can be used in any situation where the magnetic flux is changing through a closed conducting loop. While giving the correct answer, it seems to me that for the following ...
1
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1answer
37 views

What is the gravitational effect inside a electromagnetic shield due to an external electromagnetic field?

I am new in General Relativity. I know that electromagnetic field (or, the electromagnetic energy tensor, $T^{ik}=1/4\pi[1/4F_{mn}F^{mn}g^{ik}-F^i_lF^{lk}]$) can affect gravitation. Now if we take a ...
0
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1answer
29 views

Does heating an electromagnet cause change in its magnetic field?

Does heating an electromagnet cause change in its magnetic field as well and vice versa?
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0answers
11 views

Second-order correction in Quantum-Confined Stark effect

In the wikipedia article, there is a second-order correction in the Quantum-Confined Stark Effect. I could not understand how it was solved. I did not understand the meaning of 2(0) and 1(0) and how I ...
0
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1answer
52 views

Ampere's loop inside capacitor

The figure is a view of one plate of a parallel-plate capacitor from within the capacitor. In the question, we are required to rank the 4 paths (a, b, c and d) according to the value of $\oint ...
2
votes
1answer
285 views

How does the speed of a DC motor depend on the size of the coil?

I have to build a simple electric motor by attaching a magnet to a battery, extending the terminals of the battery (with stiff wires so they could act as supports), and placing a coil of wire on top ...
3
votes
1answer
43 views

Do the relations between E/B and D/H contain higher order multipole terms?

Jackson writes in section 1.4 (third edition) that \begin{align*} D_\alpha &= \epsilon_0 E_\alpha + \left(P_\alpha - \sum_\beta \frac{\partial Q'_{\alpha\beta}}{\partial x_\beta} + \ldots \right) ...
-21
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4answers
2k views

Do We Need Maxwell's Equations Since They Fail to Account for An Experimental Fact at Least in One Occasion?

This question is an outgrowth of What is the difference between electric potential, potential difference (PD), voltage and electromotive force (EMF)? where @sb1 mentioned Faraday's law. However, ...
1
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2answers
334 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 ...
0
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1answer
46 views

Cryptic remark in physics revision guide

I am currently revising for my AP physics and I couldn't understand one of the end-of-section summary notes. It says: "Remember that the direction of magnetic field is from North to South, and that ...
8
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2answers
3k views

How do superconducting materials float in magnetic field?

The movie Avatar got me interested in the subject, but so far I only found sophisticated articles loaded with unfamiliar words. Is there a simple way to explain how magnetic field affects ...
1
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3answers
197 views

Who foots the (magnetic) energy bill?

Gravitational attraction and electrostatic attraction/repulsion are intrinsic properties of matter, any particle (electron, proton) for some unknown reason can produce KE at a distance. But magnetic ...
2
votes
1answer
30 views

Do changing magnetic fields always produce solenoidal electric fields?

Since the curl of E is the time derivative of B, $\nabla \times \vec{E} = -\frac{\partial B}{\partial t}$ Do changing magnetic fields always produce solenoidal electric fields? For instance a ...
2
votes
3answers
77 views

Electric field or static electric field around a plugged-in lamp cord (when lamp is not turned on)?

When an electrical cord from, say, a lamp, is plugged into an AC wall socket, I'm aware that an electric field forms around the entire length of the cord and even before the lamp switch is flipped on. ...
0
votes
2answers
395 views

Torque per unit length on infinite rotating charged cylinder

For homework I have the following question, but I am stuck on finding the torque on the cylinder. An infinite cylinder of radius $R$ carries a uniform surface charge $\sigma$. We start rotating the ...
2
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2answers
64 views

Transition from 4-potential to E and B [on hold]

In my lecture notes there is a step that i cannot follow: $$\frac{i}{2}[\gamma^{\mu},\gamma^{\nu}] (\partial_{\mu}A_{\nu}-\partial_{\nu}A_{\mu})=: \sigma^{\mu\nu}F_{\mu\nu}=i\vec{\alpha} ...
1
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1answer
76 views

Poincaré' lemma and EM potential $A^{\mu}$

My lecturer said that given the sourceless Maxwell's equations $$ \partial_{\mu}\, ^ *F^{\mu\nu} = 0 $$, we can find a solution $$ F^{\mu\nu} = \partial_{\mu}A_{\nu} - \partial_{\nu}A_{\mu},$$ that ...
3
votes
4answers
102 views

What is the meaning of EM field having curl?

We know that the "sourceful" fields like gravity or electric static field due to charge are all curl=0. But both E/M fields have curl and they mutually "curling" into each other, making them kind of ...
3
votes
1answer
30 views

What's the difference between “Ohmic dissipation”, “Joule heating”, “ion drag” and “resistive heating”?

The following terms are sometimes used to refer to ... more or less ... the same thing by different people and in different contexts (electronic circuits vs. plasma physics, etc.): Ohmic ...
1
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2answers
34 views

Magnet gyroscopic force spin

I was wondering whether a magnet exerts any measurable gyroscopic effects. I understand that magnetism is caused by alignment of spins of electrons which have angular momentum. (I realise that that ...
1
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1answer
2k views

Proving Ampere's Circuital Law

How to prove Ampere's Circuital Law in case of any conductor. My text gives the proof of only the special case when the conductor is long and straight. I am trying to prove it, but haven't been ...
0
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0answers
37 views

Solutions to Dirac equation in 2+1 D [on hold]

Hello I solved the Dirac equation in 2+1 D taking two different gamma matrix representations. rep.A: $$\gamma^0=\sigma^3, \qquad\gamma^1=i\sigma^1, \qquad\gamma^2=i\sigma^2.$$ rep.B: ...
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votes
1answer
21 views

Faraday's Law: Current loop and proton

A single circular loop of wire with radius R carries a large clockwise current I(loop)=I0, which constrains a proton of mass M and charge e to travel in a small circle of radius r at constant speed ...
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0answers
46 views

Direction of emf and it's physical sense?

I have a situation where there exist a time varying magnetic field and a circular loop place perpendicular to it. Let us assume that the magnetic field is $\vec B=\frac{B_0 t}{\tau}$ . I tried to ...
1
vote
0answers
26 views

Is there an electric field in the direction of a uniform current?

In an infinite plane where uniform current is passing through,is there any electric field? Because i know that charge produces an electric field but in a uniform current and because it is an infinite ...
2
votes
1answer
34 views

Difference between electric and magnetic field (relating to EEG & MEG)

I study cognitive neuroscience and I periodically run into physics related questions in the context of neuroimaging technologies. My question specifically refers to electric and magnetic fields that ...
3
votes
1answer
37 views

Could the 4 Forces Split Off or “Decay” into Other Forces in the Distant Future? [duplicate]

From my basic understanding of popular-level physics articles and books and such, the 4 forces (Electromagnetism, Gravity, Strong and Weak Force) used to be 1 force in the early universe, then split ...
0
votes
1answer
161 views

How to compute speed without knowing mass or charge values?

A gold nucleus is $460$ fm ($1$ $fm = 10^{(-15)} m$) from a proton, which initially is at rest. When the proton is released, it speeds away because of the repulsion that it experiences due to the ...
-1
votes
0answers
26 views

Is it possible to defy the earths magnetic field, allowing you to hover? [duplicate]

I am just trying to figure out that if you can defy the Earths magnetic field, allowing you to hover. Some friends and I have theorized that if you put on heavy-weight magnets, and add an electronic ...
9
votes
3answers
384 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 ...
0
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1answer
95 views

Reflection coefficient as a function of frequency

I am trying to relate the equation for reflection coefficient in oblique mediums to the frequency but can't figure out how the frequency affects the reflection of light. $n_1$= intrinsic impedance of ...
1
vote
1answer
129 views

Using a circuit to make a magnetic balance to weigh objects

I understand that this is a homework question, but I am learning about magnetic fields and things like that and this certainly wasn't covered in the material, so my question is more about the actual ...
2
votes
0answers
102 views

What exactly is NASA's proposed mechanism for “propellantless” “EM Drive” propulsion? [duplicate]

Of course, this question runs perilously close to this site's prohibition against discussing non-mainstream physics. However, the accepted answer in meta about what is acceptable and what is not ...
0
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0answers
25 views

How to find the terminal angular velocity of the given figure? [on hold]

I have posted the picture of question, can someone please tell how to do it ?
2
votes
1answer
102 views

Moving electric charges

I just wanted to double-check these three statements, as I'm not entirely sure I understood them completely: 1) A stationary electric charge (let's say a proton) produces electric field. 2) A moving ...
12
votes
5answers
2k views

Is current in superconductors infinite? If they have 0 resistance then I (V/R) should be infinite?

I learned many years ago that according to Ohm's law, current is equal to voltage divided by resistance. Now if superconductors have zero resistance then the current should be infinite. Moreover the ...
-4
votes
1answer
53 views

Light - What is it? [on hold]

First of all, please excuse my bad english. I am from Germany. I am interested in, what light really is. I know, there is a range in the spectrum, and this range is visible. This is defined as light. ...
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0answers
21 views

Meissner effect and levitation

In a field less than critical field, decreasing the temperature below the critical temperature will eliminate the magnetic field inside a superconductor and increase the magnetic field around it. ...
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1answer
13 views

Do electric sinusidal waves rotate 45 degrees too when light pass through a hollow cylindrical magnet or just the sinusoidal magnetic waves do that?

Do electric sinusoidal waves rotate 45 degrees too when light pass through a hollow cylindrical magnet or just the sinusoidal magnetic waves do that?
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0answers
13 views

AC current through induction on both sides

When you rotate a coil in a magnetic field one side cuts downwards the other cuts upwards. When you move a piece of wire up and down you get an AC current or pdf which you can see on a voltmeter as ...