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.

learn more… | top users | synonyms (6)

5
votes
2answers
261 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 ...
12
votes
3answers
8k views

Protons' repulsion within a nucleus

Do the protons inside the nucleus repel each other by the electrostatic force? If they do, why doesn't the repulsion drive the protons apart so that the nuclei get disintegrated?
1
vote
2answers
2k views

What makes a nucleus unstable?

My question is simply that - what makes a nucleus unstable? What exactly causes a nucleus to start breaking apart in the first place? Is it the Coulomb force between the neighboring protons? I'm just ...
1
vote
1answer
41 views

Why don't the nucleus break apart? [duplicate]

The nucleus of an atom is made up of protons and neutrons. So if there are many protons inside nucleus even though there are neutrons why don't this protons may repel away breaking nucleus apart? The ...
3
votes
0answers
50 views

Number of Independent postulates in Electrodynamics

We know that there are two ways to get charge conservation in electrodynamics by using the following action: $$S[A]~=~\int\! d^4x {\cal L},$$ $$ {\cal L} ~=~{\cal L}_{\rm Maxwell} + {\cal L}_{\rm ...
5
votes
1answer
976 views

Magnetic Levitation : Confusion with Lenz's Law

Considering the above diagram, I am failing to see why the aluminium ring floats upwards rather than oscillates. I understand Faraday's law, but I am struggling with Lenz's law - as there is an ...
8
votes
4answers
264 views

Maxwell Equations don't give unique Electric Field?

Consider the class of electric fields given by $$\mathbf{E}=\begin{cases} \ln (Cr)\hat{z} & 0\leq r < R\\ 0 & r> R \end{cases}$$ where $C$ is a constant and $r$ is the polar-distance ...
3
votes
1answer
333 views

Does the Lorentz force law explain Flemings left hand motor rule and the right hand dynamo rule?

the Lorentz force on a charged particle $F=qv \times B$ can explain Flemings left hand rule (motor rule) and the right hand (dynamo rule) In the left hand rule, the direction of the current gives ...
0
votes
4answers
7k views

What is the difference between electromagnet and solenoid?

What is the difference between electromagnet and solenoid? Both these terms seem as the same thing to me. The only difference that I can find seems to be that an electromagnet contains a soft iron ...
-2
votes
0answers
23 views

Charging Copper ball bearing

I want to conduct an experiment to observe the trajectory of charged particles in applied magnetic and electric field. Thus, I want to charge a copper bead/ball bearing. How can I electrically charge ...
1
vote
1answer
46 views

Good book to grasp physics [duplicate]

I'll try to be clear. I'd like to ask you if you know some books or chapters in textbooks which explaining the "meaning" of some physical phenomena or entities. I may try to be more specific using ...
0
votes
1answer
38 views

Basic EM: Infinite energy of magnetic field?

I thought I was done with EM, but then I encountered a strange question: A ring of test mass (m) and test charge (q) with radius R is sitting still in space filled with a uniform B-field in the ...
2
votes
1answer
47 views

Magnetic force and torque for 2D neodymium ball simulator

I'm trying to write a simulator for neoballs. I'm doing only 2D for a start, in which each ball has a magnetic moment, m, that is in the x-y plane. The simulator will use time-step integration at 60 ...
2
votes
1answer
49 views

Induced motional EMF where the wire is stationary and the field source is moving?

If a wire is moving in a magnetic field $B$, there is a Lorentz force acting on both the postive and negative charges, separating them to create an electric field, which is a great explanation to aid ...
3
votes
0answers
82 views

Question on the Derivation of Lorentz Invariance of Electric Charge

On Zwiebach's "A First Course in String Theory" textbook, there is a problem 8.2 that derives the invariance of electric charge given a few assumptions: 1. Conservation of Maxwell 4-current $j = (c\...
1
vote
2answers
336 views

Why not use the magnetic scalar potential?

In electricity and magnetism, we use the scalar potential to derive the electric field and the vector potential to derive the magnetic field because $\nabla\cdot B=0$ and $\nabla\times E=0$. I was ...
1
vote
1answer
34 views

Alpha Particle moving through a magnetic field [closed]

How would I find the acceleration of an alpha particle moving through a magnetic field given the force of the magnetic field, the charge, the initial velocity and the strength of the magnetic field.
-1
votes
0answers
44 views

How 7.0 Tesla can yield better image resolution with less teslas?

I am studying the book 7.0 Tesla MRI Brain White Matter Atlas 2nd edition by F. Calamante for 1.5, 3.0 and 7.0 Tesla images, respectively I do not understand how the image resolution can be greater ...
2
votes
2answers
63 views

Why would a moving infinite region of magnetic field exert an electric force?

I'm confused about the Lorentz force in relation to the frame of reference. There are many questions about that here, but I still don't get it. I've tried to break it down to the most basic example. ...
1
vote
2answers
61 views

Will rotating magnets slow down one another when interacting?

Will rotating magnets slow down one another when interacting? Two discs are spinning. Both have separate magnets attached. Disc one has a diameter of $8~\mathrm{cm}$ and disc two has a diameter ...
3
votes
2answers
228 views

Definition of Ampere

On Wikipedia it says: This force is used in the formal definition of the ampere, which states that it is "the constant current that will produce an attractive force of $2 × 10–7$ newton per metre ...
5
votes
5answers
247 views

Energy density with complex permittivity?

Question What is the correct form of the energy density when we have a complex permittivity (such as in a dielectric)? Additional information A complex permittivity means that we have: $$\vec ...
0
votes
0answers
19 views

Why are magnetic quantum numbers assigned like they are assigned in the paper on WS2 monolayers?

Now I'm reading the following paper: Sie, E.J., et al., 2015. Valley-Selective Optical Stark Effect in Monolayer WS2. ‎Nat. Mater., 14, 290-294 And they assigned magnetic quantum number to valence ...
0
votes
1answer
310 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 ...
3
votes
1answer
158 views

Voltmeters and changing magnetic fields

A changing magnetic field pierces the interior of a circuit containing three identical resistors. Two voltmeters are connected to the same points, as shown. V1 reads 1 mV. What does V2 read? ...
3
votes
1answer
102 views

Find magnetic field of parabaloid-spiral particle path

Given the components of the particle's path (tcos(at), tsin(at), R(x^2 + y^2)), where a and R are constants, how can I find the magnetic force causing this? I would like to understand if this can be ...
1
vote
2answers
53 views

Is energy only transmitted through electromagnetic and particle radiations? [closed]

Which are the other ways of transmission if any? If energy does not require any medium for transmission(as for sunlight reaching earth, the heat too), is it transmitted in quanta in particle radiation ...
0
votes
1answer
213 views

What is the magnetic flux density “outside” the solenoid when AC current is passing through it?

I know that there are well defined equations explaining the magnetic flux density in the solenoid. However what about magnetic field outside the solenoid? How is the magnetic flux density related ...
1
vote
1answer
22 views

Does the direction of magnetic field in a solenoid point from north to south anywhere? [closed]

Is there any case where the direction of magnetic field inside a solenoid points from its North Pole to its south?
1
vote
1answer
835 views

calculating magnetic field strength for a very small electromagnet

I am trying to calculate the magnetic field (in tesla/gauss) of an electromagnet that is very small and has very few windings. For example 12 windings over 0.003 meters. I know this is not going to ...
2
votes
1answer
71 views

Confusion regarding conductors and their behavior

Consider two thin plates and one thick plate(all conducting). A and C are the thin plates, B is the thick one. Now B is inserted between the parallel plates A and C. A is given a charge 2q and C is ...
2
votes
2answers
84 views

Why do different vector potentials in Landau levels problem lead to different quantum mechanical ground state wavefunctions?

Consider a charged particle (electron) moves in xy plane under a magnetic field pointing along the z direction, i.e., $\vec{B}=B\hat{z}$. As a consequence, we can write down three different gauges- ...
5
votes
1answer
357 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
vote
1answer
28 views

Why can't diamagnetic and paramagnetic materials be magnetized?

Ferromagnetic materials can be made into permanent magnets. How come diamagnetic, and paramagnetic materials be made into magents? Does it have to do with the valence electrons, and how they are ...
10
votes
2answers
217 views

Charged Accelerometer in Orbit

It is well known that an accelerometer (or any other object for that matter) in a gravitational orbit will register nearly zero acceleration. According to this answer, this is because the object is, ...
0
votes
1answer
565 views

Why do we always consider the area of inner solenoid while calculating the mutual inductance of two coaxial solenoids?

While we know that flux linked to an area is the dot product of the area vector of the loop(and not the actual area encountering the field lines) and the magnetic field linked to it, why then do we ...
1
vote
1answer
75 views

What do we mean by speed of light dependent on direction?

I have a statement in textbook saying: When the speed of light is independent of direction, the secondary waves are spherical. When is it dependent on direction and how will the secondary ...
4
votes
0answers
64 views

Landau quantization: degeneracy of first level

In some books the degeneracy of one Landau level in a two-dimensional gas of free electrons is calculated in the following way: Note: The electron spin is not considered. Number of states of a free ...
0
votes
2answers
170 views

Two different electrostatic charges in rotation

It's a theoretical problem. I mean I drawn a circle and I consider this circle exist. I would like to understand how works this device composed of a circle and two charges: Two charges fixed on a ...
2
votes
1answer
196 views

Cylindrical capacitor in an electric circuit

I've come across a tricky question and would appreciate some hints or explanations as to why the given solution is the way it is. The question reads as follows: A coaxial cable consists of a wire ...
1
vote
2answers
738 views

Why toroid has no magnetic pole?

Our teacher taught us about Gauss law of magnetism, $\phi_B=\oint_s{\small{}}\vec {B} .\vec {ds}=0$(being valid because magnetic field line form closed loop, right?) which denies the existence of ...
1
vote
2answers
99 views

What is the physical meaning of Lorenz gauge condition? [closed]

What is the physical meaning of Lorenz gauge condition? And what part of the solutions we throw?
1
vote
1answer
42 views

Electromagnetic lifts, and work

Magnetic field doesnt do any work, so when an electromagnet lifts, say a car or something, where is the energy coming from?
2
votes
1answer
304 views

What kinds of behavioural anomalies can a zero-field-cooled (ZFC) / field-cooled (FC) split indicate?

If a material shows a spiltting in the ZFC and FC curves, is it necessarily superparamagnetic, or could there be any other reason for the irreversibility? I have heard spin glasses also show ZFC-FC ...
2
votes
0answers
86 views

Does the motion of a free electron in a plane wave electromagnetic field have analytical solutions?

Does the motion $[x(t), y(t), z(t)]$ of a free electron in plane wave electromagnetic field have analytical solution (oscillating E and B fields)? The Lorentz force is : \begin{align}m_e\begin{...
0
votes
2answers
2k 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 ...
1
vote
1answer
146 views

Do magnets also attract super conductors?

If a superconductor can be repelled by a magnet (or, well, locked in a certain position by the magnet) can a magnet be attracted to a superconductor. What would happen if you take a peace of metal, ...
2
votes
0answers
77 views

Is radiation perpendicular in brehmsstrahlung?

wiki says: Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic ...
0
votes
1answer
1k views

Why moving charges cause magnetic field (module and direction)?

Why an constant electric current in a wire produces a magnetic field, that circles that wire? I know that this question was posted before. However, all answers talk about Maxwell equations, axioms, ...
0
votes
1answer
24 views

What would occur between the parallel plates of the capacitor?

In capacitors dielectric materials are sometimes inserted between the parallel plates. What would happen if a diamagnetic, or a paramagnetic material is inserted between the parallel plates of a ...