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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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\...
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2answers
332 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 ...
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1answer
33 views

Alpha Particle moving through a magnetic field [on hold]

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.
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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 ...
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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. ...
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2answers
60 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 ...
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2answers
226 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 ...
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5answers
246 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 ...
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0answers
16 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 ...
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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
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1answer
157 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
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1answer
96 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 ...
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2answers
52 views

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

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 ...
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1answer
212 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 ...
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1answer
18 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?
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0answers
77 views
+100

Penetration of a “figure-8” EM field created by a dipole antenna

Note: this is more of an EM-fields question than a signal strength question. I am doing some experiments to measure Received Signal Strength Indicator (RSSI) variations of a transmitter/receiver pair ...
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1answer
820 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 ...
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2answers
45 views

Possiblity for particle to be carried by a wave such as a photon or electromagnetic wave/waves in general for means of transportation/travel? [closed]

I am wondering whether it is possible for a photon/EM wave to carry a very small particle like an elementary particle or smaller on it like a highway carries a car traveling on the highway or an ocean ...
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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 ...
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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- ...
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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 ...
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1answer
26 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 ...
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2answers
214 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, ...
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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 ...
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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 ...
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0answers
60 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 ...
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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 ...
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1answer
195 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 ...
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2answers
723 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 ...
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2answers
98 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?
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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?
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1answer
302 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 ...
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0answers
81 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{...
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1answer
46 views

How to kill magnetic field [closed]

If we pass current through a copper coil , magnetic field is formed around the coil , so how to kill that magnetic field without changing output of current
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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 ...
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1answer
145 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, ...
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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 ...
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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, ...
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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 ...
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0answers
25 views

B field, reaction to magnetic medium

I've read 'if you put a magnetic medium into a conductor loop the B-field will change everywhere while if you put an electric charge into a dielectric sphere the dielectric sphere won't have any ...
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1answer
22 views

What is the value of induction magnetic field? [closed]

Q: Two coencenteric metalic shell has inner radius $r_1$ outer radius $r_2$. We place along axis infinity wire has $\lambda$ charge in per unit length. The inner region of metalic shells inserted with ...
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0answers
100 views

Electron beam confined by electric and magnetic Fields (Edited) [closed]

An electron beam is described with the equation $\dot{r}=f(r,\theta)$ is confined by a magnetic field $\vec{B}$ and an electrical potential $V$ in a radios $a$. how to find $a$ in terms of $\vec{B}$ ...
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1answer
114 views

Experimentally finding units of physical quantity?

Say you had a new physical quantity you wanted to determine the units for. How do you go about this? For the strength of an electromagnet for example, you could carry out a simple experiment like the ...
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4answers
2k views

To what extent can speed of light be reduced? [duplicate]

Light slows down upon entering different transparent objects, and the ratio is taken as refractive index of the object. If light can be slowed down, then is there a limit up to which it can be slowed ...
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2answers
51 views

Does a displacement current have to be included in the Biot Savart Law?

Consider two spherical balls given charges of equal magnitude and opposite signs. Now, we connect them with a conducting straight finite wire. A current will flow in the wire till the charges on the ...
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3answers
61 views

Electron deflected by moving magnet

My original question was far too long, so I am editing it down to this much smaller version: The Lorentz force for an electron traveling through a uniform magnetic field with no electric field is $F=...
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0answers
19 views

Does removing half of toroidal ferromagnetic core significantly reduce flux density in the remaining half?

Assuming a DC current in a cable has induced a magnetic field around the cable, and a ferromagnetic toroid is used to concentrate the field; if half of the toroid were cut away (so the cable is no ...
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1answer
113 views

Relativistic explanation of Radiation

I ask this question again in a simpler, shorter form. Maxwell's equations can be derived with Special relativity starting from the Coulomb's law. Therefore all the phenomena of classical ...
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1answer
38 views

Why can't weak core-less electromagnets be used in conjunction with strong permanent magnets to perpetually harvest energy?

Why can't weak core-less electromagnets be used in conjunction with strong permanent magnets to perpetually harvest energy? Magnets will attract ferrous materials but some materials like copper or ...
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0answers
20 views

Time derivative of total angular momentum in Dirac equation

I would like to calculate the time derivative of the total angular momentum of a Dirac particle in an electromagnetic field $(\vec A, \phi)$: $$\vec J = \vec r \times (\vec p - \frac{q}{c} \vec A) + \...