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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How to get the magnetic field strength in space near a solenoid

I am trying to find the magnetic field strength along the axis of a solenoid. If I know the strength of the field at the center of the solenoid, and I know the distance from the center of the ...
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224 views

Forces on a wire moving in a magnetic field?

When a wire is placed inside a magnetic field and current starts to flow in that wire, that wire experiences the Lorentz force = $IL \times B$ and beings to move. Aside from induced motional - ...
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12 views

A force in a homogeneous magnetic field

Semi infinite parallel conductors, while on them without friction slides a crossbar . Perpendicular the plane of the conductors is a homogeneous magnetic field. To what extent of force we must act on ...
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15 views

understanding magnetic dipole moment and torque

I would like yo know if I understand things correctly: Magnetic moment, which is a constant value for a solenoid or magnetic dipole, determines the torque it will experience in a magnetic field. (I ...
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11 views

What is the typical strength of the electric field in a particle accelerator?

I am working on a research project involving the closed orbits of hydrogen in the presence of an external electric field and I am curious what a reasonable approximation for the electric field ...
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4answers
63 views

Magnetic moment - magnetic field relation without free currents

I'm trying to understand magnetostatics in the presense of ferromagnetic material. But I'm ending up in a contradiction: Lets take a piece of iron: Assuming that we don't care about the hysteresis ...
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3answers
73 views

What can change a photon's frequency?

I found this question: Is it possible to apply force to a light particle? As it states, gravity can change the frequency of light by changing its momentum. My question regards other phenomena that ...
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1answer
193 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 ...
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27 views

Reflectivity of Metals - real and imaginary part of k-vector and complex dielectric function

De electric field component of an electromagnetic wave that is traveling in the $x$-direction is given as $$E(x,t) = E_0 e^{i(kx - \omega t)} $$ with $E_0$ the amplitude and $k$ the wave vector. From ...
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22 views

Current and current density

Current, $I$, is generalised as: $I=\iint_{A}^{} \vec{J}. d\vec{A}$. I know that current density always points in the direction of flow of positive charge. I wonder if the infinitesimal element, ...
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22 views

What exactly happens in an electric circuit?

After reading chapters on Electrostatics, I have been having lots of problems in understanding current electricity.I will be trying to tell what my problems are by telling what I already know.Please ...
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3answers
119 views

Technical question about 2-forms

A technical question about the electromagnetic tensor, but before that, it is know if, say, instead of being $$F_{\mu\nu}=\partial_{\mu}A_{\nu} - \partial_{\nu}A_{\mu}$$it were ...
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39 views

Why are cereals attracted by a magnet?

Many cereals (from the supermarket) are attracted by a magnet. This can be seen by crushing the cereals into a very fine powder and approaching a strong magnet. The usual explanation is that cereals ...
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2answers
38 views

Lorentz force Law

I want to ask a fundamental question about Lorentz force equation. Why is it important to incorporate both electric and magnetic forces into one single expression? I know magnetism is electricity's ...
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1answer
24 views

Constant charge density and magnetic field

Suppose we have an arbitrary number of point charges in a vacuum, described by a constant charge density $\rho$. Can they be the sources of a magnetic field $\mathbf{B}$? My intuition is that they ...
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24 views

Question about Gauss' law and dipoles

I've got a question about dipoles and Gauss' law: Consider an electric dipole $p=ql$ in free space, centred at the origin of a Cartesian reference frame. Using Gauss’s Law, discuss if the electric ...
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3answers
138 views

Induced EMF of a rectangular loop should be zero?

Considering the shape of a rectangular loop in a changing magnetic field: The induced $\epsilon$ would be zero? Since a rectangular loop is a combination of wires in series to create such a shape. ...
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22 views

Why do magnets vibrate near an AC source?

Why do they vibrate and simply not get pulled closer or further from the source of emf? This may be related to the shape, but if I take a small N45 disc magnet and put it near an electrical ...
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0answers
17 views

What is the speed of the charge in this setup of spherical conductors? [on hold]

Two concentric spherical conductors respectively have radius $r_1 = 0.145\ \mathrm{m}$ and $r_2 = 0.207\ \mathrm{m}$. The inner sphere carries a charge of $-6.00 \cdot 10^{-8}\ \mathrm{C}$ . An ...
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23 views

Semi infinite parallel conductors [on hold]

Semi infinite parallel conductors separated by a distance of 0.5 m are at the endpoints associated with the resistance R = 4, while according to the two without friction slides bar by weight: 0.1 kg ...
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33 views

The approximation of the potential field of a half wave dipole antenna

I'm currently deriving an approximation of the potential field of an dipole antenna in the far-field in two dimensions, and I have some troubles. Since using two methods give two different answers. ...
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1answer
34 views

Influence of spacetime curvature on electromagnetic wave propagation [duplicate]

Classical physics assumes that spacetime is evenly distributed in the sense that Coulomb's Law predicts that a charged particle will create a spherically symmetric electric field around its location. ...
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1answer
143 views

Are magnetic hysteresis losses relevant to alternating currents flowing in a wire?

Say we have an AC in a magnetically lossy material, like iron. Because of iron's relatively high permeability, skin effect will be more pronounced than it is in say, copper, so this iron wire isn't so ...
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Faraday's law - does the induced current's magnetic field affect the change in flux?

I've had this conceptual problem with Faraday's law and inductance for a while now. Take the example of a simple current loop with increasing area in a constant field (as in this answer). So ...
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30 views

Holding magnet near CRT monitor damages the screen?

Few days ago I tried keeping my magnet in front of my CRT monitor screen and it was showing some cool effects like showing so many different color but then when I removed it ,that blurr effect was ...
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20 views

Amplitude of electric field produced by satellite dish [on hold]

A telecommunications satellite is in geostationary orbit and it has a 1 m transmitting dish optimised for coverage of Europe. If the output power from this dish is 100 W, very roughly estimate the ...
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10 views

Calculating work needed to move a charge between plates of an uncharged capacitor [on hold]

My first guess was zero work because there's no electric field(I think). E.g. a parallel-plate capacitor $4.9 cm$ on a side, spaced $1.0mm$. The work needed to move $6.8\mu C$ seems to be $1.1J$ And ...
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855 views

Apparent violation of Newton's 3rd law and the conservation of angular momentum for a pair of charged particles interacting magnetically

Consider a system of the two identical point positive charges situated in the space (isolated from influence of any other external fields) as shown in the figure.Particle1 is at (a,a,0) and Particle2 ...
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21 views

Finding the magnetic field [on hold]

A mechanical pendulum is made of a metal rod, one end of which is attached to a string. The mass of the rod is $m$ and the string length is $l$. The oscillation period of the thus obtained pendulum ...
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1answer
318 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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9 views

from control torque to magnetic moment

I am designing a system which generates a magnetic field to make itself move in space (a cubesat with magnetorquers). This magnetic field is known as the magnetic control torque, i.e. it will torque ...
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1answer
18 views

How can electric field accelerate fog dissipation?

I should emphasize that I view the problem as a 'research' rather than a 'textbook' level. However, as I am almost unfamiliar with the subject it may be that the answer is elementary and well-known. ...
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1answer
9 views

Magnetic field size from electric field [on hold]

An electric charge oscillates about the origin along the y axis. A distant observer is located at a point on the +z axis. You observe the wave from a given location and find that at a given ...
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0answers
14 views

How can I tell apart the composition of my soft ferrites

I have an assorted bunch of soft ferrites. I need to know which ones are MnZn and which ones are NiZn, so I can sort them for usage at different frequencies.
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1answer
874 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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3answers
143 views

What is supposition of equilibrium? How do Rayleigh, Jean know the electromagnetic wave in equilibrium behave?

In a cavity of dimension L, the wave must give zero amplitude at the wall, means wave equation has zero amplitude. Why? Answer from hyperphysics "since a non-zero value would dissipate energy and ...
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1answer
91 views

Understanding magnetic force on charged particle

if we put any charged motionless or static particle in the constant magnetic field, then why does it don't feel a magnetic force? Mechanism by which electric and magnetic fields interrelate I have ...
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2answers
45 views

Potential difference inside charge distributions

Always when I think about potential difference inside a symmetrical charge distribution, I think of it being equal to zero because it's dependent on the field and I imagine the directions of each of ...
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0answers
21 views

Magnetic field due to long current carrying conductor at a point near its end [duplicate]

How can one show that the magnetic field due to an infinite long current carrying conductor (length a) at a point near its end is given by $$B = \frac{\mu_0I}{4\pi a}$$ using Ampere's Circuital ...
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1answer
35 views

Net force between two Electromagnets considering Back EMF [on hold]

First this Q might be trivial thus apologies. Consider two bar electromagnets that can simulate bar magnets M1 and M2, placed end to end at distance D apart (North Pole of M1 facing South Pole of ...
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1answer
67 views

What are the proposed theoretical explanations for the EmDrive?

The EmDrive is a proposed propulsion mechanism for spacecraft in which some form of microwave device provides the propulsion. The consensus from the physics community, including many voices on this ...
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4answers
7k views

Displacement current - how to think of it

What is a good way to think of the displacement current? Maxwell imagined it as being movements in the aether, small changed of electric field producing magnetic field. I don't even understand that ...
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2answers
214 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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1answer
142 views

How does back-emf oppose the input-current?

A circuit, has current $A$ flowing at a certain $V$. When there is a change in magnetic-flux, based on Faraday's law of induction & Lenz's law, we know that there is change in Potential ...
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1answer
142 views

Force of a solenoid on a ferromagnetic material

As I was trying to find an answer to another problem, I was informed that the equation I was using would not work. The equation I was using was $$F = (NI)^2\mu_0\frac{\text{Area}}{2g^2}$$ If this ...
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0answers
21 views

Why is emf 0 for coil rotating in field?

Say you have a coil (that can carry a current) rotating in the field between two magnets. When the area of the coil is perpendicular to the field lines the emf induced in the coil is 0, why is this?
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2answers
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 ...
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58 views

What is the electric field inside a hollow sphere?

If you have a conducting hollow sphere with a uniform charge on its surface, then will the electric field at every point inside the shell be 0. The reason the electric field is 0 at the center is ...