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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1answer
20 views

How does parallel plate lose energy when deflecting charged particles

Say you have a parallel plate setup, each plate is charged with ±Q of charge, and is then disconnected from the power source and is insulated from the environment. There is no way the plates can be ...
0
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1answer
18 views

Equation relating voltage and minimum rpm needed for generating 1V using a magnet around copper wire

I was wondering if there is any equation or law or something that relates the minimum rpm needed when a magnet spins around a copper wire to generate 1 volt? In other words, how fast does the magnet ...
0
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0answers
20 views

Electromagnetism(CLOCK RULE)

Can anybody explain why the clock rule has been designed the way it is? I mean , why is it that the polarity is North pole, if current flows in the anticlockwise sense as viewed from above and ...
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0answers
62 views

Rigorous proof of Ampère's law from the Biot-Savart law

Let us assume the validity of the Biot-Savart law for a tridimensional distribution of current:$$\mathbf{B}(\mathbf{x})=\frac{\mu_0}{4\pi}\int_V ...
0
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1answer
35 views

What does it mean to be unique in terms of vector potentials?

I have a feeling this may be a stupid question... let me proceed; I was in an electro-magnetism lecture, we were looking at the magneto-static maxwell’s equations: $$\triangledown\cdot\textbf{B}=0$$ ...
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2answers
29 views

Ohm's law experiment

I was going through my physics laboratory manual. In the Ohm's Law Experiment, the book states a few precautions without any reasoning. 1.The wire whose resistance has to be determined should ...
1
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3answers
35 views

Helical motion of charged particle in external magnetic field

When an electron moves in uniform external magnetic field, with velocity not perpendicular, I understand that the magnitude of force is only due to the perpendicular component of velocity and is ...
1
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2answers
41 views

How does a point charge interact with a Gaussian surface?

A spherical Gaussian surface encloses a point charge $q$. The point charge is moved to to a point away from the center of the sphere. Does the electric field at a point on the surface change? ...
0
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1answer
19 views

Magnetic field of a current carrying wire reduce/increase exterior magnetic field?

If we place a wire within a uniform exterior magnetic field ($B_x$) that's perpendicular to it's length, and allowed current to flow withing the that conductive medium, there is another magnetic field ...
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2answers
20 views

Why is the electric potential of a point from a point charge of +Q positive?

According to the formula, $$V = - \int \vec E \cdot d\vec r.$$ The electric potential of a point charge is $\frac{KQ}R$? However, when I perform the integration, my answer is $-\frac{KQ}R$. Since ...
1
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1answer
32 views

Energy of electromagnetic wave

Its given here that energy density of an electromagnetic wave is $$\vec S=\frac{1}{\mu}(\vec E\times\vec B)$$ How is the above expression derived? And when did energy become a vector? I though work ...
1
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1answer
67 views

Light is/produces a electromagnetic wave, but is the opposite also true?

A similar question has been asked, and to some extent the answer is: you need an antenna with $L \approx 200nm$. But it's not entirely clear to me if this is completely hypothetical, as "Marek" also ...
2
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1answer
49 views

Which of the properties of particles are intrinsic properties and why?

For macroscopic objects it's clear that - once observed - the observed property does exist for a while, even if we are no longer observing it. That has to do with the complexity and stability of such ...
0
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3answers
32 views

What is the electric field exactly on the surface of a conducting sphere?

Within the sphere, the electric field is 0, but is the electric field still 0 exactly on the surface?
3
votes
1answer
34 views

Why does energy flow between a high voltage transmission line and linemen approaching it on a helicopter?

The video High power line workers shows linemen servicing a live high voltage transmission line. As the helicopter approaches the line, the lineman reaches out with a metallic wand that is ...
0
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0answers
20 views

Dielectric Constant and Frequency [on hold]

How dielectric constant of a parallel plate capacitor varies by varying frequency in physical terms?
0
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1answer
42 views

Why does magnetic field and not electric field invert in a reflection? (related to another question)

When you are talking about an elctromagnetic wave that reflects on a surface (for example here), why do you say that the reflected magnetic field inverts and not the electric field? \begin{align} E_+ ...
0
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1answer
55 views

What is “wrong” with this microwave?

Metal objects in microwave ovens are known to be somewhat dangerous. However, my microwave has the following label stuck inside: It seems to imply that this particular oven forbids heating drinks ...
0
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0answers
28 views

Relationship between the g-factor and the gyromagnetic ratio?

From what I have read the g-factor and the gyromagnetic ratio are two different quantities. It appears to me that the gyromagnetic ratio is the ratio between (any) angular momentum and the magnetic ...
0
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0answers
14 views

Electric field produced by polarized medium at the surface of a conductor

Suppose we have some conducting sphere of radius $R$ and charge $Q$ submerged half way into a medium with permittivity $\epsilon_1$. I have deduced that the electric field produced by the sphere will ...
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0answers
21 views

relation between permittivity and IMPEDANCE? [on hold]

DETERMINATION OF Permittivity FROM IMPEDANCE MEASUREMENT USING AGilent ANALYZER
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0answers
14 views

What could be the cause of a phase shift between the torque and the angular speed in DC generator?

Introducing some variations in the form of sinusoids in the electromagnetic flux of a DC generator connected to a hydraulic turbine runner I obtained some strange results for the phase shift between ...
6
votes
1answer
106 views

Why is a stellarator-type nuclear fusion reactor so oddly-shaped?

My first impression: It's a mess. Why is it shaped like that? I can't find any info about its shape other than it's a special arrangement of magnetic coils. (link) Here's some more images of it on ...
0
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1answer
31 views

Ferromagnets - Permanent?

When researching about the Curie-Temperature and magnets in general, something got me confused. What is the difference between a ferromagnet and a permanent magnet? In a ferromagnet the spins are ...
0
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1answer
61 views

Two concentric conducting shells connected by wire with modified Coulomb law?

This question is related to this one, but is a bit different. Consider two concentric and conducting spherical shells connected by a wire. Inner shell has radius $b$ and (unknown) charge $q_b$ while ...
4
votes
2answers
113 views

Do physical entities with one dimension equal to 0 exist?

Within mathematics, a lot of entities with one or more size equal to zero exist and are fluently manipulated. For example, within $\mathbb R^4$, the entity defined as $z=0, t=2$ is a plan, the one ...
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0answers
37 views

What does this coordinate transformation in the Wave equation mean?

My tutor derived the following of which I do not understand the transformations (2.1) and (2.2): $$\Delta\vec{E} - \frac{1}{c^{2}} \frac{\partial^{2}\vec{E}}{\partial t^{2}} = \frac{4\pi}{c^{2}} ...
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1answer
26 views

I need proof for dW=eIdt [on hold]

I need help to understand from where this equation is formed inorder to continue to study energy stored in an inductor.
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0answers
25 views

Radius of solenoid on the magnitude of magnetic field

I'm working on a project and have made a solenoid. I want to know the effective parameters on the magnitude of the magnetic field. $$B=μNi$$ In this equation, the radius of the solenoid don't have any ...
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0answers
41 views

Landau levels in uniform magnetic field

Intro Landau levels are obtained by gauging the vector potential to be $$ \vec{A}=\left(-By,0,0\right) $$ By which the Hamiltonian: $$ H={1\over 2m}\left(\vec{p}-q\vec{A}\right)^2 $$ can be ...
0
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1answer
19 views

How do I show for an ideal transformer $M^2=L_1L_2$?

I've been stuck on this problem for about an hour. In an ideal transformer, the same flux passes through all turns of the primary and of the secondary . Show that in this case $M^2=L_1L_2$, where ...
0
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1answer
47 views

Why can't electrons leave negatively charged objects in a vaccum?

If an object is negatively charged, and electrons repel each other since they have like charges, why doesn't that negatively charged object "kick away" electrons? Why doesn't diffusion occur? Why ...
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0answers
57 views

Self dual electromagnetic fields [on hold]

Has anyone developed (or is working on) a self-dual EM field in toroidal coordinates? If so, please reference. Self dual refers basically to the EM field tensor but the practical meaning is that the ...
1
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0answers
23 views

Charged rotating sphere in Maxwell-ChernSimons theory

Does anybody know what the electromagnetic field of a charged rotating sphere in five dimensional Maxwell-Chern-Simons theory is? This theory has action \begin{equation*} S = - \frac{1}{4} \int d^5 ...
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0answers
35 views

Magnetic field of one electron

I want to reproduce a calculation of the magnetic field at the atomic nuclei produced by one electron in hydrogen: Maxwell gives us: $\vec \nabla \times \vec B = \mu_0 \vec j$ Doing the surface ...
1
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1answer
43 views

Physical meaning of multipole moment

Is there a physical interpretation for multipole moments? For a quantity governed by the Laplace equation ($\nabla^2 \omega = 0$), I understand that the general solution is given by the multipole ...
1
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0answers
25 views

Surface bound charge density in dielectric displacement

In Griffiths, it derived that the electric potential caused by Polarization $\bf {P}$ is written $$V({\bf r}) = \frac{1}{4\pi \epsilon_0} \int_{\mathcal{V}} \frac{\hat{\gamma} \cdot {\bf P}{({\bf ...
1
vote
1answer
36 views

How does electrical energy gets converted to sound energy?

I have seen that inside headphones there is a magnet with a coil of thin wire around it. There must be longitudinal waves coming out of it that is why we can listen to audio. There must be pressure ...
4
votes
2answers
35 views

Stored energy and momentum in a magnetic field?

How is it possible for a magnetic field to store energy and momentum? And can deliver it to another object(charge/dipole) or field? I can't depict it. I know it occurs but how? For example, the ...
0
votes
1answer
43 views

Motion of Particle due to Lorentz Force

So my professor gave us the following question: A particle with electric charge $Q$ and mass $M$ is initially traveling with velocity $v_0$ in the $x$ direction at time $t= 0$. There is a constant ...
0
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0answers
11 views

Data analysis for a magnetic hysteresis loop

I'm a physics undergrad and I have MOKE data for a magnetic material (thin permalloy film on a silicon substrate). Here is one of the hysteresis loops I obtained, plotted using python: The form of ...
0
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2answers
39 views

How to attract liquid metal with magnets

I would like to know if there is any way that I could make mercury or another liquid metal be attracted or repelled by magnets. Absolutely any solution would be okay with me. For example... can I ...
1
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1answer
25 views

What causes the magnetic field around a wire with current to be circular and perpendicular to the flow of current?

I understand that a moving charge will produce a magnetic field, but what causes them to appear circular and perpendicular to the flow of current, as they are?
0
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0answers
20 views

magnetic friction

If you take attracting magnetic fields, or opposing fields, and rotate them in opposite directions in close proximity, would this not generate an increase in temperature? This would also conclude that ...
0
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0answers
23 views

EMF produced by mutual induction

In the phenomenon of mutual induction, when primary coil is connected to an A.C. suppy, does the induced EMF in secondary coil also keeps changing with the changing magnetic flux?
0
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1answer
27 views

Resistance in A.C. circuits

Why do electromagnets offer maximum resistance in A.C. circuits?
1
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1answer
36 views

Potential energy of a spherically symmetric charge density in a spherically symmetric electrostatic potential

I'm interested in calculating the potential energy of a spherically symmetric charge density in a spherically symmetric electrostatic potential. More specific, I'm currently trying to calculate the ...
0
votes
2answers
57 views

Induced Current in a wire

I am just surprised why EMF and Current are not induced in an unclosed wire. I know Maxwell's equation is only defines closed integral. But it doesn't means that it is not generated in a unclosed ...
0
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1answer
47 views

induced Current I

I am working with the Problem which I've seen in a book. That Problem was : Direction of induced current I can't find any meaningful explanation for this problem in any source. So, I am working with ...
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0answers
16 views

what is the relation between Magnetic Flux and repulsive force it produces? [closed]

I have two robots and I need to move them using Magnets(Either electromagnet or Permanent magnet). The robot of weight 20 kg has to move the robot of 5 kg using the magnetic repulsive force. How can I ...