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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234 views

Does a 4-current J determine a unique maxwell-faraday F tensor up to isometry?

Maxwell's equations on a pseudo-Riemannian manifold $(M,g_{ab})$ say, $$d_a F_{bc} = \nabla_{[a}F_{bc]} = 0,$$ $$\nabla_a F^{ab} = J^b,$$ where $d_a$ is the exterior derivative, $\nabla_a$ is the ...
0
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1answer
1k views

Electrodynamics textbook that emphasizes applications

Which textbook in Electrodynamics which emphasizes practical applications and real life examples would you recommend for undergraduates?
2
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0answers
64 views

Electrodynamics using exterior calculus

Can anyone suggest resource for full course of electrodynamics using exterior calculus?
2
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1answer
50 views

Earnshaw Theorem for ionic solids

Is a single molecule of sodium chloride (say) or a cluster of molecules of NaCl unstable, although macroscopically NaCl is in fact, stable? How can I reason this based on Earnshaw's theorem?
5
votes
2answers
783 views

Effective aperture of isotropic antenna

I have always taken for granted that 'the aperture of a loss-less isotropic antenna is $\dfrac {\lambda^2} {4\pi}$'. On a whim, I tried to look up how this expression was derived, but so far I have ...
1
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1answer
50 views

How can Drift Velocity of an electron causes change?

Assume we have a conductor that has current flowing and its placed in a magnetic field so that it experiences the Lorentz force, and it gains Kinetic energy due to that force. Would drift velocity ...
0
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1answer
20 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 ...
2
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1answer
44 views

What type of magnet to use? [closed]

I am asking this question to find out practical sizes of magnets, but this will be in a fictional setting. Say I have a person dressed in iron armor, medieval knight-style. Are there types of ...
0
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2answers
85 views

Magnetic field by a solenoid?

I am really not able to understand why is it that the current,while finding the magnetic field in the solenoid (ideal one), is not $I$ the current flowing throught the solenoid but $nI$, where $n$ is ...
0
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1answer
20 views

Deflection of magnetic compass sorrounded by alternating poles

What happens to the deflection of the magnetic compass if it is surrounded by south poles and north poles in alternating direction of a magnet around it in a circular pattern. Will it be deflected in ...
2
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1answer
43 views

Proving the equivalency of the potential energy of a system of charges and the work required to assemble a system of charges

This is a very cool, and highly beneficial problem in my opinion. I feel as though truly understanding this proof would broaden anyone's conceptual understanding of electric potential. My textbook ...
3
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2answers
95 views

How are electromagnetic waves differentiated?

I would like to know how the signals for remote controlled cars, radios, etc.. That use radio waves are told apart from each other. I know that the radio waves are modulated to encode data and the ...
1
vote
1answer
50 views

Quantitative relation between two charges moving in parallel in two frames of reference

The relevant question is here. The accepted answer may have explained my question in a descriptive manner. However, I want to see how things are related quantitatively. Imagine we have two charges ...
1
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0answers
26 views

Magnetic needles in elastic media

I am studying a system with magnetic needles in a elastic media. The (two-dimensional) system is composed of $N$ magnetic needles distributed in an elastic matrix with Young's modulus is $E$. The ...
1
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3answers
96 views

Current through two inductors after a long time

I'm having trouble with visualizing the following problem, which is asking me for the final, steady current in both inductors $L_{1}$ and $L_{2}$. I was thinking that after a long time, essentially ...
13
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5answers
2k views

Is the EmDrive, or “Relativity Drive” possible?

In 2006, New Scientist magazine published an article titled Relativity drive: The end of wings and wheels1 [1] about the EmDrive [Wikipedia] which stirred up a fair degree of controversy and some ...
0
votes
1answer
44 views

EMF due to self-inductance? [closed]

I'm trying to calculate the induced $-V$ in a DC circuit when current starts to flow, and reaches it's maximum value. The inductance of the conductor is 1000nH it carries large amounts of current ...
3
votes
2answers
51 views

Showing that Coloumb and Lorentz Gauges are indeed valid Gauge Transformations?

I'm working my way through Griffith's "Introduction to Electrodynamics". In Ch. 10, gauge transformations are introduced. The author shows that, given any magnetic potential $\textbf{A}_0$ and ...
-3
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1answer
38 views

Does magnetism only work on ferrous objects?

Ordinary magnets can attract only ferrous objects (correct if i am wrong), but celestial bodies attract almost anything (including light). Are these different types of magnetism?
0
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2answers
92 views

How can I calculate the relative permeability of iron?

I'm a high school physics student, and part of our project requires us to make an electromagnet. We have an iron core, and it will go inside a solenoid. The problem is, we don't know what the relative ...
0
votes
1answer
42 views

Dielectric boundary

I am trying to determine why electric field may be confined to a certain region if there is a large difference in the permitivity for example if electric field flows through water and then reaches a ...
3
votes
0answers
53 views

Electrons and energy [duplicate]

I'm an a-level student. While studying quantum physics I came across this statement:"According to electromagnetic theory an accelerating charge emits energy". The bottom-line was that an electron ...
1
vote
1answer
39 views

Flaw in the radius of a spherical electron model (classical electron radius)

My textbook asks me to derive an equation for the potential energy ($U$) of sphere ($r_0$) filled with an electric charge of uniform density ($\rho$), expressed in terms of the total charge $Q$. The ...
2
votes
1answer
143 views

Energy-Momentum Tensor under Lorentz Transformation

In relativity, the symmetric energy-momentum tensor is given by $$ T^{ij}, $$ where $T^{00}$ is the energy density and $\frac{1}{c}T^{10}$ is the momentum density. Thus: $$ \left(\frac{1}{c}T^{00}dV, ...
15
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2answers
1k views

Why is there no Gravitational Magnetic Field?

We think that the electric field and gravitational field operate similarly with their corresponding charges/masses. With just a difference that the electric field is sometimes attractive and sometimes ...
0
votes
1answer
37 views

How is Magnetic force on a current carrying conductor $Blb$

I was reading an answer about torque acting on a rectangular current carrying loop kept in a uniform magnitude field B. Force acting on each sides is $F_1$, $F_2$, $F_3$, $F_4$. It's written here : ...
0
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2answers
601 views

Potential energy from opposing magnets repelling each other with a gap of 1 mm

I have two powerful rare earth magnets, that are separated by a distance of 1 mm. I applied energy to bring them closer to each other, hence increasing the potential energy. Now, when one of the ...
1
vote
0answers
56 views

Is mechanics needed in order to learn electromagnetism? [closed]

I'm a college student who wants to learn physics. I want to get started with electromagnetism as soon as possible. Is it okay to skip over mechanics, or is mechanics a big requirement before starting ...
11
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3answers
3k views

Does special relativity make magnetic fields irrelevant?

I've heard that special relativity makes the concept of magnetic fields irrelevant, replacing them with relativistic effects between charges moving in different velocity frames. Is this true? If so, ...
1
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1answer
34 views

Uniformly charged sphere's electric field

I am facing this topic for the umpteenth time in my college career and, of course, every teacher has explained it in a different way. In this course, to find the expression of the electric field of a ...
1
vote
2answers
57 views

Does a magnetic field sustain a current in a loop?

A particular current produces a constant particular magnetic field. If the fact holds under scrutiny, why doesn't a magnet held within a loop sustain a particular current? Are the electron spins ...
3
votes
2answers
1k views

Where does the energy come from when a current heats a wire (resistor)?

I'm trying to figure out an example from a textbook (Demtröder -- Experimentalphysik 2, pg. 198) where the energy transport caused by a current is depicted: Assume you have a wire (with some ...
3
votes
1answer
58 views

Why are atomic quadrupole moments calculated using nuclear spin?

It's my understanding that electric quadrupoles interact with the gradient of an electric field, and I understand roughly how this works. I am trying to calculate the interaction between an atomic ...
1
vote
1answer
84 views

Is there inductance to a DC circuit?

When a DC circuit is carrying current, large amounts or small, is there induced-emf due to the inductance? Or is it only applied to AC circuits?
0
votes
1answer
48 views

In which loop does the induced emf remain constant during the passage out of the magnetic field region?

A rectangular loop and a circular loop are moving out of a uniform magnetic field to a field-free region with a constant velocity V as shown in the figure. Explain in which loop do you expect the ...
1
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1answer
81 views

What would Maxwell's Equations be if we had magnetic charges and magnetic currents?

Mind you, we still have electric charge and electric currents. But, what would Maxwell's equations look like if we had to take magnetic charges and magnetic currents into consideration? Would there be ...
2
votes
1answer
40 views

Evaluate singularity in numerical iterative physical optics integral

My goal is to implement a code to numerically compute the radar cross section (RCS) of an open-ended PEC cavity by the approach of "iterative physical optics" [1]. The idea is to iteratively compute ...
-1
votes
0answers
29 views

E-Field from Magnetic Flux Density

I'm currently trying to go through the book "2008+ Solved Problems in Electromagnetism" by Syed A. Nasar to try to build up my (very) weak ability to solve E&M problems—and having a... really ...
0
votes
2answers
108 views

How to measure vector potential such as that of magnetic field?

If we want to measure gravitational potential energy of a body-earth system, we can simply do it by measuring the body's height from ground. How could we can measure vector potential such as that of ...
0
votes
0answers
4 views

Verification of Fourier transformationn of Io-sinh function [migrated]

I try to match. What could not match $I_o-\sinh$ pair developed by Ben Logan, transform pair also published in The Practical Application of the Fourier Integral Campbell, George A .Sir J.F. Kaiser ...
1
vote
3answers
214 views

Conservation of energy in electromagnetism?

How is energy conserved in electromagnetism? Is it hinted only from Lenz's law or from Maxwell? Also, if a solenoid's flow of current produces a magnetic field, and from the magnetic field forces are ...
1
vote
1answer
107 views

Why is there a voltage drop across a superconducting inductor?

If there is no electric field inside a perfect conductor, then why is there a potential difference across an (superconducting) inductor?
1
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3answers
148 views

why the inner product between divergence-free current$\vec{J}$, (satisfied $\nabla\cdot\vec{J}=0$) and a gradient field$\nabla \varphi$ is zero?

I read a statement saying that the inner product between divergence-free current and a gradient field is zero. Divergence-free surface current is $\nabla\cdot\vec{J}=0$, and $\vec{J}$ could be ...
1
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2answers
36 views

Uniformity in a solenoid

I know the magnetic field strength increases as the number of turns in the solenoid increases. However, I've learnt the field inside the solenoid is usually nearly uniform. So, does the number of ...
0
votes
1answer
37 views

Transformer leakage reactance

In practical transformers there is always a leakage reactance. If it is possible to construct such a transformer (ideally thinking) having zero leakage reactance will the transformer action work? or ...
6
votes
5answers
547 views

Can we say there are 2 EM radiation types?

EM radiation seems to come from two different sources: According to the Maxwell, by acceleration of electrons According to the Bohr, by jumping of electrons between energy levels? Are these two, ...
0
votes
1answer
53 views

Magnetic Induction

Consider the figure above in which there is an electric current (constant) in R1 (consider it to be made of a conductor). Obviously there will be a magnetic field induced in R2 (consider it to be ...
3
votes
2answers
340 views

Does a current-carrying wire running through the centre of a solenoid experience force?

Imagine looking at a solenoid from above. Current passing is through it in a clockwise direction. The direction of the field lines therefore is towards the bottom of the solenoid. Now pass a straight ...
0
votes
3answers
425 views

Moving conducting bar in (changing) magnetic field

In my exam today I've been given this problem, yet even with the results at hand I simply can't warp my head around it; Given the picture below, a bar is placed on two conducting rails with a ...
3
votes
1answer
65 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 ...