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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110 views

cgs Gauss' system of units

I had never seen this system until today, and I'm really confused. I've read the wikipedia article about it but I still don't know how to change between this and the international system. For example, ...
11
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2answers
221 views

What mechanisms allow conductors to be transparent?

An electric field in a conductor causes charges to redistribute so as to cancel out the original field, bringing the field to zero. This is, I think, a common argument for why conductors are generally ...
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3answers
7k views

Why is copper diamagnetic?

Cu has an unpaired electron in 4s, but it is diamagnetic. I thought that it has to be paramagnetic. What am I missing?
4
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1answer
1k views

Will molten iron stick to a magnet?

I've known for a long time that if you heat a magnet, there is a point it loses its magnetism (the Curie temperature). It isn't clear to me if this applies to induced magnetism like iron sticking to ...
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4answers
300 views

Why in $F = iLB$, $L$ is a vector but $i$ is not?

I learned $F = iLB$ recently. However, I don't understand why $L$ is marked as a vector but $i$ is not. For a normal rod, how should I define the direction of length vector $L$? And if I reverse the ...
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1answer
210 views

Derivative of covariant EM tensor

I cannot seem to prove that the derivative of the duel tensor = 0. $$ \frac{1}{2}\partial_{\alpha}\epsilon^{\alpha \beta \gamma \delta} F_{\gamma \delta} = 0. $$ Writing this out I get (for some ...
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2answers
2k views

Do protons exchange photons with electrons?

I'm sorry for this question but, I just don't get it. According to the electromagnetic field theory, electrons repel each other by exchanging photons. How do protons attract electrons, by photon ...
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2answers
111 views

Why is $\vec j\cdot \vec e$ the joule dissipation?

I always see $\vec j\cdot \vec e$ as Joule's dissipation and I don't understand why. For example, if we have a uniform electric field $\vec e=e_o\vec u_x$ and we release an electron in it, it will ...
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2answers
250 views

What is the maximum power available from a magnetic field?

I just want to validate something I inferred from studying Griffiths (1999). The instantaneous magnetic field $\vec{b}(t)$ at a distance $r$ from a long infinite conductor carrying a current $i(t)$ ...
3
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1answer
385 views

What breaks the symmetry between the electromagnetic and weak nuclear force?

I know the electromagnetic force is mediated by a photon and the weak nuclear force is mediated by two massive bosons. Are there any other insights into why the masses are so different?
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2answers
28k views

Has any permanent magnet motor been proven to run?

I have read lots of articles about permanent magnet motors, some of which claim the possibility and other which refute it. Is it possible to have a permanent magnet motor that runs on the magnetic ...
6
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1answer
4k views

How do 2 magnets spin by themselves if positioned precisely?

A few years ago I went to a museum, where there was a board with 2 bar magnets, on a pole each so they could rotate. If you rotated them so the lined up with the same poles (N) facing each other, ...
2
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2answers
505 views

Electron model under Maxwell's theory

I was not able to recall my memories, so: What is the formula that states the frequency of electrons revolving around nucleus is equal to the frequency of light (or photon) emitted (or radiated)? (I ...
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2answers
107 views

Where's the energy in coupled magnetic fields?

Suppose I have two identical current loops superimposed upon one another, but with their currents in opposite directions. The coefficient of coupling $K=1$, and therefore the mutual inductance $M ...
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5answers
10k views

What non-metal is attracted by a magnet?

Are there any non-metal objects that are attracted by magnets?
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1answer
131 views

Possible to generate an electromagnetic field from moving an object through a magnetic field?

Is it possible to generate an electromagnetic field from a stationary rare earth magnet with a hole in it by passing an object through the hole? By definition, a Lorentz Force is the force on a ...
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1answer
28 views

Why coercivity increase in after depositing organic on top of Ferromagnet [closed]

I had deposited Fullerene on top of Cobalt thin film which result in increase of coercivity.
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0answers
509 views

How does a Fresnel rhomb work (half and quarter wave plate)?

How does a Fresnel rhomb work (half and quarter wave plate)? I am aware of birefringence, which creates a phase shift of $\Delta\phi=\dfrac{2\pi\Delta nL}{\lambda_0}$. But this doesn't explain how a ...
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2answers
2k views

Make a semi transparent mirror with copper

The question: How would you make a semi transparent mirror (50% reflection, 50% transmission) with glass with a layer of copper. For light $\lambda$ = 500nm Try to be as realistic as possible What ...
4
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1answer
209 views

Can we tell from periodic table that which materials are ferromagnetic?

I want to know that because I want to see whether ferromagnetic materials relate to the number of outer most electrons. If there is no direct relationship, could you please give me a list of the ...
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1answer
162 views

Is it possible to charge a capacitor to any potential? Does it depend on the dielectric placed between them?

Is it possible to charge a capacitor to any potential? Does it depend on the dielectric placed between them? Or, are there other factors affecting it?
4
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1answer
448 views

Monochromatic wave

If we have an EM wave like this one: $$E=\begin{pmatrix}1\\i\\0\end{pmatrix}e^{-i(\omega t-kz)}$$ The wave has clearly only one frequency $\omega$, but is it monochromatic? My doubt is that it's ...
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2answers
129 views

Are electrons moving? If so relative to whom? and why is there no magnetic field?

electrons on earth moving with us, due to rotation of earth, revolution of earth, sun and our galaxy right? Then, why is there no magnetic field around a piece of copper wire?
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0answers
366 views

What is the current induced in a straight wire when a magnetic field is turned from on to off?

Consider a straight wire which runs along the y-axis. A B-field exists which is uniform and only in the x-direction: $$\vec{B}=(B(t),0,0)$$ The B-field at time $t_0$ is 'switched off' in ...
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3answers
905 views

Does the energy of a magnetic field decrease when it moves a conductor carrying a current?

When a charged particle moves in an electric field, the field performs work on the particle. Thus, the energy of the field decreases, turning into kinetic energy of the particle. Does the magnetic ...
2
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1answer
353 views

Reason behind cohesive and adhesive force

What is the real cause behind the cohesive and adhesive forces?
3
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4answers
721 views

Why is the energy density of the electric field produced by two unlike charges positive, even though their potential energy is negative?

Consider two stationary charges, one positive the other negative. Their potential energy is clearly negative. So you would expect that the energy density of the associated electric field would also ...
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1answer
2k views

Magnetic Fields created by Current-Carrying Wires

The drawing shows two perpendicular, long, straight wires, both of which lie in the plane of the paper. The current in each of the wires is I = 5.6 A. What is the force and torque acting on each wire ...
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1answer
60 views

Summing up magnetic fields

In the case of a complex shape filiform distribution of current, are we allowed to determine the magnetic field created by sections of the distribution and then summing them up, like we do with a ...
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1answer
191 views

Should the electric potential of a positively charged sphere be negative?

Because the indefinite integral of the electric field results in a negative value? (As the function is proportional to $r^{-2}$? I've got to be missing something... Help please!! Thanks! Also, I ...
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5answers
109 views

Is there a more scientific term for “obstruction of EM waves”?

When EM waves pass through things like rain and hail, they can be "obstructed" and bounced back or absorbed, rather than passing through. I'm conducting an experiment on this effect, and wondered if ...
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1answer
2k views

Mutual Inductance and the Dot Convention

Can anyone please explain me, the dot convention in coil systems (Mutual and self inductance) with some related images to understand..?
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0answers
50 views

Should a 1D Guassian wave oscillate?

I wrote a few lines that numerically solve Maxwell's equations. The result is a moving wave that looks like a single pulse. This looks strange to me because I expect waves to move in oscillator ...
4
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4answers
2k views

Can an object between two magnets reduce their attraction/repulsion?

If you have two magnets (not in contact) and then put a non-magnetic object in between the two magnets, does that decrease the attraction/repulsion between the two magnets? It seems that it wouldn't, ...
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3answers
838 views

Can the path of a charged particle under the influence of a magnetic field be considered piecewise linear?

Ordinarily we consider the path of a charged particle under the influence of a magnetic field to be curved. However, in order for the trajectory of the particle to change, it must emit a photon. ...
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1answer
77 views

Regarding Electromagnetic Plane and Maxwell equations

I asked this on the math.stackechange but I was told that it might be a good idea to ask here too since my problem is physics/math! Here is the question: Hello everybody I am kind of struggling with ...
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1answer
236 views

Are normal light waves more dangerous than WiFi radiation? [closed]

I'm getting sort of tired of all the "wireless hysteria", so I thought this might be a nice comment. To the extent of my knowledge, WiFi uses microwaves, which have a lower frequency and hence carry ...
1
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1answer
340 views

Faraday's Law and magnetic monopoles

The magnetic monopoles does not exist which can be shown by $ \int {\vec{B} \cdot d\vec{A}} = 0 $. But in Faraday's Law of electromagnetic induction, we clearly show the EMF induced is the time rate ...
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1answer
313 views

Conservation, Maxwell tensor

Can someone please explain to me how the conservation of the energy momentum tensor $$\nabla_\beta T^{\alpha \beta}=0$$ imply the conservation of the Maxwell tensor $$\nabla_\beta F^{\alpha ...
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1answer
928 views

why define relative permittivity and relative permeability?

I am reading something about electomagnetic field and the first introduce the free space permittivity and permeability for the electric field and magnetic field. And later when discussing the field in ...
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0answers
125 views

Einstein +Maxwell 's tensor

Why is it true that we can deduce that Einstein's GR equations coupled with Maxwell's EM equations may be written in the form $$R_{ij}=C(F_{ik}F_j^{\,\,k}-{1\over 4}g_{ij}F_{mn}F^{mn})$$ without ...
3
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1answer
145 views

Absorption cross section for direct dissociation and predissociation - Photoelectric absorption

Why the absorption cross section in direct dissociation process is wide and structureless while the absorption cross section in the predissociation process is structured and containing lines which are ...
4
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2answers
889 views

How does lightning “know” where to go?

If lightning comes down in, say, a large flat field with a lightning rod sticking out of the middle, the lightning will strike the rod. How does it "know" the rod is there? Will it always strike the ...
2
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2answers
326 views

Magnetic properties of matter

When a dielectric is placed in an electric field,it gets polarized. The electric field in a polarized material is less than the applied field. Now my query is, when a paramagnetic substance is kept ...
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1answer
128 views

Thermodynamics and electromagnetic fields

The energy density in an electromagnetic field is given by: $u = (1/2) <\vec{E}^2 + \vec{B}^2>$ where $<,>$ denotes the average over time. In a cavity it holds that $u= u(T) = ...
3
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2answers
2k views

Greens function in EM with boundary conditions confusion

So I thought I was understanding Green's functions, but now I am unsure. I'll start by explaining (briefly) what I think I know then ask the question. Background Greens are a way of solving ...
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1answer
54 views

What role has our Moon played in creating a persistent geomagnetic field?

The question comes from a comment by Mark Rovetta on my earlier question about the Earth's core going cold.
3
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3answers
954 views

Maxwell equations invariant under Lorentz transformation but not Galilean transformations

Why Maxwell equations are not invariant under Galilean transformations, but invariant under Lorentz transformations? What is the deep physical meaning behind it?
5
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1answer
758 views

Angular momentum and EM wave

Is there any sense in saying that circularly polarized EM waves have angular momentum?