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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1k 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 ...
4
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1answer
210 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 ...
1
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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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1answer
355 views

Reason behind cohesive and adhesive force

What is the real cause behind the cohesive and adhesive forces?
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0answers
367 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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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
193 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 ...
19
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1answer
708 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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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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2answers
3k views

What is the force between two perpendicular wire? [closed]

What is the force between two perpendicular wire carrying current, one to the north and one to the east?
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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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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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1answer
303 views

Aharonov-Casher effect for charged particles

All the explanations of the Aharonov-Casher effect seem to imply it only "works" for neutral particles with a magnetic moment. This seems to stem from the duality of the A-C effect with the more known ...
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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 ...
0
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1answer
238 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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2answers
199 views

Scale-down modeling for radar cross-section measurement

Scale-down models for targets are used for radar cross-section measurements of huge-size objects. Is there any upper limit to the scaling factor on downscale modeling as a $100~\text{m}$ length of ...
10
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3answers
843 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. ...
1
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1answer
341 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
935 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 ...
0
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1answer
314 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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0answers
126 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 ...
1
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3answers
484 views

Ferromagnetism and electrical conductivity

Are ferromagnetic materials good conductors of electricity? If yes, please explain with proper examples.
6
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4answers
707 views

Influence of charged particle's own electric field on itself

I read this in my textbook: A charged particle or object is not affected by its own electric field. Since I find this completely unintuitive and my mind is yelling "wrong! wrong! how could a particle ...
6
votes
1answer
504 views

Explosively Pumped Flux Compression Generator = EMP strong enough to destroy my phone from a mile away?

Explosively Pumped Flux Compression Generator detonated a mile away or so, what damage would be done to my phone, and other electronics? I would imagine it would be similar to a CD in the microwave ...
2
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2answers
721 views

Should I use Coulombs law when magnets attract/repel?

When magnets attract to each other or repel. Should I use Coulombs law? If not, why not? Some would say that I shouldn't because: "Coulomb's law deals with static charges and force due to them. ...
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 ...
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3answers
5k views

Induced emf in AC generator

The induced emf in a coil in AC generator is given as: $$\mathbb E = NAB\omega \sin \theta $$ $\omega = d\theta/dt$ Now, when the angle between the normal of plane and magnetic field is zero ...
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3answers
6k views

conservation of energy in Lenz's law

When we bring opposite poles of two magnets together, they attract each other (or vice versa). Now, we can say that the kinetic energy gained by the magnets is due to the attractive force. Similarly, ...
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) = ...
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2answers
1k views

Electric potential energy and speed

If we have electric field and we put electron there , the electron will move in the opposite direction as the electric field. My question is electron in that direction will speed up or slows down ? ...
0
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1answer
55 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.
5
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2answers
675 views

Einstein Field Equations and Electromagnetic Stress-Energy Tensor

My question is: if we write Einstein field equations in this form: $$R_{\mu\nu} - \dfrac{1}{2}g_{\mu\nu}R=8\pi \dfrac{G}{c^4}T_{\mu\nu}$$ Then the left hand side is one statement about the geometry ...
1
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1answer
138 views

Does a photon have a north and south pole?

A photon has an oscillating magnetic and electric field. Is the magnetic field a dipole?
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1answer
92 views

Can electromagnetic momentum be introduced at pre-university level as for electromagnetic energy?

Electromagnetic energy is introduced at pre-university level, starting with static electric energy followed by static magnetic energy. But the introduction of electromagnetic momentum usually has to ...
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1answer
317 views

a metal bar on a pair of conducting rails that carries a current

This is a homework question, and I solved it already, but something bugs me. So the problem is stated as following: A metal bar of mass M sits on a pair of long horizontal conducting rails separated ...
6
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0answers
114 views

Is the search for a Simple-group-based Electro-Weak theory over?

Just wondering: We know that, in its current form of the $SU(2)_L\times U(1)$, the electroweak theroy rides a wave of huge success. However, is it not possible that the correct simple group ...
1
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1answer
454 views

How many fundamental fields / constraints are in Maxwell's Equations?

I've seen electromagnetics formulated in the potentials ($\bf A$, $\varphi$) (and their magnetic counterparts) in the Lorenz gauge for a long time. Justifications for using these include that they ...
2
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3answers
1k views

Maxwell Stress Tensor in the absence of a magnetic field

I'm having some trouble calculating the stress tensor in the case of a static electric field without a magnetic field. Following the derivation on Wikipedia, Start with Lorentz force: $$\mathbf{F} = ...
2
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2answers
181 views

Radiated power from a 'source volume' outside which charge & current are zero (i.e. derive radiated power from Jefimenko's equations)

In classical electrodynamics, what is the radiated power from a generalized source (consisting of charge density $\rho$ and current density $\vec{J}$) in vacuum? Let us define $V_s$ to be the ...
0
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1answer
149 views

what is the difference between constant and changing magnetic and electric fields? How do they occur? How do they form an electromagnetic wave?

what is the difference between constant and changing magnetic and electric fields? How do they occur? How do they form an electromagnetic wave?
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 ...
7
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3answers
2k views

Why is visible light used in Optical fibers (instead of other EM waves)?

Why aren't other electromagnetic waves used in optical fibres instead of visible light? Is it because the wavelength of light fits the internal reflection/refractive index of the material used for the ...
4
votes
2answers
890 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 ...
11
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5answers
627 views

Form of the Classical EM Lagrangian

So I know that for an electromagnetic field in a vacuum the Lagrangian is $\mathcal L=-\frac 1 4 F^{\mu\nu} F_{\mu\nu}$, the standard model tells me this. What I want to know is if there is an ...
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0answers
101 views

How to calculate how weak does a magnet get when you get an other magnet closer to it?

I heard that when you take two magnets and get them closer together so they reject each other (north pole to north pole or south pole to south pole) they weakens. Does anybody knows how to calculate ...
2
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1answer
1k views

Length of wire required for solenoid to produce desired magnetic field

In a question: To construct a solenoid, you wrap insulated wire uniformly around a plastic tube 12cm in diameter and 50cm in length. You would like a 2.2 A current to produce a 2.6 kG magnetic ...
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3answers
4k views

Similarity between the Coulomb force and Newton's gravitational force

Coulomb force and gravitational force has the same governing equation. So they should be same in nature. A moving electric charge creates magnetic field, so a moving mass should create some force ...
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1answer
188 views

A rod filled with water and charged particles is rotated in magnetic field

Assume we have a rod of diameter $d$ and length $l$. It is filled with a mixture of water and, say, sodium chloride. Thus there are positively charged sodium ions and negatively charged chloride ions ...
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7answers
2k views

What happens to the magnetic field in this case?

As far as I know, it's possible to create a radially polarised ring magnet, where one pole is on the inside, and the field lines cross the circumference at right angles. So imagine if I made one ...