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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14
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8answers
10k views

Is gravity just electromagnetic attraction?

Recently, I was pondering over the thought that is most of the elementary particles have intrinsic magnetism, then can gravity be just a weaker form of electromagnetic attraction? But decided the ...
1
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1answer
39 views

Need an analytic model for describing a region with fast transition of magnetic field from a constant value to zero

I would like to describe analytically the electromagnetic fields in a region with the magnetic field changing rapidly from a constant value to zero. It's more or less like the fringe region of the an ...
0
votes
2answers
355 views

Why do electricity generators have to work harder for higher loads?

Maybe this is a silly question, but why does, say, a gasoline-powered AC generator have to use more gas depending on the load? Let's say I have a 120VAC generator and either a 1A or a 10A load, and ...
8
votes
2answers
4k views

Is it possible to kill a human with a powerful magnet?

I'm asking in terms of physics. Can powerful magnetic induction rearrange spins of my body in such way I will die? How? Or maybe it can rip all iron from me, which would make my blood cells useless? ...
2
votes
1answer
43 views

Taylor expansion of translated fields

First of all, I would like to say that I am somewhat new to four-vector notation. I have a function of a four-vector that I want to expand. $$ A_\mu (\mathbf{x} + \mathbf{x}_0) = A_\mu (\mathbf{x}) ...
1
vote
1answer
50 views

Can magnets destroy notification light on phone? [closed]

I'm posting this question here because it's about magnets and electronics. Please tell me if it's better suited to another site! I've been messing around with some reasonably strong magnets around my ...
2
votes
3answers
1k views

Curl of an electromagnetic wave

I can't understand the concept of the curl of an electromagnetic wave. $$ \nabla \times E = -\frac{\partial \textbf{B}}{\partial t} $$ All of the examples I find show a current through a conductor, ...
7
votes
2answers
528 views

Counting the number of propagating degrees of freedom in Lorenz Gauge Electrodynamics

How do I definitively show that there are only two propagating degrees of freedom in the Lorenz Gauge $\partial_\mu A^\mu=0$ in classical electrodynamics. I need an clear argument that involves the ...
9
votes
1answer
744 views

Recovering all of Maxwell's equations from the variational principle

Whether you can get the first couple of Maxwell equations from a variational principle? In the second volume of the Landau theoretical physics said that it is impossible.
1
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1answer
563 views

How is magnetism ''conducted'' through a non-magnetic metal?

I have a ball of metal about an inch in diameter and a concave disc of another metal (which is magnetic) around the ring of the disc (about $12 {\rm mm}$ in diameter). I don't know which metals they ...
1
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1answer
78 views

Electromagnetic waves and group velocity

I have three questions about electromagnetic waves and was wondering whether anybody here could comment on these things: Wikipedia says that there are no longitudinal EM waves, although TM and TE ...
0
votes
1answer
270 views

How do I create a direct current with a magnet? The magnet is not to be moved in the direction of the wire [closed]

How do I create a direct current with a magnet? The magnet is not to be moved in the direction of the wire. In fact I'm looking for the contrary to the drawing from ...
0
votes
0answers
306 views

Why are magnets attracted to negative side in battery

I have been experimenting with magnets, and built an Homopolar motor however it works but the magnet only attracts the negative side of the AA battery (the flat side of a battery) which I thought was ...
1
vote
2answers
3k views

How is the direction of Magnetic/Electric Lines of Force Known?

It is shown that the direction of magnetic line is from north to the south and that of the electric line is from positive to negative. How do we/scientists know that the imaginary lines of force or ...
0
votes
1answer
591 views

How are the E and B fields inside a coaxial cable related to the current and voltage?

Conceptually, I understand that coaxial cables operate in a TEM mode due to the central conductor, and hence the electric field is directed radially outward from the voltage $V_0$ at the centre of the ...
1
vote
2answers
79 views

Energy of an EM Wave and its temperature and amplitude

I'm trying to understand why classical physics fails to explain black body radiation. I'm confused. According to Boltzmann, energy calculation for em wave is based on temperature. According to ...
2
votes
2answers
79 views

Speed of light in different parts of the universe

Do we have enough evidence to claim that speed of light remains the same at 'c' even at the most distant parts of the visible universe?
2
votes
1answer
571 views

How to transform material permittivity tensor from Cartesian coordinates to another orthogonal coordinate system?

I have a material specified by a permittivity tensor written in Cartesian coordiantes: $$\begin{pmatrix} \epsilon_{xx} & \epsilon_{xy} & \epsilon_{xz}\\ \epsilon_{yx} &\epsilon_{yy} ...
1
vote
0answers
31 views

How to isolate and verify the source of optical absorption/effects?

This is similar to my other question, but not the same -- that one was about the energy ranges of various absorption mechanisms, and this one is more about experimental techniques to find them. Let's ...
1
vote
1answer
58 views

Electricity voltage & current [closed]

Why does current return to its source?
1
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0answers
27 views

If lead's electron configuration ends with 6p$^2$, shouldn't it be paramagnetic? [duplicate]

Doesn't lead, according to its electron configuration, have unpaired electrons? Yet why isn't it paramagnetic?
3
votes
3answers
132 views

At an instant, does a system of gravitational charges exhibit equivalent behavior to a time-reversed system of like electric charges?

Question: In principle, does a system of gravitational charges exhibit equivalent behavior to a time-reversed system of like electric charges? (At a single instance in time?) Additional Notes: I am ...
-2
votes
1answer
140 views

Maxwell equations [closed]

$$\oint B.dl = \mu_0\left(I+\epsilon_0\frac{\partial\Phi_E}{\partial t}\right)$$ Please explain the applications , and implications of the modified Ampere's circuital law with Maxwell's addition. ...
2
votes
1answer
74 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 ...
7
votes
1answer
4k views

Why is the magnetic field stronger at the edges of a bar magnet?

I have been performing an experiment at school in which I test the force on an iron surface from the magnetic field of an electromagnet. The electromagnet has a rectangular iron core. The theory ...
0
votes
1answer
614 views

Constructive Interference of Electromagnetic Waves

So I was wondering if Electromagnetic wave has the same property of interference as normal waves. I understand that both the electric and magnetic parts of the wave would have to be in the same ...
7
votes
5answers
595 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, ...
3
votes
1answer
233 views

Solving electromagnetic vector field using the Lagrangian

Given an action of the form \begin{equation}S=-\frac{1}{4}\int d^4x\eta^{\mu\nu}\eta^{\lambda\rho}F_{\mu\lambda}F_{\nu\rho}\end{equation} where ...
4
votes
4answers
770 views

How do electrons actually move in a circuit?

Last year, we were taught about electricity, about how electrons move in a closed circuit. But as our teacher had not taught us about electric fields yet, she gave us a simplified model of motion of ...
1
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0answers
85 views

Is the dielectric constant of an insulator always positive?

I know there are many examples of metals with a negative dielectric constant at optical frequencies. But what about insulators?
0
votes
1answer
75 views

Understanding frequency of a signal

I'm trying to understand how can one transmit different signals on a same copper wire using different frequencies. How does the electrons actually interact with the frequency? And how does the ...
1
vote
0answers
30 views

Sphere in homogenou magnetic field

I have to find out the magnetic dipole moment of sphere in homogenou magnetic field in the limit $\mu_r\rightarrow+\infty$. I don't know what does the condition $\mu_r\rightarrow+\infty$ mean. Does ...
4
votes
2answers
1k views

How do I actually calculate the Lorentz transformation of a field strength tensor

Say now I have an arbitrary field strength tensor $F$, and I want to boost it according to a Lorentz transformation matrix $(\Lambda)$ The transformation is given by $$ F^{'\mu \nu} = ...
-2
votes
1answer
1k views

why does alternate current attract and direct current repel [duplicate]

Does AC current attract and DC repel our body? If the potential difference remains same will still AC and DC will give us same type of electric shock?
4
votes
2answers
4k views

Is magnetic force non-conservative? [duplicate]

If magnetic field is conservative, then why not the magnetic force? My professor thinks it is non conservative but he couldn't explain to me why?
1
vote
1answer
298 views

What are 'two way' magnets and how do they work?

Thomas & Friends Take-n-Play series connect to each other via something described as 'two way' magnets: the new models use special 'two-way' magnets As a parent of an owner of two engines of ...
0
votes
2answers
313 views

Does a simple copper wire with an AC current create EM waves?

I want to know if a household AC current flowing through a simple copper wire will radiate EM waves? If yes, up to what range and can they be called radio waves?
2
votes
2answers
412 views

Why is this not a violation of parity invarance for EM

I read that Wu's experiment illustrates that parity violation is possible for weak processes. In that experiment, when Co-60 undergoes beta decay, the emitted electrons come out opposite to the ...
1
vote
1answer
441 views

Finding speed of light by $c=f\lambda$?

When considering EM radiation as waves it is said that it is electric and magnetic fields that oscillate with time. Therefore $f$ is not frequency of distance but of electromagnetic fields. I have ...
1
vote
3answers
184 views

Definition of Electromagnetic field?

Definition of Electromagnetic field? What is the Electromagnetic field at a given point in space due to a point charge being accelerated non-uniformly? Is there a single equation that can give me ...
3
votes
0answers
290 views

Attraction and repulsion of electron spin ups and electron spin downs

Alright, we know that copper is a diamagnetic material, which has paired electrons. These paired electrons have different spin. I'm specifically interested in what is going on with the electrons in a ...
2
votes
1answer
127 views

What does it mean for an electromagnetic structure to be resonant?

There are many electromagnetic structures used in microwave engineering and EM devices. For example, patch antennas, metamaterials made from unit cells, etc. When they design structures like patch ...
4
votes
2answers
611 views

Why my simple electromagnetic experiment does not work?

I make a setup (see attached picture). ONE coil (1" diameter x ~120 turns), which is separated with a gap, is mount on two sides. Each side has an iron rod (1" diameter x 6" long) to increase the ...
0
votes
2answers
158 views

motion of an electron near a conducting wire [closed]

An electron is ejected from the surface of a long thick straight conductor carying a current, initially in direction perpendicular to the conductor. The electron will: a) ultimately return to the ...
1
vote
0answers
51 views

What would happen if…? (Scenarios involving a ball in an electric field) [closed]

First I shall define two table tennis balls: Ball $A$ is coated with a conducting material and ball $B$ is an insulator. Then I'll define two scenarios: Scenario $I$ is a ball held from a ...
1
vote
0answers
63 views

Difference between action-at-a-distance and a field according to Maxwell?

My question is more on a historical note that involves Maxwell’s equations. Besides the information that I have obtained from textbooks, I am mainly getting it from History of Maxwell's Equations and ...
0
votes
1answer
73 views

Can an electron change direction in a conductor?

Can an electron change direction in a conductor? Or is it restricted to do so? Why can't it change direction do to the atoms of the conductor itself? Why if the change in the electron's direction ...
1
vote
2answers
87 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 ...
1
vote
2answers
267 views

What will happen to a solenoid with a bar magnet core?

Suppose I wrap wire around a bar magnet, creating a solenoid such that the poles of a bar magnet and the poles of solenoid are opposite to each other. If I were to hold a compass near one side, which ...
0
votes
3answers
80 views

What will happen to compass with north all around it?

If I keep a compass with bar magnets all around it, all of equal strength and a compass in a center then what will happen? Will the compass start rotating?