Tagged Questions

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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3
votes
1answer
255 views

How much power can be drawn from stray electromagnetism in the atmosphere?

I know this probably varies quite a bit from place to place on earth. But just some rough estimates: if I were to pull power via multiple antennae tuned to a variety of different frequencies, how much ...
3
votes
1answer
4k views

When does voltage drop occur?

Why or when does it occur in a circuit? What does it imply when you speak of a voltage drop across a resistor? (Obviously, it probably means that the current's voltage before the resistor is higher ...
2
votes
2answers
977 views

Does space have to be filled with charged particles to carry electromagnetic waves?

I'm a newbie here so have mercy. I'm studying electromagnetic waves. This is the propagation of energy via the vibration of charged particles, as I understand it. A charged particle could be like ...
3
votes
5answers
3k views

How to find the poles on a spherical magnet?

To any nonmagnet, the whole sphere is a magnet. To another spherical magnet though, there is a rough area on the surface where it is strongly repelled. Given a spherical magnet, how should the poles ...
3
votes
2answers
615 views

Paramagnet: Negative specific heat?

for a simple paramagnet ($N$ magnetic moments with values $-\mu m_i$ and $m_i = -s, ..., s$) in an external magnetic field $B$, I have computed the Gibbs partition function and thus the Gibbs free ...
2
votes
3answers
2k views

Is visible light and radio waves made of the same thing?

I understand that there is such a thing as the electro magnetic spectrum, and that light and RF are both on it, so dose that mean that they are made of the same thing? Just at different frequencies.
4
votes
2answers
357 views

Interference of EM Waves with Orbital Angular Momentum

If you have two coherent collinear e-m beams of same frequency and polarization, but 180 degrees out of phase, they will destructively interfere. If you introduce orbital angular momentum of L=3 ...
12
votes
4answers
4k views

How long does a permanent magnet remain a magnet?

I have a bunch of magnets (one of those game-board thingies) given to me when I was a school-going lad over 20 years ago, and the magnets feel just as strong as it was the day it was given. As a ...
3
votes
2answers
156 views

speed of an electromagnetic wave

I have this equation $$\dfrac{\partial E_z}{\partial y^2} - \mu_0 \varepsilon_0 \dfrac{\partial E_z}{\partial t^2} = 0$$ Why is $v^2 = 1 / (\mu_0 \varepsilon_0)$ ?
1
vote
1answer
162 views

The energy carried from one winding of a transformer to another, in quantum terms

I have read in wikipedia this statement "The energy carried from one winding of a transformer to another, in quantum terms is carried by virtual photons, not real photons" (wikipedia src: virtual ...
0
votes
1answer
623 views

Changing magnetic flux graph?

In regards to a graph of the changing magnetic flux in a generator such as this one: a) The equation of the graph should be $\Phi = BA \cos \theta$. As $\theta=\omega t$ (angular velocity*time), ...
1
vote
3answers
523 views

What was meant by the 'ponderomotive force' as understood by Minkowski?

Skimming through Minkowski's famous 1907 paper, he uses the term ponderomotive force. What does he mean by this?
10
votes
3answers
5k views

Where can I find simulation software for electricity and magnets?

Is there easily-available* software to simulate coils, solenoids, and other magnetic and electromagnetic devices? I'd like to play around with some design ideas, such as Halbach arrays, but physics ...
3
votes
3answers
3k views

Why does current alternate in an AC generator?

I understand how generators work, but I can't for the life of me conceptualize why the current in an AC generator reverses every 180 degrees!!! I understand that, using the right hand rule, it can be ...
2
votes
2answers
4k views

Why exactly does current carrying two current wires attract/repel?

When to parallel wires carrying currents in same direction I1 & I2. http://www.youtube.com/watch?v=43AeuDvWc0k this video demonstrates that effect. My question is, why exactly does this happen? ...
4
votes
1answer
723 views

What is the effective spring rate of a magnetic spring

Consider a magnetic spring as seen on this YouTube video, but ignore gravity. If I wanted to calculate the effective spring rate (Force vs. Deflection) curve for the top magnet, how would I go by ...
1
vote
2answers
345 views

Pollen Particle Attracted to TV due to Which Force?

A pollen particle has no charge so I cannot understand how the Lorentz force $\bar{F} = q \bar{E} + q(\bar{v} \times \bar{B} )$ could explain the event. I speculated that it is because of the electric ...
3
votes
5answers
1k views

What is the origin of the Dirac delta term in the dipole electric field?

I am a bit lost how one has deduced the formula for electric field with electric dipole because of some inconsistency between different sources. The Wikipedia article contains a delta function in the ...
0
votes
1answer
1k views

Using Ampere's circuital law for an infinitely long wire & wire of given length

According to Ampere's Ciruital Law: Now consider two straight wires, each carrying current I, one of infinite length and another of finite length ...
2
votes
1answer
103 views

Mangnetic Flux summing up like Kirhoff?

You have a coil over an iron hearth. There is a current in coil which creates the flux $\phi_{1}$. The flux then distributes over the wider area in the iron (using wrong word?): $\phi_{2}$ the flux ...
1
vote
2answers
2k views

Magnetic field in the Centre of Circular loop wire with Current?

By biot-savart: $$\bar{H} = \frac{I}{4\pi} \oint \frac{d\bar{l} \times \bar{r}}{r^{3}}$$ so $$\bar{H} = \frac{I}{2a} \hat{n}$$ Please, explain the last implication. I cannot find such integral to ...
9
votes
4answers
3k views

Can superconducting magnets fly (or repel the earth's core)?

If a superconducting magnet and appropriate power supply had just enough $I\cdot s$ (current $\cdot$ length) so that when it was perpendicular to the earth's magnetic field, the force of the ...
3
votes
2answers
6k views

Why is the magnetic field inside a solenoid constant?

Why is the magnetic field along the length of a solenoid constant? (preferably in relatively simple terms) Thanks!
4
votes
2answers
887 views

What role does electrical charge play in black holes?

Not having studied General Relativity, I have sometimes been puzzled by references to the behaviour for "classic" black holes — as they are popularly portrayed — as being true for black ...
3
votes
2answers
718 views

References for real life applications on advanced EM

For EM (freshman level physics) and advanced EM (Junior/Senior level) to help students appreciate the material, I am looking for books/websites that contain: 1-applications of electricity and ...
4
votes
1answer
415 views

Question about Rayleigh scattering

To quote from Wikipedia on elastic scattering, "In this scattering process, the energy (and therefore the wavelength) of the incident photon is conserved and only its direction is changed." How does ...
9
votes
1answer
3k views

What is the source of Earth's magnetic field?

I saw on Discovery channel that the source of Earth's magnetic field is the molten iron and metals in the earth's core. The spinning of these liquid metals produces the same effect as electric current ...
6
votes
1answer
581 views

Magnetic field from a half-cylinder [closed]

I am preparing for an exam, on this problem I had the opposite direction of the magnetic field. A conductor-cylinder with radius R has been cut in half ($\phi \in [0,\pi]$) A DC current $I$ runs ...
1
vote
1answer
231 views

Lenz' law versus $-\frac{d\Omega}{dt}$

I am preparing for an exam, on one problem my answer differ from the solution. The current $i(t)=I_0e^{-\alpha t}$ runs in a long straight conductor along $\hat z$. Point A,B,C,D forms a triangle. ...
3
votes
2answers
354 views

How to measure faster than light electric energy?

According to relativity,nothing can break light barrier.But a recent preprint shows energy transmission of commercial electric power (f=60Hz) is faster than light. (It is not the drift velocity of ...
3
votes
1answer
171 views

Scattering off a random magnetic field?

Here is another old exam question I'm wondering about: A proton moving in the $-x$ direction encounters a region of space with a magnetic field that randomizes the direction of the particle. The ...
1
vote
0answers
138 views

Reflected electromagnetic wave relation

If incident electromagnetic wave is given as: $$\begin{align*}E_i&=A_e \cos(\omega t + bz)\\ H_i&=A_h \cos(\omega t + bz)\end{align*}$$ What would be relation for REFLECTED wave? Does it ...
3
votes
3answers
534 views

Does the wavelength always decrease in a medium?

I was studying a GRE Physics Test problem where optical light with a wavelength of 500 nm travels through a gas with refractive index $n$. If we look at the equations for wave motion and index ...
2
votes
1answer
83 views

One point to change electric field

Can there be a charge configuration in space such that at any instant of time I can change the electric field at one and only one point?
19
votes
4answers
911 views

Why does only one side of a neon lamp glow?

When applying DC to a neon lamp, only one electrode glows: The voltages across the lamps are left: DC (left lead positive), middle: DC (right lead positive), and right: AC. But... why? The ...
4
votes
2answers
379 views

Must the action be a Lorentz scalar?

Page 580, Chapter 12 in Jackson's 3rd edition text carries the statement: From the first postulate of special relativity the action integral must be a Lorentz scalar because the equations of ...
4
votes
1answer
138 views

Photons in a “wrap-around” universe

This question was inspired by: How are photons "consumed"? Imagine I have some number of photons, $N$, each of frequency $\nu$, moving randomly in a spherical "wrap-around" universe of ...
2
votes
1answer
261 views

Practical physics: where does this current come from?

It's been a while since school and I don't remember the right laws and formulas. So I come here. :) I've got a power cord extender running from the wall outlet to my washing machine. It's about 3 or ...
10
votes
2answers
750 views

If you run an electric current through a wire loop, do the accelerated charges radiate?

Does an accelerated charge always radiate? For example the current electrons in an electric circuit when moving through a turn they are accelerated, do they radiate because of that acceleration? If ...
1
vote
0answers
75 views

Scaling an electron microscope to fit within 1 cubic cm when running multiple parallel scans [closed]

Yes, I'm serious. What components correspond to an individual scan vrs what components can be cascaded across all objects? Ex: The vacuum chamber can be cascaded ( colocate the sample chambers ...
3
votes
1answer
135 views

3 current loops with non-conservation of momentum, when one specifically considers SIGNs

Thought experiment I. Consider two loops of wire, 2 small dipoles B and C , with a common axis z (facing each other) and (say) 30 cm apart B to C. At the speed of light, information (including a ...
11
votes
1answer
197 views

Theorems on instability of classical systems of charged particles?

Classically, a hydrogen atom should not be stable, since it should radiate away all its energy. I remember hearing from my favorite freshman physics prof ca. 1983 about a general theorem to the effect ...
5
votes
1answer
326 views

2 electromagnetic loops, with light-speed constraints: Is Newton's Third Law violated?

I'm a retired Physics Professor, but I admit I'm puzzled by this... Consider two loops of wire, 2 small dipoles B and C , with a common axis z (facing each other) and (say) 30 cm apart B to C. At ...
1
vote
1answer
89 views

Creating the opposite of an optical lattice

Is it possible to create periodic potentials that instead of creating a well for an atom to be trapped in, repulsed by that specific location? If yes, can we use this as a means to make artificial ...
4
votes
1answer
222 views

The feeling of fuzz on the display surface of a Cathode Ray Tube Television

I was always wondering where the feeling of fuzz comes from when touching a CRT (Cathode Ray Tube) television on its display screen when the TV is turned on. Can someone please explain why I feel ...
13
votes
3answers
900 views

Where the angular momentum has gone?

I came across an interesting problem when I prepared for the preliminary exam on electromagnetism. Below is the problem in its original words: A metallic sphere of mass, $m$, and radius, $a$, ...
3
votes
1answer
304 views

Lorentz invariance of a frequency- and wavelength- dependent dielectric tensor

Suppose we have a material described by a dielectric tensor $\bar{\epsilon}$. In frequency domain, this tensor depends on the wave frequency $\omega$ and the wave vector $\vec{k}$. Clearly not all ...
1
vote
1answer
2k views

How to calculate the exit velocity of a coil gun projectile?

First off, what quantities need to be factored in? Voltage and current through the coils, the magnetism of the projectile, the magnetic fields, etc.? Next, how would you calculate the speed of the ...
9
votes
4answers
653 views

fraction of magnetic energy stored outside a solenoid

If I have a long solenoid, e.g. length $l$ and radius $r$ with $l = kr$, where k >> 1, with a nonpermeable (e.g. air) core, how much of the magnetic energy is stored outside as compared to inside? If ...
4
votes
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, ...