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
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0answers
112 views

A correct explanation for the levitation of a superconductor above a magnet [duplicate]

I teach high school physics and I'm trying to put together a correct explanation for the levitation of a superconductor above a magnet without a high level of quantum mechanics (but consistent). I ...
3
votes
4answers
587 views

Can a magnet be tuned to attract only to one other magnet?

So think magnet X is only attracted to magnet Y , but magnet A is not attracted to either magnet Y or X. Only X and Y can attract each other???
5
votes
1answer
192 views

Help me understand eqn. 28.6 from Feynman

Feynman says that the electric field, $\bf{E}$, can be written as, $$\mathbf{E} = \frac{-q}{4 \pi \epsilon_0} [ \frac{ \mathbf{e}_{r'}}{r'^2} + \frac{r'}{c} \frac{d}{dt} (\frac{\mathbf{e}_{r'} ...
-2
votes
1answer
91 views

What is the alternative of PMG (Permanent Magnet Generator) in a Horizontal Axis Wind Turbine? [closed]

I need to make a report about a HAWT (Horizontal Axis Wind Turbine) and list the parts with their costs, but as a Permanent Magnet Generator is a costly thing so what can I use in alternative of this ...
0
votes
0answers
30 views

recharge magnet strength again

I have a speaker that have lost magnet strength. So I want to know is there any way can I recharge magnet strength again to this speaker. Truly, I don't have physics knowledge much therefore please ...
0
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1answer
180 views

Magnetic field, alternated current and EMF

If I apply an alternated current to a solenoid and insert into it a smaller solenoid, I could measure the induced EMF (electromotive force) and study how it changes in relation to the frequency of ...
0
votes
0answers
35 views

Viscous force in a conductor

I was told that in a conductor there is a dissipative force and if Ohm's Law is valid for a component, the component has to be dissipative. I'm trying to prove it, but I have some problems. I know ...
15
votes
2answers
787 views

Is Newton's universal gravitational constant the inverse of permittivity of mass in vacuum?

Is it possible to consider Newton's universal gravitational constant, $G$, as inverse of vacuum permittivity of mass? $$\epsilon_m=\frac {1}{4\pi G}$$ if so, then vacuum permeability of mass will ...
1
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1answer
192 views

Direction of magnetic force when magnetic field and velocity are not in same plane

We know from Flaming's Right Hand Rule how to calculate the direction of the magnetic force given the magnetic field and the velocity are in the same plane. Now suppose they are not in the same plane. ...
5
votes
3answers
360 views

Does a magnet contain (and potentially produce) energy?

Very quick question, does a magnet contain energy? The general consensus seems to be, it does not. And this is generally confirmed by the fact that it would break the first law of thermodynamics. ...
4
votes
1answer
269 views

Fourier transformation, electric field and magnetic field to have a shielding lattice against particles

With Fourier-Series Expansion, we can write a function as sum of many non-repating different frequncied different amplituded sine and cosine functions. Lets assume we know electric-field and ...
0
votes
1answer
111 views

Reflection of a Plane Wave at Oblique Incidence

I am still trying to prepare for my electromagnetics preliminary exam, but I find myself stuck on understanding the origins of Eqs. 10.105a, 10.105b, 10.106a, and 10.106b on page 453 of Elements of ...
3
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0answers
153 views

Angular momentum of particle in dipole magnetic field

Basically I'm just trying to find the expression for the angular momentum of a particle of mass $m$ and charge $q$ in a dipole magnetic field. In cylindrical coordinates, ...
3
votes
0answers
67 views

Quantum Cyclotron Frequency - Why is it off by a factor of 2?

Say you have a magnetic field $\vec{B}=(0,0,B_0)$. Then the Schrodinger Equation Hamiltonian for a spin-2 particle of charge $e$ moving in this field is: $$H = ...
1
vote
1answer
129 views

Accelerated charged particles produce electromagnetic radiation, but holes (the charge carriers) do not. Is this correct?

Holes are treated as particles in solid-state physics, so I've had some trouble with reasoning through this properly.
1
vote
3answers
223 views

potential difference problem?

As far as i know potential difference between 2 points is defined as the negative line integral of electric field between those 2 points. I also know that when magnetic field changes curl of electric ...
2
votes
1answer
109 views

I'm reading this in a senior year physics text book, need to know if this is true, and if so then how? :

"Even if a magnet is broken into atoms, each atom shall be a complete magnet. If the atom is further broken into electrons, protons, neutrons, etc. even then each particle shall behave like a complete ...
1
vote
2answers
110 views

Tale of two magnets

Please help. My example: You take a sheet of steel and suspend it in the air. Then you take a permanent magnet (i.e. a neodymium magnet) and attach it to the sheet of steel and from that you suspend ...
0
votes
1answer
426 views

Direction of the induced electric field

Assuming there is a time varying magnetic field (B), how to determine the direction of the induced electric field due to B. and to which parameters does it depend? P.S. determining the electric field ...
-2
votes
1answer
247 views

Is the thought on this problem on LC circuits correct?

I have the following problem and I want to know if my thought is correct: In one $LC$ circuit, on which $C=4,0\mu F$, the maximum potential difference across the capacitor during the oscilations is ...
2
votes
3answers
251 views

How Does $\epsilon$ Relate to the Dampened Harmonic Motion of Electrons?

I realize that the permittivity $\epsilon$ of a substance is easily calculated based on diffraction angles, but I am not satisfied with merely measuring it experimentally. I wish to understand its ...
0
votes
2answers
595 views

Electromagnetic wave propagation through two lossless dielectrics

In Elements of Electromagnetics (Sadiku, 3rd edition, Section 10.8), the author says to consider two lossless dielectric materials joined at an interface $z=0$. Here two lossless dielectric materials ...
1
vote
1answer
167 views

Battery and current confusion?

How exactly does a battery produce a current in the circuit connected across its ends? I dont want to know the chemical reactions in the battery core, but just the essence of it. I believe it doesn't ...
0
votes
0answers
49 views

Blocking electro-magnetic waves between two near field antennas

Its been a while since I studied physics, so pardon my ignorance. I have two near field antennas. The range of the antennas is about 2-3 cm. I am looking for a way where I can block and unblock the ...
0
votes
1answer
169 views

does matter radiate energy? [duplicate]

Since we know that all accelerated charges radiate energy and we also know that all matter is made up of protons and electrons which are all the while doing accelerated motion.So from this can i ...
6
votes
1answer
144 views

Can we write the electromagnetic potential covariantly in terms of the four-current?

In the Lorenz gauge, we have a beautiful relation between the four-current and the four-potential: $$\Box A^{\alpha} = \mu_0 J^{\alpha}$$ To get $A$ in terms of $J$, however, we have to use a ...
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1answer
298 views

Why is the Maxwell Stress Tensor symmetric?

What is the physical meaning of the Maxwell Stress tensor symmetry?
2
votes
1answer
97 views

How to derive magnetic moment for integrated circuit level?

Imagine we have a circuit on the xy-plane, with a random geometry (it just need to be closed). I want to calculate the magnetic moment of this setup: $$\vec{m} = \frac{1}{2} \int_V d^3 x' \vec{x}' ...
2
votes
1answer
358 views

What is the meaning of a constant magnetic scalar potential?

Let a spherical shell of inner radius $a$ and outer radius $b$ have a uniform magnetization $\mathbf{M}=M\,\hat{\mathbf{z}}$, $\hskip2in$ I've found that the magnetic scalar potential ...
1
vote
0answers
727 views

Calculation of magnetic force magnitude from a parmenant magnet

I was wondering how strong a magnet should be in order to be able to horizontally attract 0.2 Kg of pure iron at a distance of 0.3 M. Since I know nothing about magnets, I started looking for ...
0
votes
1answer
97 views

LINAC: why are they being accelerated?

I have read the wiki page and I have seen this image, but I still don't understand why they are being accelerated. Could you easily explain it to me? Thanks a lot
1
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0answers
68 views

How to size an electromagnet

How can I size an electromagnet? For example if I would to attract a mass of $x$ kg what are the calculations that I must do to size the ferromagnetic core and of course the solenoid? And thus, how ...
41
votes
8answers
2k views

How can Magnets be used to pick up pieces of metal when the force from a magnetic field does no work?

I learned that the force from magnetic fields does no work. However I was wondering how magnets can be used to pick up pieces of metal like small paperclips and stuff. I also was wondering how magnets ...
2
votes
0answers
137 views

Current to cancel Earth's magnetic field

I'm making a lab report where I've used a Helmholtz coil and a magnet to find Earth's magnetic field intensity in the area by relating the frequency of oscillation of the magnet when a certain current ...
1
vote
1answer
218 views

Energy Change in a Capacitor by Inserting Dielectric

This is a practice Physics Subject GRE problem. A parallel-plate capacitor has plate separation $d$. The space between the plates is empty. A battery supplying voltage $V_0$ is connected across ...
3
votes
1answer
384 views

Magnetic field insulators

I was wondering if there is any way to stop the magnetic field, without having the insulator turned into magnet. Let me present this as a simple case, there is a magnet to the left and a piece of iron ...
2
votes
0answers
361 views

Relationship between current, voltage, magnetic field in a torus

I'm trying to find the relation between current density $\boldsymbol{j}$, voltage $v$, and magnetic field $\boldsymbol{B}$ for the time-harmonic approximation in a cylindrically-symmetric coil (a ...
3
votes
1answer
655 views

Why do black lights show bacteria and proteins?

Why are blacklights used to show proteins and bacteria? Is it related to their ability to make white colors glow? Don't black lights just radiate in the ultraviolet/purple range?
4
votes
1answer
134 views

What is an anapolar moment?

I just read this: Anapolar Dark Matter I'm not sure i've heard the term 'anapolar' before, so i google and i found this: http://en.wikipedia.org/wiki/Toroidal_moment This confuses me, my ...
3
votes
2answers
101 views

How to calculate resistance of bunches of wires

I figure that the resistance of multiple wires would work just like adding in parallel, a bunch of wires that is. But couldn't any wire just be viewed as a bunch of wires, albeit small ones? I know ...
11
votes
5answers
526 views

Why are wires in simple circuits approximated as equipotentials?

I just answered this question: Voltage in a circuit by asserting that any two points joined only by a wire, but no other circuit elements, are at the same value of the electric potential. It is, of ...
1
vote
2answers
269 views

What is wrong with this form of the Maxwell-Faraday equation?

What is wrong with this form of the Maxwell-Faraday equation? $$\oint \vec{E}\ \partial \vec l= \bigcirc \hspace{-1.4em} \int \hspace{-.6em} \int \frac{\partial \vec B}{\partial t}$$ "Line integral ...
1
vote
3answers
274 views

Question regarding inverse relation of resistance with area of cross section

It is said that resistance is inversely proportional to area of cross section. But greater area will have greater electric flux, and greater electric flux will have greater magnetic flux, and greater ...
0
votes
0answers
62 views

Electrical properties of molten gallium arsenide

I'm looking for the resistivity and magnetic permeability of molten gallium arsenide, but can only seem to find the values for the solid material at room temperature (e.g., Wikpedia). Not even ...
8
votes
1answer
838 views

How do I derive the Lorenz gauge from the continuity equation?

I was reading my old electromagnetics book (Elements of Electromagnetics, by Sadiku, 3rd edition) and after the author explained what the Lorenz gauge is mathematically and why it is useful in ...
0
votes
0answers
41 views

Electric Field of Electret

I've read that a method to measure Radon, is to use an Electret Ion Chamber. In particular here can be seen that, a voltage drop can be $2V$ in a single day. I believe this very much, yet I don't ...
5
votes
1answer
181 views

Aharonov-Bohm Effect in Torus

I had a very brief introduction to the Aharonov-Bohm effect in class. The lecturer introduced the notion that $H(\Phi=\Phi_0)$ and $H(\Phi=0)$ gives identical energy spectrum and that the Hamiltonians ...
0
votes
1answer
1k views

Can a light be bent by a magnetic field?

I'm struck with two competing ideas on the question in the title. Listing #1: http://van.physics.illinois.edu/qa/listing.php?id=2009 Q: "How far can a magnetic field bend light?" A: "Unfortunately, ...
11
votes
2answers
631 views

What could magnetic monopoles do that electrically charged particles can't?

I understand the significance to physics, but what can a magnetic monopole be used for assuming we could free them from spin ice and put them to work? What would be a magnetic version of electricity? ...
10
votes
2answers
1k views

Is Biot-Savart law obtained empirically or can it be derived?

Well, there's already a question like this here so that my question could be considered duplicate, but I'll try to make my point clear. Is there a way to derive Biot-Savart law from the Lorentz' Force ...