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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1answer
49 views

Time changing potential gives rise to “force”?

Imagine a charged particle inside a Faraday cage (i.e. charge on outside, zero electric field inside, but non-zero electric potential on the inside). Suppose the charge distributed on the outside of ...
0
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0answers
12 views

Electrodynamic near fields around black holes

For the standard Schwarzschild black hole, the temperature of Hawking radiation is simply related to the Schwarzschild radius as $kT = \hbar c/(4\pi r_S)$, meaning that the typical wavelength of ...
1
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0answers
31 views

Does the Lorentz force law explain Flemings left hand motor rule and the right hand dynamo rule?

the Lorentz force on a charged particle $F=qv \times B$ can explain Flemings left hand rule (motor rule) and the right hand (dynamo rule) In the left hand rule, the direction of the current gives ...
0
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1answer
20 views

Poynting vector from 1st term in Lienard-Wiechert field

I start with 1st (non-radiative) term from Lienard-Wiechert fields: $$ \vec{E} = q (1-v^2) \frac{\vec{R_{t'}} - \vec{v}R_{t'}}{(R_{t'} - \vec{v}\vec{R_{t'}})^3} $$ $$ \vec{H} = - q (1-v^2) ...
42
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5answers
9k views

Why doesn't light, which travels faster than sound, produce a sonic boom?

I know that when an object exceeds the speed of sound ($340$ m/s) a sonic boom is produced. Light which travels at $300,000,000$ m/s, much more than the speed of sound but doesn't produce a sonic ...
0
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1answer
34 views

Concerning railguns and magnetic fields

I'm part of a group working on a Physics II project based on electromagnetism, and my group decided to create a proof-of-concept railgun, shown here: ...
1
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1answer
29 views

Physical meaning of the separation constants in Laplace's Equation for Electrostatics

In Electrostatics, if we consider a region without charges the electrostatic potential $V$ obeys Laplace's Equation $\nabla^2 V = 0$. We can tackle this with separation of variables. In cartesian ...
0
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1answer
14 views

What are the general rules for determine on which two ends of a bar magnetic are its poles located?

I am wondering. Maybe somebody be nice enough to put a explanation that doesn't involve too much math. I did notice that for bar magnets, their poles are not always on the two ends separated by the ...
4
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0answers
40 views

From Liénard-Wiechert to Feynman potential expression

When studying the potential of an uniformly moving charge in vacuum, Feynman proposes to apply a Lorentz transformation on the Coulomb potential, which reads in the rest frame $ \phi'(\mathbf r',t') ...
1
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1answer
46 views

Motional emf with rod

We know that $\mathcal{E} = -N \frac{\mathrm{d}\phi}{\mathrm{d}t}$. When we have a rod such as the one on the left moving through a constant magnetic field, how is it the case that the flux is ...
0
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0answers
11 views

Is there a magnetic force affecting point charges outside of a toroidal coil with sinusoidal current?

To clarify my question a bit more, if we have a toroidal coil with sinusoidal current flowing through wires around the core of the toroid, is there a magnetic force that affects charges outside of the ...
4
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2answers
235 views

Maxwell's equations invariant under all linear transformations?

Maxwell's equations in tensor notation read: \begin{align} \partial_\mu F^{\mu\nu} &= J^\nu \\ \partial_{[\lambda}F_{\mu\nu]} &= 0 \end{align} Consider doing a general coordinate ...
3
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0answers
43 views

MRI and precession

A lot of explanations of the quantum mechanics of MRI discuss the precession of a proton in an external magnetic field, for example here: http://www.physicscentral.com/explore/action/mri.cfm Doing ...
0
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2answers
70 views

Why do we feel hot because of sunlight? [closed]

sunlight , light generally , is an electromagnetic wave which turns into heat when it contacts a matter (solid,liquid,etc..) is that right ?
0
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2answers
57 views

why do electromagnetic waves have no charge?

i would have thought that because the electric and magnetic fields oscillate, the charge could be positive or negative between 0 and 1 inclusive at any one point in time. i cannot see any explanation ...
0
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0answers
19 views

langevin theory of diamagnetism

According to langevin diamagnetic theory when an electron orbiting the nucleus is subjected to an uniform external magnetic field (say perpendicular to plane of orbit) then the angular frequency of ...
1
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1answer
65 views

Shouldn't the currents be time-continuous here?

My intuition is that the current upon an inductor (say, a solenoid) will always be time-continuous, without "sudden changes". But below is a case that seemingly contradicts this point of view. There ...
4
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3answers
88 views

What makes waves propagate?

Why do electromagnetic waves propagate? I have searched a lot about EM waves, but I am still unable to understand what is driving them. Could you explain?
0
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0answers
9 views

Relation between frequency, speed, and coverage range

From http://superuser.com/a/901075/9265 about wireless network: Higher frequency signals degrade over shorter distances, but can carry data more quickly. I wonder what theorem/theory in Physics ...
1
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1answer
38 views

Question regarding charge and acceleration

From a stationary charge electrostatic fields arise. From a moving charge, magnetostatic fields arise. From an accelerating charge, EM waves arise. So i wonder -- what about a non-constantly ...
2
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1answer
46 views

Special relativity, electromagnetic fields, charge and Q

Is it true, were Coulomb's constant k to be several orders of magnitude smaller, that there would be no (or increasingly negligible) magnetic fields generated by moving charges? The reason being the ...
0
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1answer
34 views

Current direction and magnetism

So using the right hand rule, I understand current is clockwise. Because I read the magnetic field emerges from the north pole, I thought the answer was a, that the north pole is on the top. But the ...
1
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3answers
228 views

What does it mean that a magnetic field's flux vanishes through any closed surface?

I'm reading the Britannica guide to Electricity and Magnetism, and I came across the following quote: A fundamental property of a magnetic field is that its flux through any closed surface ...
4
votes
4answers
112 views

A current through a wire produces a magnetic field around it. Is the reverse possible?

If somehow a magnetic field around a wire can be made to exist that is identical to the magnetic field produced when a current passes through the wire, will a current be produced in the wire?A thought ...
0
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2answers
62 views

When does $\mathbf n\times(\nabla V_2-\nabla V_1)=0$ imply $V_1=V_2$

I was reading a paper on electrohydrodynamics which has the following sentence (in my own words): At the interface/boundary, the requirement of continuity of the tangential component of the ...
1
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1answer
29 views

Neutral $K$ and $B$ mesons decay to 2 photons?

The neutral pion $\pi^0$ decays almost exclusively to 2 photons, $\pi^0 \rightarrow \gamma \gamma$, which got me thinking: Can we have $K^0 \rightarrow \gamma \gamma$ and $B^0 \rightarrow \gamma ...
0
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2answers
47 views

Is there no electric field inside a conductor?

I came across this statement while studying electric currents and I am confused: "There is no electric field inside a conductor. Hence no current can flow through it". Is there a fallacy in this ...
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0answers
12 views

Eddy currents are out of phase with respect to the field generated by a coil?

I have a coil from which a sinusoidal current (low frequency, few kHz) should generate a precise AC magnetic field in the surrounding space. Another coil intercepts this field and the corresponding ...
-1
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0answers
7 views

Superconductivity induced by magnetic flux or temperature fluctuations (+/-)

Does anyone know of an experiment with magnetic flux rotations that can make a difference in local temperature? I would really appreciate it, I need it for my theoretical model of some things and ...
0
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0answers
25 views

Magnets quantum locking/levitating [duplicate]

How does cooling a magnet allow it to quantum lock/levitate? I have seen it in videos but do not know how it works.
1
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2answers
46 views

Are magnetic field lines level sets?

I have been learning a bit about level sets. After doing this, I looked at a diagram of magnetic field lines and noticed they don't intersect rather like the lines on closed curve level sets. My ...
0
votes
2answers
34 views

Why does a voltmeter give a positive reading for a path in the same direction as the electric field?

Straight from my textbook: If the direction of the path from initial location to final location is the same as the direction of the electric field, the potential difference is negative. Yet a ...
1
vote
1answer
29 views

In a noiseless environment, how accurate do today's transmitters send EM waves?

Suppose that there is no external noise in the environment. How accurate are today's TEM wave transmitters in such a case? So if we want to send $200\cos(1000\pi t)$, can transmitters send exactly ...
5
votes
1answer
63 views

Charge of a moving particle [duplicate]

Is there an experiment that measures the electric charge of a moving particle, therefore proving "experimentally" that it is indeed the same as a static particle?
1
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1answer
37 views

The force on the northern hemisphere

I am reading Griffiths' Introduction to Electrodynamics. On page 364, example 8.2 (4th edition), he calculates the force on the northern hemisphere of a ball with total charge $Q$ spread uniformly. ...
0
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1answer
28 views

Induced electric field

Let's consider a thin cylinder of radius $r$ with a charge in is outer surface. It is made of an isolator. Let the magnetic field be parallel to its axis. If the magnetic field changed by $dB$ in time ...
2
votes
1answer
55 views

How does an infrared thermometer actually calculate temperature?

I am slightly confused about infrared radiation and the equations related to it. $P = A \epsilon \sigma T^4$ (1) and $B_{\lambda}(\lambda,T) = \frac{2hc^2}{\lambda^5} \frac{1}{e^{\frac{hc}{\lambda ...
3
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0answers
67 views

Maxwell's equation in curved spacetime - how come? And experimental evidence?

I'm trying to understand the generalization of Maxwell's equations to curved spacetime. In FLAT (Minkowski) SPACETIME: If we define the "four-potential" as $$\ ...
1
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2answers
39 views

Does the gravitational force of one object interfere with that of another?

Assume we have two iron spheres, Sphere A and Sphere B, with identical masses floating in the vacuum of interstellar space separated by some distance from each other. The gravitational force of each ...
1
vote
1answer
56 views

Magnetic moment of an iron-core solenoid

I'm currently developing a Simulink model for the attitude control system (ACS) of an undergrad-developed CubeSat. The ACS uses magnetorquers for attitude actuation. The magnetorquers are iron-nickel ...
0
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2answers
40 views

Path of a proton in a magnetic field

I read somewhere that a proton takes a semicircular path and exists a magnetic field when it enters. I did some research and that seems to be the case. From my understanding, an electron in a magnetic ...
1
vote
1answer
71 views

Why does Ohm's law ignore the Lorentz force?

For example, the usual derivation of the complex dielectric constant of metals (using the Drude model) makes use of the Ohm's law in the Maxwell's equations, but what is never mentioned is why they ...
0
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0answers
28 views

How can you visualize hard axis of magnetization?

I think the main problem here is my understanding of the hard axis of magnetization. The hard axis is the "bad" direction of the magnetization i.e. the direction inside a crystal, along which large ...
2
votes
1answer
61 views

Gravity vs. Electromagnetism Scenario

Imagine a two dimensional world where there are only two electrons. They are set right beside each other. Of course, immediately they will start to separate, being repelled. My question is, as they ...
0
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0answers
28 views

How to repeat the Biot-Savart experiment for the direct current in a straight wire and the corresponding magnetic field without side effects?

As I read Biot and Savart took a compass needle and this needle make some rotation when the current was switched on and off. As it was clear that a changing current induces a magnetic field the ...
0
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1answer
38 views

Are the electrons spin and his magnetic dipole moment unambiguously connected?

Is the angle between the spin orientation and the magnetic dipole orientation for all electrons and under all circumstances the same?
0
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0answers
14 views

Expanding the H and D field in orthogonal modes

Currently I am doing my bachelor thesis on the behaviour of Q-factors in 3D microwave cavities. One step in that process is to approximate the behaviour of the 3D cavity by comparing the cavity to an ...
4
votes
1answer
36 views

Interference and windows

The other day i was learning about interference patterns with the effect of a bubble making a rainbow on the surface. I learned that the reflections from both sides of the soap can interfere ...
0
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0answers
13 views

Electromagnetism Books and Lectures [duplicate]

I am an undergraduate at mathematics.And this semester i took E&M as one of my courses because i found it interesting .So i am looking for a good introductory book that will keep my interest as ...
1
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2answers
73 views

Obtaining electric field of an uniformly charged sphere surface without using gauss law [closed]

How can i obtain the electric field due to a uniformly charged sphere surface without using gauss law on a point outside the sphere, im stuck not knowing what infinitesimal surface i shall consider so ...