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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814 views

What is the difference between the Balmer series of hydrogen and deuterium?

In my quantum mechanics textbook, it claims that the Balmer series between hydrogen and deuterium is different. However, I was under the impression that the Balmer series $$H_\alpha, H_\beta, ...
2
votes
1answer
217 views

Isn't the front edge of a wave, kind of “information” which travels faster than light?

Considering the definition of phase and group velocities, We know group velocity can't exceed C but phase velocity can be infinitely high. Assume a monochromatic electromagnetic wave traveling with a ...
2
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0answers
37 views

How do we generalize limitations on the propagation speed of sinusoidal waves to limitations for transmitting any kind of information?

Our calculations on EM waves basically deal with harmonic waves and through such calculations and relations, we derive some relations and limitations for wave like phenomena. Of these relations are ...
4
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2answers
431 views

About an electrostatics integral and a delta-function kernel

I'm having trouble with an integral and I would like some pointers on how to "take" it: $$ \int \limits_{-\infty}^{\infty}\frac{3\gamma a^{2}d^{3}\mathbf r}{4 \pi \left( r^{2} + ...
0
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0answers
50 views

The electron and classical dynamics

Can I use Newtonian laws of dynamics and kinematics applied to electrons (like $F = ma$ and $s = s_0 + v_0 t + at^2/2$)? If not, why not? I know that everything in principle behaves in a quantum ...
0
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1answer
63 views

Energy spent by motor in a period

Consider a single coil that is turning in a constant and uniform magnetic field thanks to a motor. The normal to the coil is given by: $${\bf u}(t)=\sin (\omega t){\bf u_x}+\cos(\omega t){\bf u_z}$$ ...
2
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1answer
179 views

Hamiltonian formulation of single particle in an electromagnetic field

Consider a charged particle in a static electromagnetic field. Suppose that the domain is simply connected so that the second law of Newton's dynamics reads: $$ ...
1
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2answers
203 views

Sliding force less or equal?

Why is the force required to slide a magnet off a steel plate A LOT less than the force required to directly pull it off? The force required to pull the magnet can be: 20lb While the force required ...
2
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1answer
167 views

Expression of electrostatic field

An electrostatic field is characterized by the fact that it depends only by $r$, isn't it? If it is true, I don't understand why this expression, given in cylindrical coordinates, $${\bf ...
0
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0answers
31 views

Uniform distribution of charge on a plane and on a bar: conditions to have no motion of the bar

On the plane z=0 there is a superficial charge distribution such that $\sigma$ is constant. Near to the plane, there is a bar, charged uniform with total charge q. At the extremities the bar has two ...
4
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2answers
306 views

When studying electrodynamics do we assume Maxwell's Equations or derive them?

This question is because something made me confused. I always thought that the idea behind electrodynamics was to postulate some things, like Coulomb's law in electrostatics and so on, and then ...
11
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1answer
206 views

Do image charges radiate?

Suppose I have a charge moving back and forth above an infinite, grounded, conducting plane. Can I calculate the total radiated power by using image charges? That is, are the scalar and vector ...
4
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2answers
2k views

How can a conductor be grounded yet there are induced charges on it?

A classic example for the method of images is the following, quoted from Griffiths's Introduction to Electrodynamics, page 121: "Suppose a point charge $q$ is held a distance $d$ above an infinite ...
4
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3answers
496 views

Maxwell equation boundary conditions on a conducting sheet

I'm having difficulties solving boundary conditions for an infinitely thin conducting layer in a presence of an alternating field. I use the Maxwell equations: $\nabla \cdot \mathbf B = 0$ $\nabla ...
3
votes
1answer
401 views

inductance value of a single turn thin circular coil

the question may seem simple but I haven't found any fitting formula yet. The problem is the following: consider a single-turn, circular coil made of reasonably thin wire (diameter of the wire much ...
1
vote
2answers
258 views

Deriving the Lorentz force from velocity dependent potential

We can achieve a simplified version of the Lorentz force by $$F=q\bigg[-\nabla(\phi-\mathbf{A}\cdot\mathbf{v})-\frac{d\mathbf{A}}{dt}\bigg],$$ where $\mathbf{A}$ is the magnetic vector potential and ...
3
votes
1answer
253 views

Time reversal and the solutions to Maxwell equations

The standard Lienard-Wiechert potentials describe the electromagnetic field at a point $P$ at time $t$ due to an arbitrarily moving charge $q$ at the retarded time $t-r/c$. An electromagnetic ...
2
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1answer
394 views

Why are magnetic fields only produced by moving charges? [duplicate]

Why do charged particles only produce magnetic fields while in motion?
9
votes
4answers
2k views

Can the Lorentz force expression be derived from Maxwell's equations?

The electromagnetic force on a charge $e$ is $$F=e(E+v\times B),$$ the Lorentz force. But, is this a separate assumption added to the full Maxwell's equations? (the result of some empirical ...
0
votes
1answer
364 views

Are Lorentz force and maxwell's equations independent? [duplicate]

The Lorentz force and Maxwell's Equations gives answers to many physics problems, and the answers given by both methods are consistent. For example, consider the problem of a conducting rod of ...
5
votes
1answer
171 views

Is an alpha particle's curvature in a magnetic field visible with a homemade cloud chamber?

I'm trying to do some calculations to see just how strong a magnet you'd have to have, in order for curvature to be noticeable in a rudimentary cloud chamber, with lead-210 as an alpha particle ...
2
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0answers
82 views

value of $\omega$ in nonlinear equation

IN this article the author written a nonlinear equation as $$-\frac{\partial^2 \phi}{\partial t^2}+ \nabla \phi= \phi+ \sum_{k=2}^\infty g_k \phi^k$$ They have scaled the time as, ...
1
vote
1answer
134 views

What is the mass in $\vec{\nabla}\cdot \vec{B}_g=\text{something}$ called if it could be non-zero?

Maxwell's equation for gravity has $$\vec{\nabla}\cdot \vec{B}_g~=~0,$$ see Gravitoelectromagnetism in analogy with the electrodynamics. What is the mass called that needs to make these equations ...
3
votes
3answers
1k views

formula for transparency of very thin film of metal

Is there formula that gives transparency of very thin film of given metal (tens of nanometers) to the visible light/light of given wavelength ? Which properties of metals are needed for the formula ? ...
1
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1answer
439 views

Covariance of the Dirac equation and Infinitesimal Lorentz transformation

Okay, I've got really desperate now. I've spend 10 hours of work (for a few days) trying to prove 'trivial' equation in Quantum Electrodynamics. To anybody who want to write an answer for my questions ...
0
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2answers
312 views

An Electromagnetic Paradox?

The above diagram represents an isolated system with two masses $M$, at position $X$, and $m$, at position $x$, connected together by an extended spring. Each mass is connected by rigid rods to ...
10
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2answers
15k views

Effects of a very large magnetic field on the human body

Ever since reading about the NHMFL I have always wondered about this and asked several people without getting a good satisfactory answer. My question is, considering the simplest case let's say a ...
1
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2answers
175 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 ...
4
votes
1answer
387 views

Does weather obstruct microwaves and radiowaves?

I read an article ('How airplanes use radar' at Bright Hub.com) that stated that airplanes use radio waves for communication because they aren't obstructed by weather. Is this true?
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0answers
61 views

Maximise magnetic force at one point while minimizing at another

Is it possible to have a magnet that attracts one object strongly, but not another object behind that first object? Given an (electro-) magnet M above two thin parallel steel sheets P1 and P2 in the ...
2
votes
1answer
482 views

An objective question from electromagnetic induction

A bar magnet is released from rest along the axis of a very long, vertical copper tube. After some the the magnet : (a) will stop in the tube. (b) will move with almost constant speed. ...
3
votes
1answer
3k views

Is there any relationship between Gravity and Electromagnetism? [duplicate]

We all know that the universe is governed by four Fundamental Forces which are The strong force , The weak force , The electromagnetic force and The gravitational force . Now, is there any ...
2
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3answers
184 views

What is the interpretation of the Chern-Simons electromagnetic spin density?

Hans de Vries (who happens to be a no-longer-active physics.SE user) has an online book (referenced below) in which ch. 6 is a presentation of an object he calls the Chern-Simons current, ...
0
votes
1answer
151 views

EM vector potential

We can write the electromagnetic field tensor as $$\begin{bmatrix} 0 & -E_x/c & -E_y/c & -E_z/c \\ E_x/c & 0 & -B_z & B_y \\ E_y/c & B_z & 0 & ...
0
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0answers
143 views

Particle inside a charged shell

Imagine that I have a particle of charge $q$ at the center of a spherical insulating shell of charge $Q$ and radius $R$. Both the particle and shell are initially at rest. Now I apply a force ...
2
votes
2answers
95 views

Does the Lorentz force change under a boost?

I have a question about the Lorentz force on a charge $q$: $$\mathbf{F}=q(\mathbf{E}+\mathbf{v}\times\mathbf{B}).$$ I understand that if one performs a Lorentz boost then the electric field ...
11
votes
3answers
988 views

There is no such thing as magnetism?

Here's an interesting "proof" that there is no such thing as magnetism. I know the answer but I love this so much I had to ask it here. It's a great way to confuse people! As we all know, $$\nabla ...
3
votes
1answer
83 views

The lab have a constant electric field and a constant magnetic field, what is the electric and magnetic field inside a conductor and far from it

The following is an old question from an exam in a Physics $2$ course I am taking, I have tried to solve the question and after I thought I got the answer I looked at the solution and saw it isn't ...
0
votes
1answer
416 views

Pressure experienced due to magnetic force?

A current $I$ flows in a long thin walled cylinder(parallel to the axis) of radius $R$. What pressure do the walls experience? This is the 263rd problem in Section III from the book 'Problems In ...
6
votes
2answers
614 views

Can water be magnetized?

This may be a stupid question, so feel free to shoot it down. Assuming all atoms have a magnetic moment, I would assume the water molecule too would have a resultant magnetic moment; ergo, it may be ...
3
votes
1answer
45 views

What would happen if the electric field was cut to a cyclotron half-way?

After accelerating say a proton, half way, if the alternating electric field was cut, would the proton maintain a perpetual circular motion at the same radius? I know that any charged particle moving ...
1
vote
1answer
221 views

Maximum electric field in a circle

I have a question as follows: "Show that $E_x $ on the axis of a ring charge of radius $a$ has its maximum value at $x = \pm a /\sqrt{2} $. Sketch $E_x$ versus x for both positivve and negative ...
11
votes
3answers
7k views

What is the highest possible frequency for an EM wave?

What is the highest possible frequency, shortest wavelength, for an electromagnetic wave in free space, and what limits it? Is the answer different for EM waves in other materials or circumstances? ...
12
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0answers
480 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 ...
1
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2answers
99 views

Time evolution of the worldlines of 2 particles

Suppose I have a lab frame that is freely falling in a gravitational field of the Earth -- assume non-homogeneity-- and a uniform constant electric field. There are 2 test particles in the frame -- ...
1
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1answer
270 views

Local gauge invariance and fields

I have one question about local gauge invariance of the spinor and scalar theories. For the scalar complex field with lagrangian $L_{0}$ requirement of local gauge invariance leads us to the ...
1
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6answers
2k views

No magnetic field from a static charge - Is there a simple physical argument to show why?

For a charge moving in an electric field $\vec E$, its equation of motion is given by the electric part of the Lorentz force $$\frac d {dt}\gamma m \vec v = e\vec E$$This comes from the conservation ...
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2answers
1k views

Could you use earth's magnetic field to build a hover skateboard? [closed]

In "Back to the Future" they use levitating anti-gravo skateboards without wheels which will be difficult to build, because there is no theory on how anti-gravity could be created. A possible ...
0
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0answers
36 views

Effects of magnetic fields on our bodies [duplicate]

I worked 20years in a very high magnetic field at an Aluminum foundry. I'm now in my 50's and I don't know or am I aware of any negative effects from this. Is there anything I should be on the look ...
-1
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2answers
263 views

Conceptually, why does an electric field not require a charge flowing through it?

My son has been asking me a lot about electromagnetism and so I got to read up on it.One of the interesting and hard to grasp (for me at least) concepts is that an electric field will exist even when ...