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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Should there be a linear force acting aside from torque?

Assume to identical motors are placed vertically on-top of a table and they are not fixed able to move/fall etc...and they have gears attached to them and are attached to one another like this image: ...
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1answer
25 views

Finding the vector potential of an infinite cyclinder with uniform surface current

This is the second part of a problem in Griffith's "Introduction to Electrodynamics 4th Edition" (problem 6.16). The first part was to find the magnetic field inside a coaxial cable (2 concentric ...
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17 views

Back force after capacitor is discharged

You have a mechanical hand crank generator connected to a capacitor. You charge the capacitor by cranking it (you obviously have a back current as you crank it so it is difficult to crank). At a ...
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22 views

Electromagnetic field to shield from EMP?

Could an electromagnetic field be use to protect against an EMP? As I understand, an EMP is an Electro Magnetic Pulse, so a sufficiently powerful field should be able to deflect one, right?
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3answers
211 views

Are electrons held together by vacuum energy?

If one models the electron as a hollow spherical conductor with charge $e$ and radius $a$ then its electrostatic energy is given by: $$E_{em}=\frac{1}{2}\frac{e^2}{4\pi\epsilon_0a}$$ However if one ...
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2answers
137 views

Em induction, sliding rod with resistance?

Consider the digram below: This consists of a rod sliding with velocity $v$. I want to find the voltage developed between $A$ & $B$. To do this I drew this 'equivalence circuit': From this ...
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39 views

Elementary electromagnetism problem - Electric field generated by points that form an equilateral triangle

I am a newbie at physics and i just started learning electromagnetism at the university. Some help would be very welcome on the following questions (my question and/or result and/or partial work will ...
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1answer
83 views

Symmetry argument for a toroid?

When using Ampere's law for a toroid (in the toroid and around a circular path) please can someone explain the symmetry argument (or an alternative argument) which allows us to assume the field is ...
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20 views

Formation of Magnetic Field from Moving Charge

Is there a derivation of the magnetic field from the electric field of a charge q moving with speed v? (I know that there are some online, but my limited knowledge in multivariable calculus does not ...
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35 views

Matter like properties of electric field

First of all I would like to know the origin of the idea of electric field. How exactly was the idea of electric field conceived. Let us consider two charged particles exerting forces on one another . ...
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31 views

Charged black hole

It is known that there is solution of Einstein's equations for charged black hole. Reissner–Nordström metric in case of non-rotating charged black hole and for rotating charged black hole is a ...
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1answer
30 views

Energy of anihilation in the form of heat

Suppose you have a closed system. Within it is 2 protons, an anti-proton, 2 electrons, and a positron. All particles are barely above absolute zero in temperature. A proton collides with the ...
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26 views

The affect of high frequency and high voltage AC on induction

I am designing an induction system to wirelessly transfer power a short distance. I was planning on using resonant inductive coupling to increase the efficiency. The Inductance of my coil is about ...
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1answer
19 views

Magnetic Flux Quantization

I remember a few years back in my introductory AC/DC circuits class reading a little bit in the back of the book about magnetism. By this point, I was well aware that magnetic fields are commonly ...
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28 views

calculating magnetic field strength for a very small electromagnet

I am trying to calculate the magnetic field (in tesla/gauss) of an electromagnet that is very small and has very few windings. For example 12 windings over 0.003 meters. I know this is not going to ...
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2answers
40 views

Intuitive picture of diffraction without fictitious secondary wavelet

This question has been asked in a similar way but no satisfactory answers came up. Huygen's principle relies on the fact that if we consider an aperture, every element dS on the surface of the ...
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1answer
25 views

Explanation of force amplification inside a solenoid

For a system being actuated by a motor, the force can be amplified by gearing. The energy is being used for force instead of distance, so it produces more torque but moves slower. For a system being ...
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21 views

Generation of electromagnetic waves [duplicate]

I came across this while searching about generation of electromagnetic waves-Does this mean that if I vary the electric current following through a conducting wire, it will radiate electromagnetic ...
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1answer
51 views

Faraday's Law and Lenz's Law: Is there any theoretical explanations on why changing magnetic field induces an electric field?

This is a more specific extension to this question I came across today One certain aspect of Faraday's Law always stumped me (other than it is an experimental observation back in the 19th century) ...
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1answer
20 views

Does displacement current occur in an inductor?

We have learned in school that displacement current comes about due to a change in electric field flux per time in a capacitor (Ampere-Maxwell Law). Does the same displacement current come about in an ...
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1answer
154 views

Solution of the wave equation [closed]

I am dealing with the solution of the wave equation in two different cases represented in figure by Case A and Case B. The two figures were obtained by shining a slab with an incident wave coming from ...
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1answer
43 views

If a charge needs potential difference to move

Let's consider a circuit with a ideal source of emf and a resistor. Consider the segment of wire that connects emf with resistor. If we measure potential difference at any point of the wire, this will ...
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24 views

Voltage induced in coil

Okay, so I have two coils. A ring of radius $R$ is stationary at origin. A coil of $N$ turns and area $A$ carries current $I$ and is moving along the $x$ axis such that $x = x_0 + cos(\omega t)$. ...
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1answer
38 views

2 level atomic system interacting with Black body radiation. Relaxation time issue

I am studying the transient regime of a 2 level atomic system ($N_1,N_2$) interacting with a blackbody radiation from a source at constant temperature $T_{nr}$. The initial state of the atomic system ...
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1answer
38 views

Can you create a powerful magnetic field out of confined circulating electrons only?

Would it be possible, regardless of how efficient it was to do something like this, or why you would actually do it, to generate a really powerful magnetic field simply by producing a cycling electric ...
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2answers
43 views

Wouldn't $D = \epsilon E$ mean that a higher permittivity constant gives a higher electric flux? (And what exactly is the displacement field?)

A quote from the Wikipedia page for Permittivity: More electric flux exists in a medium with a low permittivity (per unit charge) because of polarization effects. This makes sense to me. My ...
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26 views

Does ratio of magnetic length to geometric length of a bar magnet always remain the same?

The magnetic length of a bar magnet is nearly 0.84 times that of the geometric length(Physical length), which looks quite valid. What would happen if I cut the same bar magnet along the transverse ...
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42 views

Pulling a bar through a magnetic field

This is a question on magnetic flux and induced voltage. I believe that this problem's answer key is incorrect, and I want to determine whether my opinion is sound. I also want to explore one ...
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0answers
43 views

Can a(n electro)magnetic field be amplified to overcome the residual strong force between protons-neutrons in a nucleus?

My knowledge of physics is cursory at best, I never studied it in high school but I enjoy reading about it as a layperson, especially the posts here on PSE. I don't understand any of the complex math ...
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46 views

What is the most general form for a plane wave?

In the context of electromagnetism, what is the most general form of a plane wave. My suggestion is: $$E=\sum_i{\vec E_ie^{i(\omega t+\vec k \cdot \vec r+\phi_i)}}$$ where $\vec E_i$ are constant ...
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2answers
320 views

Reflected and refracted light have same frequency as that of the incident light frequency. Why?

My text book says- When a monochromatic light is incident on a surface separating two media, the refracted and reflected light both have the same frequency as the incident frequency. Can anyone ...
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1answer
53 views

Is it possible to make an electromagnet w 2 like ends?

HELP! Is it possible to create an electromagnet with one continuous wire with 2 like poles (i.e. both ends either north or south)?. Visualising it with the right hand screw rule for current ...
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2answers
28 views

Is the screen in CRT connected with a positive pole?

I don't understand this passage would you clarify it? "These electrons are then freed (liberated) from the metal and are then picked up by the screen, which is connected to a positive pole called the ...
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30 views

Gauge invariance and non-commuting second derivatives

I'm currently doing a homework assignment in relativistic quantum mechanics, and one of the problems involves proving the gauge invariance of a particular lagrangian. The problem is really quite ...
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3answers
56 views

Where is the energy stored in an inductor?

In an inductor, Most text books say that the $(1/2)Li^2$ is stored in the magnetic field. But is there another way to explain this? In a capacitor I understand that all the energy that the battery ...
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21 views

Mie's scattering Theory Problem

Is Mie's theory which describes the scattering of EM wave off sphere , possible to extend to spheroidal ? object or arbitrary shape with finite series expression ? I tried to replace the radial ...
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31 views

Dipole moment of a specific charge distribution

A negative charge cloud distribute as a ellipsoid and a positive point charge situated at the centre of the ellipsoid.What should be the dipole moment of charge distribution.I mean, if the negative ...
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1answer
25 views

Wireless energy flow through earths magnetic field

Seeing that technologies such as that developed by WItricity are based on magnetic fields, is it possible to transfer electricity that way through earths magnetic field?
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1answer
57 views

Show that the current in a cylindrical conductor is uniformly distributed across it's cross section

Using relevant equations for E and J, show that the current in a steady current I in a cylindrical conductor with uniform conductivity $\sigma$ is uniformly distributed across its cross-section. I ...
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0answers
18 views

Is it possible to write the Lorentz oscillator model using Green's functions concepts?

Is it possible to write: $$\lim_{\gamma_j\rightarrow0}Im\left(\frac{1}{\omega_j^2 - \omega^2 - i\omega \gamma_j}\right)$$ which occurs, for example, in the Drude-Lorentz oscillator model for ...
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0answers
15 views

What is the transfer function in fft beam propagation for unpolarized light?

What is the transfer function in fft beam propagation for unpolarized light ? How to construct the fft beam propagation ? This is for homework. For coherent light the beam propagation is E(x,z) ...
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13 views

Biological Signal Extraction Methods

EEG / MEG Back in 1929 Hans Berger recorded the first electrical potential variations in the brain from the scalp. Great advances have been made in this science since discover, with many scientists ...
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3answers
53 views

Doppler shift and special relativity

I have been listening to an audio course on Special Relativity. In the course the teacher repeatedly says that the "laws of physics are equally valid for all uniformly moving frames of reference". ...
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2answers
84 views

Electric field 0 everywhere inside Gaussian surface

Gauss's Law shows that the electric field everywhere inside a spherical shell of uniform charge density is $0$. Suppose we have a surface which divides space into two disjoint regions (an interior and ...
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0answers
52 views

How Electromagnetic Force is stronger than Gravitational Force [duplicate]

I had read this in text book..Can some explain this. When we hold a book in our hand, we are balancing the gravitational force on the book due to the huge mass of the earth by the 'normal force' ...
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1answer
55 views

Does the strength of a beam of light determine how much distance it can travel?

If I point a light torch to a cloudless, crystal clear and an absolutely pristine sky, will the light beam exit the planet and travel forever in the vacuum of space? (assuming it doesn't get perturbed ...
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1answer
60 views

Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
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33 views

How to derive De Haas–van Alphen effect?

I was reading Solid State Physics by Kittel and they manage to derive De Haas–van Alphen effect by invoking the Bohr-Sommerfeld model. This feels unsatisfactory to me. Can someone derive this using ...
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1answer
23 views

How do you show that the normal derivative of the magnetic vector potential is discontinous across a surface current?

This is from Griffith's "Introduction to Electrodynamics" 4th edition, problem 5.33. I need to show that $$ \tag 1 \frac{\partial \textbf{A}_{above}}{\partial n}-\frac{\partial ...
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1answer
29 views

Faraday's electromagnetic induction

Does the emf increase when no of turns of the same wire increased? Consider a wire of length 'l' is coiled with an area of cross section 'A' is subjected to a change in magnetic flux. an emf will be ...