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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58 views

Why can xenon gas stay ionised after the “trigger voltage” is removed?

From my understanding of a Xenon flash circuit, a storage capacitor and a trigger capacitor are charged up to about 240V (both with similar RC values, but less capacitance is needed for the trigger ...
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29 views

Why is it necessary to break symmetry to get magnetoelectric effect in multiferroics?

Spatial or time symmetry needs to be broken for both magnetic and electric to happen in the same material. But how is symmetry hindering the process?
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30 views

Can I disperse a droplet with electromagnetic fields?

If I have a droplet of water, can I apply an electromagnetic field in order to disperse the droplet into smaller subdroplets (tiny vapor) and fill a space with them? The same way that magnetic dust ...
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69 views

What unit system puts a bunch of powers of ten in Maxwell's equations?

In what unit system can we get the Maxwell equation in the following form (section 1.2 from Lewin's book Advanced Theory of Waveguide) $$\nabla\times \vec E = -10^8\mu \frac{\partial \vec H} {\...
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41 views

Excitation spectrum of heisenberg model

I understand that ferromagnetic Heisenberg model (lattice of spin variables that can point in any direction) spectrum can be deduced by a $\lambda\phi^4$ theory with $\phi$ being complex. This model ...
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47 views

EMF and Distance

As a professional underground service locator, EMF and Inductance are at the very core of what we do. Very few of us truly understand the detailed physics behind what makes it possible for us to ...
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37 views

How to describe frequency spectrum with samples?

I posted this same question in math, but nobody is answering, so I thought I'd give a try here too... To me the question is pretty straightforward, but nevertheless I have to explain its background ...
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68 views

Why do Superconductors make Magnetic Fields Bend Away

When superconductors reach their "threshold temperature" their electrical resistance turns 0. This makes the magnetic field lines not pass through the superconductor but instead bend itself around it ...
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67 views

Transfer function of a space varying wave equation

$$\frac{\partial ^2 \psi}{\partial x^2}-\mu \epsilon \frac{\partial ^2 \psi}{\partial t^2}-\mu \sigma \frac{\partial \psi}{\partial t}=0$$ Is the wave electromagnetic wave equation in lossy, source ...
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39 views

Recommended books that describe emission and absorption of light and light/matter interactions

Hello I am a Chemical Engineer that has ventured into the field of optics and I was wondering if there is any recommended book/books to understand the maths and also some answered exercises or ...
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56 views

Lack of symmetry in Poynting vector

I've noticed that there's a lack of symmetry in the definition of the Poynting vector, being defined as: $$S=E\times H$$ meaning, using the electric field cross the magnetizing field instead of the ...
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13 views

Coordinate Transformation across different media

I have an arrangement that is as follows : (1) inner cylinder(radius r1) filled with water (2) outer cylinder(concentric, radius r2>r1) made of glass I have a sensor S1 at radius r(r1) at angle beta. ...
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89 views

A classically charged point particle interacting with electromagnetism and gravity

Consider a classically charged point particle interacting with electromagnetism and gravity. The relevant dynamical variables are $\chi^\mu (\tau)$ of the particle, the electromangetic potential $A_\...
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32 views

Combining wave plates?

In the context of classical electromagnetism, is it always possible to reproduce the action of a large number of half-wave plates and quarter wave-plates with that of a single half-wave plate and a ...
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34 views

Do electrons move easier depending on the way a TIG tungsten head is grinded?

I'm in a TIG welding course, and during the course we (inevitably) messed up a few (suicidal) tungsten heads by grazing/suicide dive into the molten puddle during operation. Standard procedure is to ...
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47 views

Magnetic dipole near black hole

Usually it is said that black holes cannot have electric or magnetic dipole, only electric charge and angular momentum are allowed quantities besides mass So, it would seem that black holes behave as ...
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45 views

Question about Meissner Effect video

In this video, what material is on top of the superconductor disk that keeping it cold? Is it just a circular chunk of dry ice? http://www.youtube.com/watch?v=Ws6AAhTw7RA I'm planning on recreating ...
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113 views

Ampere's law and magnetic circuit

For magnetic circuit there is an equation NI=HL. This equation is obtained by applying Ampere's law. H is magnetic field intensity and L is the length of the circuit. In following picture the equation ...
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119 views

How can I have a voltage in hall effect with zero magnetic field?

How can we obtain a voltage with no magnetic field is applied to the semiconductor? They told me that if the points where I measure the potential difference are not at the same level I would obtain a ...
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38 views

Boundary conditions for vector wave equations

Assume the time-harmonic case of Maxwell's equations, one can obtain the following vector wave equations: $$ \nabla\times(\frac{1}{\epsilon_r}\nabla\times\mathrm{H})-k_0^2\mu_r\mathrm{H}=\nabla\times(\...
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30 views

Uniform field through a disc

Let us say that we have a magnetic field of the form $B_0 \sin(\omega t) \hat e_z$ in all space. Now let us say we put a conducting disc within this field. I want to find the electric field within ...
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33 views

Calculate characteristic frequency from atomic polarizability

With Lorentz model, the plasma frequency can be calculated from electron density $$\omega_p=\sqrt\frac{4\pi N e^2}{3m}$$ But I also found in this paper that it could also be calculated from atomic ...
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82 views

Does Lorentz force affect the magnetic field?

When a charged particle moves thru magnetic field, it is affected by Lorentz force. But does the magnetic field surrounding the particle experience a force opposite to the particles Lorentz force ? ...
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60 views

Electromagnetism and magnetic fields

Would it be possible to fill hollow non-magnetic blocks of metal with ferromagnetic fluids and use magnetic fields to move the blocks in a certain way? I am only asking this because I think that if it ...
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31 views

What does it mean to perturb the electromagnetic energy with respect to a change in a Lorentz frame?

I am perturbing the energy of an electromagnetic field using \begin{align} \dfrac{\partial U}{\partial \zeta} = \dfrac{1}{\mu} \boldsymbol{H}\cdot \dfrac{\partial \boldsymbol{H}}{\partial \zeta} + \...
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35 views

Halbach array possible shapes?

Looking at images on the web most hallbachs are either rings or blocks but I have seen a couple pictures of what look to be rectangles or pyramids. They are just illustrations though and maybe they'...
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81 views

What's the shape of electric field line in a rectangular metal under varying magnetic field

We know that,the shape of electric field line in a cylinder under varying magnetic field is circle,and I wonder,what's the situation if it is a piece of metal with rectangular cross section? I think ...
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2k views

Induced e.m.f depends on…?

In my revision guide it says that Michael Faraday did experiments that showed that induced e.m.f for a coil of wire depends on four things: Magnetic strength of the core in the coil of wire. (...
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91 views

Electromagnet experiment

I was reading about an experiment on cyberphysics.co.uk. You set up a circuit as follows: Then: The current through the circuit is varied while the number of turns of wire around the iron nail ...
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88 views

Meissner effect and levitation

In a field less than critical field, decreasing the temperature below the critical temperature will eliminate the magnetic field inside a superconductor and increase the magnetic field around it. (...
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18 views

AC current through induction on both sides

When you rotate a coil in a magnetic field one side cuts downwards the other cuts upwards. When you move a piece of wire up and down you get an AC current or pdf which you can see on a voltmeter as ...
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62 views

Physical reality of calculated wavelength from the two measurable phenomena slit distance and fringes distance?

I see some difficulties in answering this question (Have we really measured the wavelength of light?) about the measurement of the light's wavelengths with double slit interference. "... we can find ...
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35 views

How do I show $e_r$ and its first- and second time derivative?

What does a charge q moving in a circular orbit with a constant velocity (a "rotating charge") and a stationary observer that measures the E field look like? And how do I show $e_r$ and its first- and ...
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28 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 ...
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251 views

Why is the electric field inside a solenoid tangential?

I have been looking at some derivations for the electric field inside a solenoid. I know how to find it, but I don't get the symmetry argument used. This is often of the form: Since if we choose ...
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154 views

Magnetic force between two point charges

I tried to derive the magnetic force between two point-charges for iterative computation. Starting out with Lorentz force and Biot–Savart law for a point charge. $$ \vec F = q_2( - \Delta \vec{v} \...
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40 views

Where can I learn about Motors and Generators?

` Where can I learn about the functionality of motors and generators, so I can answer questions such as these? I need a good textbook that starts from the basics (eg faradays) then moves up with lots ...
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27 views

Galerkin-type weak formulation for electrokinetics

I am currently working on finite element simulations about electrokinetics. My solver (getdp) accepts directly galerkin-type weak formulation of equations. I am thus trying to write my equations in ...
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43 views

What happens to the emf generated due to a change in magnetic flux in a coil?

When we move a magnet into a coil of wire (open circuit) and then out of it, we create a emf across the ends of the coil and another emf in the opposite direction. My question is what would happen to ...
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86 views

Weizsäcker–Williams approximation

I'm having some trouble understanding the Weizsäcker-Williams approximation What I think it is is the following: I have a charged particle at high speed, close to the speed of light, at this speed ...
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43 views

Faraday's law in free space explaining away the constant vector?

Let's say that I have a plane electromagnetic wave travailing in free space, and I know the electric field part to be $\vec E$. If I am using Faraday's law to get the magnetic field part I will get ...
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49 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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63 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 Kerr–...
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55 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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66 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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83 views

Why do we define the magnetic field around a conducting rod as concentric instead of radial?

Why is it that we define the magnetic field around a conducting rod as concentric instead of being radially outwards? Ok, we know from the Lorentz force law, that the magnetic force is the cross ...
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799 views

Ed Leedskalnin's “Perpetual Motion Holder” (PMH)

Edward Leedskalnin (Coral Castle) describes a perpetual motion holder in his writings. I've built one and pretty much know how it works (I think). It is NOT a perpetual motion device by any means, ...
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93 views

Discernible “frequencies” of magnetic fields that do not affect each other

Two magnets secured to a surface close to each other in a fashion such that their constant attraction to one another is counteracted by the normal force of the securement of each. Is there a way to ...
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105 views

Why is no work done by magnetic forces in a uniform B and E field (perpendicular to one another)?

Say $B$ points in the $x$ direction, and $E$ points in the $z$ direction. Assuming a charged particle begins at rest at the origin, its motion will be a cycloid that progresses indefinitely in the $y$...
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25 views

Earth's magnetic effect on my magnet

My magnet seems to work independently of, and unaffected by, the Earth's magnetic field, yet the magnetic field of my magnet changes as I introduce some other similar magnets of varied sizes into its ...