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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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
18 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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12 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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13 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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9 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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7 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
44 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
45 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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43 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
38 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
52 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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27 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
21 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
21 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 ...
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46 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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1answer
65 views

$U(1)$ 5-dimensional Kaluza-Klein topological defects

Five-dimensional Kaluza-Klein theory is well-known to predict that the electromagnetic field can be described as a curled additional dimension over four-dimensional spacetime. That is, you only need ...
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17 views

Is there an expression for field charge analogous to field mass?

In electrodynamics, it is possible to derive an expression for the field momentum for a given moving charge distribution (e.g. a sphere with uniform velocity) and from that infer an "electromagnetic ...
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30 views

Knotted solutions of Maxwell's equations in flat vacuum - do they really exist?

The paper http://arxiv.org/abs/1502.01382 claims that such solutions exist and that a number of specialists know them since a long time. Is this paper correct? Jackson's text on electrodynamics does ...
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18 views

Electrical currents

I have a luminous electric plasma light that looks similar to a lava light only electric currents are inside instead of lava.This light is filled with electric current flowing throughout it. if I ...
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18 views

Free charge density and current in an ohmic material

I have come across what seems a paradox -or at least an exotic conclusion- regarding current conduction in an ohmic material. It is well known that free charge density can only be zero on an ohmic ...
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1answer
19 views

What kinds of behavioural anomalies can a zero-field-cooled (ZFC) / field-cooled (FC) split indicate?

If a material shows a spiltting in the ZFC and FC curves, is it necessarily superparamagnetic, or could there be any other reason for the irreversibility? I have heard spin glasses also show ZFC-FC ...
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20 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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1answer
49 views

Understanding the relationship between electric energy and force

I'm trying to do the following problem: One of the three types of radioactive decay is "β decay", during which protons decay into neutrons or vice­versa, emitting either electrons (β­) or positrons ...
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25 views

What should the magnitude of the induced voltage on the 2nd loop of a transformer be?

I'm trying to understand transformers and am reading this site: Mutual inductance and basic operation I am a little confused about how to reason about the voltages involved in a transformer. Assume ...
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1answer
58 views

How could the effective electric dipole interaction be derived

In some papers (e.g. Bernreuther equation (1.4), The electric dipole moment of the electron) you can find the electric dipole interaction defined as $$L_I=-\frac i2 ...
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1answer
50 views

Work functions of ceramics/insulators

Is anyone aware of or know of a good source or means of estimating the work function of a ceramic material? Typically, work functions are given for pure elemental metals, rather than for compounds, ...
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2answers
36 views

Why can't I connection the anode of a battery to the cathode of an other battery and get a current flowing?

If I touch the anode of a battery or connect the anode the the cathode of an other battery, none (or rather: a very small current) flows between those two. If I instead connect the connect the anode ...
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2answers
47 views

How can a magnetic field accelerate particles if it cannot do work?

A varying magnetic field can accelerate charge particles, but it is said that a magnetic field can't do any work so it should be able to speed up charged particles, right? How is this apparent ...
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1answer
63 views

I need help with divergence and gradient? [on hold]

$$A_z = \mu{\frac{e^{-jBr}}{4\pi r}}∫I(z')e^{jBz'\cos\theta}dz'$$ Midway into my question, I want to compute: $$-j\left( \frac{\nabla(\nabla\cdot A) }{w\mu\varepsilon} \right).$$ Symbols like $ w, ...
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1answer
17 views

potential inside a cylindrical shell in terms of the surface potential?

Given a potential distribution $V(\phi)$ at the surface of an infinite cylindrical shell, is there an easy way to derive the potential inside the cylinder. No charges or currents anywhere.
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1answer
26 views

Negative power measurement of transformer short-circuit test. What happened?

I have a dissectible transformer, made up of two 120-turn coils. I connect a function generator to the primary side, set the AC voltage to be constantly 10V, and short circuit the secondary side. And ...
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14 views

electron trajectory in Bfield with energy kick at midpoint

So I wanted to get some second opinions on this situation I have thinking about.Lets say an electron enters, normally, a section of bfield that is perpendicular to its motion such that the electron ...
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2answers
72 views

Derivation of Faraday's law using Lorentz force?

For a stationary loop of wire with a time varying magnetic field $\vec{B}(t)$ through it. Is it possible to derive Faraday's law, using only the Lorentz force $$\vec{F}=q \vec v\times \vec B \tag ...
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23 views

can we reuse the motion of a dc motor to power an ac generator

My question is ,when we make a simple dc motor, cant we connect another circuit to the rotating coil and make another circuit for an ac generator
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1answer
126 views

Does magnetic monopole violate $U(1)$ gauge symmetry?

Does a magnetic monopole violate $U(1)$ gauge symmetry? In what sense and why? Insofar as I know, there are at least two types of magnetic monopoles. One is the Dirac monopole while the other is the ...
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3answers
67 views

Induction and electromagnetic fields

I've got a few questions on induction and electromagnetic fields. My current understanding of induction and electromagnetic fields is that, when electricity/current flows through a wire, it creates an ...
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33 views

Difference between Reed Relay, Reed Switch and Just a Relay? [closed]

Whats the Difference between a Reed Relay and and Just a Relay, and How does it work.
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3answers
92 views

The propagation of electric field

In case of a charged particle which is travelling at a uniform velocity, the electric field due to it at a given point doesn't change instantaneously . The reason for this delay in change of electric ...
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1answer
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42 views

Force acting on a polar molecule

What kind of force does act on a polar molecule? I mean, there must be a force that is acting on the molecule to keep the negative and positive centre away ( like in H2O). Why the positive and ...
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31 views

problem on electromagnetic induction and newtonian physics

Please imagine a solenoidal toroid (i.e. a donut shaped inductor) powered by an AC voltage source. It creates a changing magnetic field which is confined to the interior of the toroid (i.e. within the ...
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1answer
25 views

Electromagnetism and static electricity

What is the difference between electromagnetism and static electricity? Also electromagnetic waves are mediated by photons , what mediates static electricity?
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22 views

Earth's magnetic field and the equator

Suppose earth is like a bar magnet with the poles acting as N and S. If a charged particle from outer space heads towards the equator, why does it get travel around the field instead of the magnetic ...
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2answers
77 views

Why does light travel as waves? [duplicate]

Why does light travel as waves instead of say just a straight line? What are the forces that make a light photon travel in a wavelike pattern?
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2answers
124 views

Are electron fields and photon fields part of the same field in QED?

I know in classical field theory we have the electromagnetic field. And Maxwell's equations show how electromagnetic radiation can propagate through empty space. I also have been reading about QED ...
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1answer
48 views

Can electromagnetic fields be used to deconstruct and reconstruct molecular bonds?

I was thinking one day and came up with a theory after reading about how scientists were studying anti-matter by using electro magnetic fields to separate matter from the anti-matter they made. It got ...
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2answers
45 views

Predicting Polarity of capacitor in the given diagram

I have to determine the polarity of the Capacitor in the given diagram. I Approached the problem as follows: When Magnet 1 moves with its North pole towards the coil, emf is induced in the coil as ...
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12 views

ratio between conduction current and displacement current

First, recall that Maxwell displacement current for a plane wave is $$ \vec j_D = \epsilon \partial_t \vec E = \epsilon \partial_t (\vec E_o cos(\vec k \cdot \vec r - \omega t)) = \epsilon \omega ...
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25 views

Time-dependent electric and magnetic field between two point charges

I would like to get expressions for the time-dependent electric and magnetic fields in the region around two moving point particles. I can't see how this is possible. Clearly you can't use Gauss' Law ...
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Space between Gas molecules

We know gas molecules are in random motion. There is obviously space between gas molecules. But I doubt that it is not vacuum,because if it was vacuum then how would the propagation of sound through ...