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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2
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2answers
96 views

What is the link between observation wavelength and spatial resolution of an instrument? [closed]

It seems that when capturing and emitting EM waves matter proceeds differently depending on the wavelenght. According to an answer from another question, EM energy is captured following three modes: ...
0
votes
5answers
87 views

Is anything coloured “infra-red”?

All our day-to-day objects are things we can see, and thus we can conclude that their colour lies in the visual spectrum. Is there matter that does not have a colour within the visual spectrum? What ...
3
votes
3answers
941 views

Why the magnetic flux is not zero?

If $\vec{\mathbf B}=B\vec{\mathbf a}_z$, compute the magnetic flux passing through a hemisphere of radius $R$ centered at the origin and bounded by the plane $z=0$. Solution The hemisphere and ...
4
votes
2answers
644 views

Deriving Heaviside-Feynman formula for the electric field of an arbitrarily moving charge from Lienard-Wiechert potential

I've been trying to derive this (which Feynman warns takes a lot of work) for a couple of days now, without success. My current best derivation which however doesn't give the right answer is: First, ...
0
votes
3answers
120 views

What is the experimental evidence that light is an electromagnetic wave? [closed]

Do we have any experimental evidence to confirm this? How can maxwells equation confirm light as an EM wave simply based on a similarity in speed? Just curious!
0
votes
0answers
369 views

Why do we apply Fleming's *left* hand rule in case of DC motors but use Fleming's *right* hand rule in case of generators?

Why do we apply Fleming's left hand rule in case of DC motors but use Fleming's right hand rule in case of generators to find the direction of induced current?
0
votes
0answers
30 views

BH-demagnetazation curve of NdFeB, N50

Is there a trusted literature source (material handbook) where someone can find the BH-demagnetazation curve of the Neodymium Iron Boron (NdFeB) permanent magnet, grade N50? Why there exists ...
0
votes
1answer
106 views

Can Gauss' and Ampere's Laws be written in terms of the divergence of an energy four-vector?

In the first 20 minutes of this video, Susskind derives the continuity equation for charge conservation: $$\dot{\rho}+\nabla\cdot\vec{J}=0$$ (Where $\vec{J}=\frac{\partial\dot{q}^m}{\partial A^m} ...
0
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3answers
2k views

Relation between complex dielectric constant and resistance

I am having a real hard time understanding how I reach the expression that relates resistance and $\varepsilon''$ as the complex part of the dielectric constant. I was trying to follow a clear and ...
0
votes
0answers
44 views

Electric Field Inside Dielectric Filled Cavity

I want to determine the electric and magnetic fields inside the following cavity: perfect conducting walls at $z = 0, z=d, x=0, x=b, y=t, -t^\prime$, with a dielectric $\epsilon$ filling ...
10
votes
3answers
669 views

Where's the energy in a boosted capacitor?

Suppose I look at a parallel plate capacitor in its rest frame and calculate the electrostatic energy, $E$. Next, I look at the same capacitor in a primed frame boosted in the direction perpendicular ...
0
votes
1answer
41 views

Tranverse electromagnetic waves

TEM waves do not exist in waveguide. Is this the correct explanation- Both curl and divergence of TEM are zero inside the waveguide and because of the boundary conditions (electric field zero at every ...
1
vote
0answers
22 views

Assumption of Positive Dielectric Value in Derivation of Dipole Field Around a Spherical Dielectric? Implications for Plasmonics

I am going through the solutions to Laplace's equation in spherical coordinates. In particular the boundary value problem of a dielectric sphere of radius $a$ and permittivity $\epsilon$ which has a ...
2
votes
1answer
50 views

momentum and energy of the rods

I have two electromagnets and I attach them to two rods(same mass) such that, first one will be attached to centre of first rod and the second one will be to the end of second rod. Now I bring them ...
0
votes
0answers
46 views

Reflectivity of gases

According to the principle of the conservation of energy, the following is true for an object: emissivity+reflectivity+transmittance=1; where emissivity=absorbtivity according to Kirchoff. When ...
2
votes
2answers
80 views

Divergence of Magnetic Field in an Electromagnetic Wave

In many text books of physics (especially those coping with Electromagnetic Theory), we see that an electromagnetic wave (EMW) is drawn as an electric field varying with a sinusoidal form and exactly ...
1
vote
1answer
99 views

Physical meaning of magnetic vector potential [duplicate]

Can anyone give me an intuition of why the magnetic vector potential $A$ is sometimes interpreted as the electromagnetic momentum ? I don't know analytical mechanics, just classical electromagnetism ...
1
vote
0answers
89 views

A-level electromagnetism experiments? [closed]

Does anybody know any A-level experiments investigating electromagnetism that can produce good data relatively simply?
1
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3answers
1k views

How could electromagnetic waves propagate through space although they have no electrons?

How could electric fields in these waves propagate through space although in space there's no electrons for the electric field to be formed? is there another type of charged particles that carry the ...
0
votes
0answers
96 views

Create radial magnetization with permanent magnets

Is there a way to create a magnetic field with permanent magnets that imposes a TRUE radial magnetization, without using a continuous ring, because it is so hard to produce? Could this be done with ...
0
votes
1answer
148 views

Is the physics of $j = \rho v$ questionable? What are the consequences? [closed]

In electrodynamics you have the quantity $\mathbf{j}$, which represents density of current per surface area. It is often said that $\mathbf{j} = \rho\mathbf{v}$, for geometric reasons. When I make ...
38
votes
7answers
12k views

Does electricity flow on the surface of a wire or in the interior?

I was having a conversation with my father and father-in-law, both of whom are in electric related work, and we came to a point where none of us knew how to proceed. I was under the impression that ...
2
votes
2answers
218 views

Why does the existence of an aether go against Maxwell's equations

I am trying to determine why there is a conflict between Maxwell's equations and Galilean relativity. The standard way I have seen it explained is: The Galilean world model says velocities transform ...
0
votes
1answer
75 views

What are magnet poles?

You see the poles of a magnet on every magnet picture, and they are said to be in the direction of magnetic field lines, but what does that mean? Is the number of electrons different on one side of ...
0
votes
1answer
144 views

Phase difference between incident plane wave incident on a dipole and radiation fields from dipole

i have an incident plane wave and a dipole, consider that plane wave incident on dipole. at this moment what happens to dipole? We know that after incident of plane wave on dipole, the radiation has ...
1
vote
1answer
123 views

How does the electric field pulse propagate during a change in the magnetic vector potential?

The electric field in terms of the electric scalar potential, and the magnetic vector potential is: $E = -\nabla\phi - \frac{\partial A}{\partial t}$, where $A$ is such that $B = \nabla \times A$. ...
0
votes
0answers
42 views

back emf in armature winding of ac generator

Emf is induced in armature windi g of ac generator when it rotate in magnetic field.And now load is connected to its terminal,due to induced emf current will flow through armature to load . Current is ...
1
vote
0answers
57 views

Magnetic field derived from a scalar function

Question: Show that $\mathbf B = \hat r \times \nabla g(\mathbf r)$, where $g$ is an arbitrary scalar function, is a plausible magnetic field. What current density $\mathbf J $ can produce ...
2
votes
0answers
334 views

Field at the tip of a rotating cone. Is (THIS STEP) in my solution right? [closed]

** Edited: I'm going to CLARIFY where and what my question is. Problem: A hollow cone(like a party hat) has vertex angle $2\theta$, slant height $L$, and surface charge density $\sigma$. It spins ...
7
votes
1answer
192 views

What does a relativistically moving capacitor “look” like? [duplicate]

Suppose I have a parallel plate capacitor with a vacuum between the plates, a voltage $V$ across them and a capacitance of $C$. What will this arrangement look like to an observer in a uniformly ...
-1
votes
3answers
339 views

Gravity and Magnetism… One and the same? [duplicate]

I asked this in another question as if it was an answer so as to use a bit of that questions popularity and possibly get some clarifications on this but I also prefer to make a question of my own, so ...
2
votes
1answer
68 views

Introduction of the vector potential $A_{\mu}$ for the local gauge invariance of the complex scalar field lagrangian [duplicate]

In Ryder, when trying to restore the local $U(1)$ gauge symmetry of the complex scalar field $\phi=\phi_1+i\phi_2$, the final Lagrangian consists of the following four parts: ...
13
votes
8answers
5k views

Is gravity just electromagnetic attraction?

Recently, I was pondering over the thought that is most of the elementary particles have intrinsic magnetism, then can gravity be just a weaker form of electromagnetic attraction? But decided the ...
1
vote
0answers
50 views

faradays law in differential form [closed]

A conducting spherical balloon is charged with a charge $Q$ constant in time and uniformly distributed on its surface. If the radius of the balloon is made to vary sinusoidally in time between a ...
11
votes
3answers
459 views

Motivation for Potentials

This is a hypothetical question about "pedagogy". Let's say I am trying to take someone who has just a very small amount of knowledge about Newtonian mechanics and convince them that the Lagrangian ...
5
votes
1answer
155 views

Faraday's law: sin and cos?

I am looking at this paper (Hanna S. A., Varhue W. J. and Titcomb S., IEEE Trans. on Instrumentaion and Measurement, Vol. 58, No. 1, 2009). They claim that the voltage generated in a loop of $N$ turns ...
1
vote
0answers
68 views

Analytic (or just qualitative) solution to *any* nontrivial electrodynamic dynamic problem?

I just realized that, after a basic course in electrodynamics, I have no idea whatsoever how to solve the most trivial non-static example in an "exact" way. By this, I mean the following: I know (at ...
1
vote
1answer
104 views

Lagrangian of Non-Relativistic Charged Particle in a Magnetic Field

I'm trying to derive the Lagrangian for a non-relativistic charged particle under the influence of a magnetic potential. I'm assuming that $F=-grad(V)$ and so by the Lorentz force we have $-grad(V)=q ...
0
votes
2answers
43 views

Using the force law to obtain total energy of an electron as a function of its radius

I am working on a problem which starts saying determine the total energy of a hydrogen atom with an electron moving with momentum $p$ at a radius $r$. For that part I got: $E = \frac{p^2}{2m_e} - ...
0
votes
1answer
63 views

Does the medium matter when calculating Optical Period?

If I consider the optical period $T$ for a given wavelength $\lambda$ in vacuum, I can calculate it as: $$ T = \frac{1}{f} = \frac{\lambda}{c} $$ However, in a particular material/medium, I am not ...
1
vote
1answer
107 views

NMR rotating frame

I'm reading about a linearly polarized field (in the context of NMR). The field is given by $$ {\bf H_{lin}}=2H_1({\bf i}cos(\omega_zt)).$$ This can be created by having a pulse field plus its ...
1
vote
1answer
39 views

How to model the impedance of a coil with a dielectric slab on the top

I have sketched what I meant in the image below. Were there no dielectric slab, the coil could have been simply modeled as an inductor. But I am not sure what the impedance can be if there is a ...
0
votes
0answers
70 views

Driving force in a homopolar motor

I can't really figure out the driving force in the following homopolar motor: http://www.youtube.com/watch?v=w2f6RD1hT6Q I am not convinced by the usual argument involving the Laplace force, so ...
0
votes
1answer
62 views

Mimic Magnetic Repulsive Resistance through a spring

I have two identical magnets and I push them towards each other so that they are repelling. Is it possible to accurately mimic this repulsion effect (magnetic energy) just using a metal spring ...
1
vote
1answer
83 views

Influence of applied voltage to an electron of a metal

I would like to ask what would happen to the potential well of an electron being trapped in a metal? If I apply a voltage trying to accelerate the electron out of the potential well. Would It make the ...
1
vote
2answers
199 views

Magnetic force as a relativistic effect?

There is something I am confused about when it comes to the force between two parallel wires carrying current, specifically why when they carry current in the same direction the wires are always ...
0
votes
2answers
345 views

Why aren't Faraday's law of induction and Maxwell-Ampere's law (without sources) symmetric?

I was wondering why Faraday's law of induction and Maxwell-Ampere's law (without sources) are not totally symmetric in the sense that Maxwell-Ampere's law has a $\epsilon_0 \mu_0$ term on the right ...
1
vote
1answer
282 views

What causes the light spectrum to appear on oil surface? [duplicate]

I have observed that when petrol or oil leaks, the light spectrum appear on it. What causes light to split into its constituent colours?
1
vote
1answer
241 views

Magnetic levitation frog example

I'm trying to figure out the strength of the magnetic field needed to levitate the frog, assuming it's spherical, has mass $5$kg and diameter $20$cm. I also assume $\vec{B}$ is uniform. Since it's ...
1
vote
0answers
22 views

What is the rate at which the field of an electron spreads? [duplicate]

There are 2 steps in the creation of a force field: (1) first the object comes into existence (2) then the field appears. In the case when a photon splits into an electron and an antielectron, these 2 ...