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.

learn more… | top users | synonyms (6)

1
vote
4answers
117 views

Where does the energy of the electric cable come from?

I'm stuck on one really simple example, I can't figure out what's happening to energy here... (This is not homework) Let's consider an uncharged electric cable, we'll model it by an infinite ...
2
votes
4answers
2k views

When to use method of images in Electrostatics?

I am a bit confused about when to use the method of images in E&M? For example, in Griffith's Electrodynamics Example 3.2, the problem reads: A point charge $q$ is situated a distance $a$ from ...
1
vote
1answer
139 views

Can someone reconcile the Boltzmann transport equation with the Maxwell equations for photons/light?

Having taking courses in both physics and nuclear engineering, I've noticed that the two fields tend to describe photons/light in two different settings. In nuclear engineering, the radiative ...
1
vote
0answers
30 views

Will any massless particle/object in the vacuum of space always travel at the speed of light? [duplicate]

The question as stated above, is "Will any massless particle/object in the vacuum of space always travel at the speed of light?"
1
vote
0answers
45 views

Fresnel equations of S and T waves and Isofrequecy Curves: Feeling confused

First of all sorry for my (probably) bad english. I've been studying propagation of light in anisotropic media from the Born and Wolf and from Landau "electrodynamics in continuous media" and I'm ...
0
votes
1answer
127 views

Finding Transversal Components from Longitudinal component for Electric and Magnetic Field in a cylindrical coordinates system

Can someone explain why this two equation are equivalent? $\nabla_T$ denotes the transverse two-dimensional nabla operator: $\nabla_T=\hat{x}\frac{\partial}{\partial ...
0
votes
1answer
86 views

Relationship Between Conductivity and Lossiness of a material

I read that a material is loss-less if the conductivity is zero. I have always learned that conductivity is a measure of how easily the material can conduct a current. Does this then mean that the ...
0
votes
0answers
19 views

a quandry into electromagnetic fields and generating usable power

I have a question, regarding generated electricity via earth's magnetic field. What if you put a coil of flat high conductive wire with short intervals between the loops on a maglev train? Would that ...
0
votes
1answer
54 views

Change in work done by dipole

How would you explain the intuition behind the equation $dW = -\tau d\phi$ for an electric dipole?
0
votes
0answers
35 views

why do everything tend to be stable? [duplicate]

Whenever I ask'why does a proton attract electron?' People say that 'because they are oppositely charged' But I want to know why do opposite Charges attract each other and tend to become neutralised? ...
4
votes
2answers
144 views

Is there something equivalent to a diode for light?

In electronics a diode is a component allowing current passing in only one direction, and blocking the other side. I'm wondering if something similar exists for visible light or other EM waves, like ...
0
votes
0answers
24 views

Why breakdown voltage decreases with increasing FWHM of an asperity?

I am running a simulation for a dielectric breakdown. I am getting breakdown voltage decreases with FWHM, which does not make sense as field enhancement is higher for a small width. Simulations solve ...
0
votes
2answers
293 views

Does a simple copper wire with an AC current create EM waves?

I want to know if a household AC current flowing through a simple copper wire will radiate EM waves? If yes, up to what range and can they be called radio waves?
0
votes
0answers
21 views

How to conserve power at the reflection boundary of perfect conductor?

How to conserve energy during the reflection ? I am taking the abs value of $\sum E_{field}$ along spacial direction and plotting at each time step. Then it is normalized to the input E filed. ...
0
votes
2answers
77 views

Photon propagation direction prediction possible after interacting with neutral hydrogen?

My current line of research deals a lot with hydrogen's Lyman-alpha emission and subsequent interactions of the Lyman-alpha photons with the surrounding hydrogen gas. My question is whether ...
1
vote
1answer
105 views

How can we define the energy stored in a (conservative) force field?

I have come to know from my textbook that energy is stored in the E-field of a capacitor, in the B-field of an inductor and so on. Take the example of an inductor. The derivation bewilders me ...
0
votes
0answers
16 views

How can two Electromotive forces exist at the same time?

This thought is derived from counter-induced $\epsilon$ in a motor, there is an applied voltage source that creates an $\epsilon$ to allow current flow from the power supply to the motor's windings to ...
1
vote
1answer
112 views

Where did earth's electric charge come from?

I get between .7 and 1.3 volts out of a 20 foot tree depending on the weather. I understand this is "telluric" electricity with pretty low amperage. Where is this charge coming from? The sun, right? ...
1
vote
2answers
83 views

selection of p substrate as wafer in typical cmos process flow

why is p-substrate typically used as wafer in the typical cmos process flow? why not n substrate?with respect to memories, Has it got anything to do with the aplha-paritcle radiation induced errors ...
0
votes
0answers
73 views

Time reversed Abraham-Lorentz reaction force

The Abraham-Lorentz radiation reaction force on a charged particle is given by: $$\mathbf{F_{rad}} = \frac{q^2}{6\pi\epsilon_0c^3}\mathbf{\dot{a}}$$ I understand the situation where one fires a ...
0
votes
1answer
113 views

Did light already experience death of the universe immediately after the Big Bang?

Let's suppose that I am on Point A with a light-beam launcher. My goal is to launch a beam of light to Point B, which is about 8,000 quadrillion light years away. My friend named Jack is at Point B ...
0
votes
2answers
204 views

Field Vectors and satisfying Maxwell's equations

If I have an electric field that its direction is parallel to the direction of the wave propagation, it will not satisfy Gauss's law for vacuum. However we can say it satisfies Gauss's law for ...
0
votes
1answer
46 views

Well-arranged equation of magnetic resonance

When a magnetic elementary particles enters a uniform magnetic field in a direction parallel to the field lines, its magnetic moment aligns instantaneously to the field direction, becoming either ...
14
votes
3answers
965 views

How to experimentally reconstruct Maxwell's equations from scratch

What are the minimal experiments would one need to perform in order to reconstruct Maxwell's equations from scratch, assuming even the concepts of $\vec E$ and $\vec B$ are unknown? While I'm not ...
0
votes
0answers
58 views

Extremely long wavelength electromagnetic radiation

High frequencies are used to eject electrons, because only electrons can really be affected at such a small wavelength scale. But could an electromagnetic wave have such a ridiculously low frequency ...
2
votes
2answers
248 views

Why does the classical electrodynamics Lagrangian density equation have a “field” term and an “interaction” term?

On Wikipedia's page on classical electrodynamics, they state the Lagrangian density equation as follows \begin{equation} \mathcal{L} = \mathcal{L}_{\text{field}} + \mathcal{L}_{\text{int}} = ...
0
votes
1answer
302 views

Magnetic Field Direction — a single one — on a current carrying wire

Take the wire on the right hand side. It experiences a force to the left of the page. To experience a force pointing left-wards, the magnetic field must point into the page. Why does it point into the ...
0
votes
1answer
73 views

Molecular field meaning in Liquid Crystal Theory

Given the Frank-De Gennes free energy $F = \int f(\boldsymbol{p},\nabla\boldsymbol{p}, ...)\ d\boldsymbol{x},$ for liquid crystals (see De Gennes-Prost, p. 107, formula 3.21), the vector ...
5
votes
1answer
197 views

Can a magnetic field be focused at a distance?

There are interesting arrangements of magnets that can strengthen the field in some places while weakening it in others. This is discussed in Can magnetic fields be redirected and focused at one ...
6
votes
2answers
647 views

Why is Xenon favored as the propellant in electric thrusters?

Most of the articles I've read on electric thrusters mention that Xenon is generally, with some exceptions, used as the propellant (or would it be termed reaction mass?). They never mention why ...
0
votes
1answer
208 views

Confusion about depletion layer in pn-junction?

Here: external link Seek: around (2:53) Since n-type and more electrons than p-type and vice versa for holes, they diffuse and in the process neutralize in the central region. How is the ...
1
vote
3answers
154 views

Do electromagnetic fields are already present all over the space? [closed]

Consider a region $R$ in space without any source of electromagnetic field. Now put a source $S$ of electromagnetic wave in the vicinity of $R$ so that at time $t=0$, $S$ starts radiating ...
2
votes
2answers
407 views

Natural Frequency of an object and the phenomenon of resonance!

I have read about the term natural frequency in quite a lot of places. But I haven't found an explanation as to what is vibrating. It was pretty awkward when I couldn't clearly answer my little sister ...
2
votes
1answer
151 views

Falling charged objects: energy conservation paradox?

Imagine that we start with two oppositely charged objects on the ground, separated by a distance $d$, with charges $+q$, $-q$ and masses $m$. We raise them both up to a height $h$. In doing so we ...
5
votes
2answers
316 views

Why would two protons repel?

I understand that two protons would repel due to them both being positively charged, however, wouldn't the strong force act on the two protons pulling them together? Would this mean that in this case ...
1
vote
1answer
206 views

The energy carried from one winding of a transformer to another, in quantum terms

I have read in wikipedia this statement "The energy carried from one winding of a transformer to another, in quantum terms is carried by virtual photons, not real photons" (wikipedia src: virtual ...
0
votes
2answers
116 views

Integrate the law of universal gravitation to get position [closed]

I'm trying to write Physics simulation software which simulates gravitational and electric forces between $n$ bodies. I know how constant acceleration is integrated to get distance, but I have no ...
1
vote
1answer
313 views

electron-gas theory of a metal (drude model) - intuition

I'm back to studying "Microelectronics" by Millman & Grabel (2nd ed.). The book makes some references to the electron-gas theory, and I found out to have some problems with my intuition. At each ...
2
votes
4answers
2k views

Potential energy of a charged ring

Consider a ring of radius $R$, and charge density $\rho$. What will be the potential energy of the ring in its self field? The best I can do: $$dq = \rho R \cdot \, d \alpha $$ $$E_p = 2 \pi R \cdot ...
2
votes
2answers
320 views

Poynting's Theorem simplified?

How can I apply Poynting's theorem to any system that has a magnetic field & electric field, to state conservation of energy? How does Poynting's theorem state conservation of energy in EM? I ...
10
votes
5answers
26k views

Why is current a scalar quantity?

Current has both magnitude and direction. As per the definition of vector defined in encyclopedia, current should be a vector quantity. But, we know that current is a scalar quantity. What is the ...
2
votes
1answer
121 views

components of mixed tensor with same indices

If my tensor $a^{\mu\nu}=$ matrix of 4*4 size (let's say, in 1+3 dimensions with mostly negative convention for the metric), what is $a^{\mu}_{\mu}$ ? Is it the trace or the vector of diagonal ...
-1
votes
1answer
55 views

How to convert EM fields to EM waves?

How a body producing electricity and magnetic fields become a body radiating electromagnetic waves?
2
votes
2answers
65 views

Magnetic Field and Flow of Vector Potential

I am sorry, when my question is not really concrete, but here we go. Consider the Hamiltonian function $$H(x, \xi) = \frac{1}{2m}\bigl|\xi - eA(x)\bigr|^2$$ corresponding to a charged particle in a ...
0
votes
1answer
112 views

Closed and Open Electromagnetic Cores

I think I may understand this concept, but I haven't been able to verify with anyone that I'm going about this the right way. So when a wire is wrapped around a core that is a closed loop, like a ...
0
votes
1answer
70 views

Why does Ampère's force only linearly depend on the distance between the two wires?

When we encountered Ampère's force law our professor stated: In the formula of the magnetic force you don't find $r^2$ as in the gravitational and the electric one because while those directly ...
0
votes
1answer
42 views

Is anything without mass an EM wave?

For the longest time I thought the distinction between matter wave and other types of wave is the non-relativist mass of the "thing" under discussion. Photons are EM wave, electrons are matter waves. ...
4
votes
1answer
323 views

What is reduced momentum in “A Dynamical Theory of the Electromagnetic Field” by James Clerk Maxwell?

I was reading Maxwell's paper titled [A Dynamical Theory of the Electromagnetic Field][1]. In part 2, section 3 ("Dynamical Illustration of Reduced Momentum"), Maxwell discusses a mechanical ...
0
votes
2answers
67 views

Traveling to Mars in 24 Days

I am trying to calculate an electromagnetic rail launch from the Moon but I need to know if humans can survive the launch. Please see the specs below. 150 miles of track Proposed final launch speed ...
0
votes
1answer
389 views

Magnetic Wire Doesn't conduct Electricity

I recently bought a 22 and 30 gauge loop of magnetic wire from radio-shack. The loops of wire were colored differently (if this makes any difference) and on the package it noted how these wires were ...