Questions tagged [electromagnetism]

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.

Filter by
Sorted by
Tagged with
2
votes
0answers
33 views

What is the radiation field of a black body with temperature $T$ translating at speed $v$?

Suppose I have a spherical black body at temperature $T$ with radius $R$. When stationary in a vacuum at zero Kelvin, it produces an incoherent radiation field according to Planck's law $$ B_\nu = \...
0
votes
1answer
40 views

Different formulas for magnetic flux

I am familiar with the formula $\phi=BA\cos(\theta)$ fo magnetic flux, where I know $\theta$ is the angle between $B$ and $A$. I came across another formula ; $\phi=Ba\cos(\omega t)$. Wanted to ask ...
1
vote
2answers
53 views

What is the exact mechanism of flow of electricity?

When a steady current flows through a conductor, the electrons in it move with a certain average ‘drift speed’. One can calculate this drift speed of electrons for a typical copper wire carrying a ...
2
votes
1answer
81 views

Why is absorbance calculated as $\log_{10}(1/R)$ and not $1-R$?

According to Lambert Beers law, the absorbance ($A$) of a sample at a given wavelength can be described in terms of its transmittance ($T$) according to: $A = \log_{10}(1/T)$ In near-infrared ...
6
votes
1answer
80 views

Do potentials make sense in relativistic quantum theory?

In Peskin & Schröder QFT, just before equation 7.93, he writes in passing, Next let us examine how $\Pi _2 (q^2)$ modifies the electromagnetic interaction, as determined by Eq. (7.77). In the ...
0
votes
3answers
128 views

How to decide if a potential is a scalar?

I know that ${\vec E}=-{\vec \nabla} V$. If a potential is scalar, I can find its electric field strength. The question is how can I know if a potential is scalar? Do all point charges result in ...
0
votes
2answers
35 views

Metal sphere in uniform Electric Field—doesn’t behave like dipole at large r?

I’m going over Griffiths EM example 3.9 (different question on this example is here Metal Sphere in a Uniform Electric field). I’m wondering where the logic for $$V\rightarrow -E_0z= -E_0r\cos(\theta)...
1
vote
1answer
58 views

EM-Wave: Calculate magnetic field $H$ from electric field $E$

In an exercise I am supposed to calculate the magnetic field from the electric field for a plane, harmonic wave in vacuum. $$\vec{E} = - E_0 \cdot \sin(\omega t - k z) \cdot \vec{e_y}$$ Using the law ...
1
vote
2answers
55 views

Magnetic fields and closed loop

It is well known that there don't appear to be magnetic poles. In Maxwell's equations this has the implication $$ \nabla \cdot \mathbf{B} = 0 $$ and results in the statement "the magnetic field forms ...
0
votes
1answer
23 views

Magnetic Field Manipulation

For a reference of my level of knowledge, I have taken an undergrad electromagnetism class, and it wasn't pretty but I passed. I have a real interest in this so I don't mind doing more research. If ...
1
vote
0answers
28 views

Is equilibrium potential relative?

If you have a half cell reaction, let's say: $Zn$ electrode and ($SO_4^{2-}$ and $Zn^{2+}$) solution The solution will get more and more postive and the electrode more and more negative until $\...
2
votes
0answers
15 views

What is the critical volume/mass of non-deuterated water that appears blue to our eyes?

A glass of water appears clear, but a swimming pool appears blue. At some point there must be a switch and I am wondering if anyone knows what that point is (or if its so gradual we can't tell). Does ...
2
votes
1answer
158 views

Why is there longitudinal response in a partially-filled Landau level?

Suppose I consider an infinite, non-interacting (so no FQHE should happen) 2DEG in the magnetic field $\vec B=B\hat z$ with a non-integer filling factor, say 0.13 or whatever. Suppose I apply an ...
0
votes
0answers
14 views

Conformal mappings in electrostatics applied to an infinite conducting fence in a uniform electric field

A long straight fence made of conducting wire mesh separates two fields and stands one metre high. Sometimes, on fine days, there is a vertical electric field over flat open countryside. Well away ...
36
votes
6answers
3k views

What's the physical meaning of the statement that “photons don't have positions”?

It's been mentioned elsewhere on this site that one cannot define a position operator for the one-photon sector of the quantized electromagnetic field, if one requires the position operator have ...
2
votes
0answers
174 views

Are there Optical Magnetic Mirrors (OMMs) which reflect via interaction with the magnetic field?

The most familiar mirror relies on reflection from a metal, where the very strong interaction of the electric field component of the EM wave with the conduction electrons produces a reflected wave. ...
3
votes
1answer
21 views

Fresnel Transmission Coefficient for Magnetic Field

Helmholtz equations for electric and magnetic fields are $$∇^2 \mathbf{H} + k^2 \mathbf{H} = \mathbf{0}$$ $$∇^2 \mathbf{E} + k^2 \mathbf{E} = \mathbf{0}$$ Obviously, if a solution is found to ...
0
votes
2answers
32 views

What is a magnetic wall?

I've been going through the "Foundations for microwave engineering" textbook by Robert E. Collin in order to study coplanar waveguides. In doing so I was introduced to Conformal mapping techniques, a ...
0
votes
1answer
50 views

Dipole moments in QFT

We say that an electron has a dipole moment (let's arbitrarily focus on magnetic dipole moment), which we can calculate classically and also add quantum corrections. Suppose we measure the electron's ...
2
votes
0answers
31 views

Shape of wavefronts for transverse modes in electromagnetic waveguide

I've been able to confuse myself about the shape (i.e. surface ripple/bend/tilt) of wavefronts for transverse modes in an electromagnetic waveguide, like a rectangular cross section waveguide with ...
1
vote
1answer
36 views

Earth's Magnetic Field and Rotation used to generate electricity in huge longitudinal coil

If we wrapped the Earth in a longitudinal copper coil, would the planet's magnetic field and rotation give current through the coil? Is this the energy solution?
3
votes
1answer
713 views

Donut shaped magnet

Suppose we have a horseshoe magnet. Now we bend it in such a way that it becomes donut shaped and poles remain in contact with each other. In this situation, what will happen to the magnet? Will it ...
1
vote
0answers
36 views

Are Cooper pairs (of electrons) entangled?

I have read this question: Does a normal torch emit entangled photons? where Quantum Journalist says: Although all the particles of a BEC are in a single quantum state, they are not ...
0
votes
1answer
42 views

In a circuit, do surface charges get influenced by external electric fields? [duplicate]

Hello ,This image represents surface charges in a circuit caused by the battery's electric field, if you were to place an other electric field source of the same magnitude (but not connecting it to ...
0
votes
1answer
50 views

What happens if permanent magnets are placed under electric field?

I understand that ferromagnets have domains that align parallel to the applied magnetic field, in which the spin of the unpaired electrons line up. But I am curious as to what happens to such magnets ...
3
votes
1answer
173 views

Do accelerated moving electrons radiate electromagnetic energy?

Do accelerated moving electrons radiate electromagnetic energy? Generally speaking, the charge should absorb energy instead of radiating energy in the acceleration stage, and release energy in the ...
11
votes
1answer
1k views

Looking for physical intuition into the Electromagnetic Tensor:

I have done some work with the electromagnetic tensor and I'm fairly good at manipulating it and using it to transform the Maxwell Equations into tensored forms. Admittedly though, I have no physical ...
1
vote
1answer
133 views

What is the frequency distribution of the electromagnetic wave emitted by a charge accelerating along a given trajectory?

Charged particles radiate when accelerated.From a few other answers from Physics Stack exchange like What is the frequency of a photon emitted by an accelerated charged particle? and How ...
0
votes
1answer
131 views

Vector potential of a linear current

First of all, I apologize in advance for my English, as I know it is not very good. So thank you for making the effort of understanding me. Well, here is my problem. I am trying to prove that if I ...
6
votes
1answer
28k views

Physical meaning of Maxwell's equations and origin of EM waves

Is it possible to describe the physical meaning of Maxwell's equations and show how they lead to electromagnetic wave, with little involvement of mathematics ?
0
votes
1answer
33 views

Energy in radar equation

In the book Skolnik, Introduction to Radar Systems, ed. 2, pag. 52 (sec. Transmitter Power) the radar equation in its simplest form is initially cited: $$R_{\max}=\left[\frac{P_t G A_e \sigma }{(4 \...
1
vote
2answers
18k views

Formula of the Radius of the Circular Path of a Charged Particle in a Uniform Magnetic Field

A charged particle $q$ enters a uniform magnetic field $\vec{B}$ with velocity $\vec{v}$ making an angle $\theta$ with it. Since the Lorentz force is perpendicular to the velocity, the particle will ...
0
votes
0answers
40 views

Is there a formula showing the relationship between the number of iron cores and the output voltage of a transformer?

I know the relationship between number of turns, current and voltage of a transformer, but is there a formula showing the relationship between the number of iron cores and the output voltage of a ...
0
votes
1answer
41 views

Justification of dropping term in Hamiltonian and expectation Values

While reading Sakurai's Modern QM, I was stuck at the point where he explains the absorption and emission of light quanta in atoms. He proceeds with Hamiltonian: $$H= p^2/2m + e\phi(x) -e/mc A\cdot p$...
0
votes
0answers
28 views

Electromagnetism: What do $B$, $H$ and $M$ field lines look like? [duplicate]

I can't understand the meaning behind this picture For the magnet (upper half of picture), it is said that $\oint$$\vec{H}\vec{dl}=0$ because there are no currents in the magnet (non Ampere's ...
6
votes
4answers
329 views

How is it possible for the induced emf to take negative values in Faraday's Law of induction?

Faraday's Law of induction states that the work done per unit charge by the (induced) electric force along a loop of wire, or the emf, is minus the rate of change of magnetic flux through the loop/...
2
votes
2answers
56 views

Is the Poynting Vector Continuous

Helmholtz equation for the magnetic field is $$∇^2 \mathbf{H} + k^2 \mathbf{H} = \mathbf{0}$$ Assuming time-harmonic waves Ampere's law can be rearranged $$\mathbf{E} = \frac{∇\times\mathbf{H}}{σ+...
3
votes
3answers
2k views

What is an induced electric field?

I have read in many books about induced current in a coil (Faraday's law), and also the motional emf across a moving conductor in a magnetic field. But somewhere I read about induced electric field ...
1
vote
1answer
129 views

How can this integration by parts from Dirac's Lectures on Quantum Mechanics be justified?

In Dirac's Lectures on Quantum Mechanics, when he is deriving the Hamiltonian for the free electromagnetic field, he performs an integration by parts: \begin{align*} H &= -L + \int \pi^\mu A_{\mu,...
0
votes
1answer
48 views

How is the electromagnetic/gravitational force transmitted? [duplicate]

So I was thinking about how a positive and a negative charge (or positive/positive, negative/negative) interact. I have read previously about how photons carry the electromagnetic force. However, how ...
2
votes
0answers
84 views

How is Electromagnetic segmented-capacitor propulsion system even possible?

The alleged invention - patent US10006446B2 - suggest allowing the creation of lifting force (4) on electrically charged rotating plate when the magnetic flux is turned on and directed towards the ...
0
votes
2answers
333 views

Vector potential of a solenoid in the Coulomb gauge

I understand the usual argument for calculating the vector potential outside of a solenoid of radius $R$ with $n$ turns per unit length carrying current $I_0$ using $$ \oint \mathbf{A} \cdot d \mathbf{...
-1
votes
2answers
57 views

Electrons glow by themselves

Electron beams moving at variable speeds emit electromagnetic radiation. In a vacuum tube, an electron beam bent by a strong magnetic field in a circular motion emits light. The luminescence does not ...
2
votes
2answers
120 views

Where does the magnetic field energy of a charged particle moving with a uniform velocity come from?

Consider a charged particle initially at rest with respect to an inertial frame. Let a force act on it so that it gains a velocity 'v'. It now produces a magnetic field that has some energy ...
1
vote
2answers
132 views

Electric charge attraction, unlike magnet attraction, can be neutralized, right?

If I put a magnet in a box of nails, it attracts lots of nails. If I put a hydrogen atom missing its electron (a hydron) near a bunch of electrons, it attracts exactly one electron and then it stops ...
1
vote
1answer
83 views

Why do we have redundant degrees of freedom?

Preliminaries: Consider the homogenous Maxwell's equations $$\partial_\mu F^{\mu\nu}=0.$$ and $$\partial_{\sigma} F_{\mu \nu}+\partial_{\mu} F_{\nu \sigma}+\partial_{\nu} F_{\sigma \mu}=0$$ Since ...
7
votes
5answers
1k views

Velocity of Object from electromagnetic field

I am wondering if someone could provide me with a formula that would tell me at what velocity a projectile can be launched from something using an electromagnetic field. The idea is much like a rail ...
0
votes
0answers
12 views

Triple infinite summation of a 3D Fourier series for Madelung Potential

I'm trying to evaluate the equation below excluding the case when $n_x=n_y=n_z=0$. I know this equation converges everywhere except where x,y, and z are all multiples of $2\pi$. I've attempted ...
2
votes
2answers
48 views

Why are there many turns in the primary coil of a step-up electricity transformer?

Since the voltage in a coil is proportional to the number of turns, the ratio of voltages is equal to the ratio of turns. So if you need a big increase in voltage, what stops you from having one turn ...
0
votes
2answers
2k views

Prerequisites for Griffiths electrodynamics [duplicate]

I want to start reading Griffiths electrodynamics during my holidays. But I am not sure of the math I need to know to start reading and understand the text thoroughly. So any advice would be ...