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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23 views

$H$ field in Faraday's law problem?

In my textbook it says, Consider a rectangular loop in the vicinity of an infinitely long wire carrying a current $I(t)$. To calculate the magnetic field, we can use the quasistatic approach, which ...
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2answers
142 views

Do transformers lose energy?

EDIT: The title should rather be how/why transformers lose energy My idea of a transformer is that it is composed of two separate wire windings around some metal core. The purpose is to ...
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1answer
44 views

Polarization type/direction of $E$ field

I have an E field given as $E(z) = [xE_a - yjE_b]\exp(-jkz)$ and I know the answer is that the field is right-hand circularly/elliptically polarized depending on whether $E_a = E_b$. Can someone show ...
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702 views

What different approximations yield Gravitoelectromagnetism and Weak Field Einstein Equations?

This question is inspired by this answer, which cites Gravitoelectromagnetism (GEM) as a valid approximation to the Einstein Field Equations (EFE). The wonted presentation of gravitational waves is ...
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1answer
51 views

How does Kohn's theorem demonstrate that a rotating microwave field can only connect the ground state with the cyclotron mode?

This is a follow-up question to Proof of Kohn's theorem. I am confused about a point in the answer given by @NowIGetToLearnWhatAHeadIs. It is noted that the perturbing Hamiltonian in Equation 12 ...
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1answer
185 views

Obtaining quantum Hamiltonian for charged particle from path integral formulation

I was working on Shankar 8.6.4, which is about obtaining in one dimension the Hamiltonian operator of a charged particle from the path integral formulation. First, I get the propagator over a time ...
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2answers
534 views

Does a rotating disk develop a potential difference between the centre and rim?

This stems from thinking about the question If a perfect conductor where to move what happens to the electrons?. Suppose we have a rotating disk with no external magnetic field, so this is not a ...
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31 views

How do I evaluate this general magnetic dipole equation for this given setup?

This shouldn't be too hard a question (mostly focused on vector multiplication) but I'm still not confident in my answer. Basically, I am looking at the force between two magnetic dipoles and using ...
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2answers
2k views

Can water be magnetized?

This may be a stupid question, so feel free to shoot it down. Assuming all atoms have a magnetic moment, I would assume the water molecule too would have a resultant magnetic moment; ergo, it may be ...
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2answers
327 views

Why do similar smooth metals seem to stick together?

We have a stack of thin (0.025in) stainless steel plates (round 2in)that seem to stick together. Surfaces are clean of oil/grease, and when we lift the top piece off the stack with a suction cup, the ...
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2answers
200 views

What will happen when lightning strikes on the surface of the deep sea?

The lightning is having enormous amount of energy, as it strikes the ground it makes various electrical and magnetic effects. If the lightning strikes a tree, that will be burst by lightning. ...
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3answers
647 views

Does the electromagnetic field “spin”?

Due to electron "spin", a small magnetic field is produced. Maxwell's equations imply that magnetic fields are due to changes in electric fields. Is the magnetic field produced then because the ...
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3answers
77 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 ...
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3answers
5k views

Is the EmDrive, or “Relativity Drive” possible?

In 2006, New Scientist magazine published an article titled Relativity drive: The end of wings and wheels1 [1] about the EmDrive [Wikipedia] which stirred up a fair degree of controversy and some ...
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0answers
59 views

By what mechanism is back-emf induced in a railgun?

I don't see what causes a back-emf in a railgun, because as the projectile moves along the rails, there is no change in field on any part of the projectile. Therefore, there shouldn't be any back-emf, ...
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1answer
3k views

How do I derive the Lorenz gauge from the continuity equation?

I was reading my old electromagnetics book (Elements of Electromagnetics, by Sadiku, 3rd edition) and after the author explained what the Lorenz gauge is mathematically and why it is useful in ...
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0answers
68 views

How to properly implement Huygen's integral?

I'm really confused by what's happening ... I'm trying to propagate a beam some distance and compare initial and final intensities. Lets say the initial electric field is just cos(200x). I then used ...
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0answers
266 views

EM wave frequency vs. attenuation

Why does increasing frequency of an EM wave (for example in sea water with positive and negative ions floating about) increase the attenuation of the wave? Is the relative permittivity changed or the ...
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0answers
103 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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1answer
157 views

NOT Perpetual Motion; Could It Work?

TL;DR The energy is not being created freely, so why shouldn't it work? We have all seen the YouTube "perpetual motion" machines. You know, the computer fans with magnets taped on them and hidden 9 ...
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2answers
67 views

Change of magnetic flux on size of magnetic field and area

Consider a solenoid with 50 turns. Each turn gives has an area $A$. When the power is switched on, each turn produces a magnetic field of 1 Tesla. What is the total change in magnetic flux when the ...
2
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2answers
389 views

Moving electric charges

I just wanted to double-check these three statements, as I'm not entirely sure I understood them completely: 1) A stationary electric charge (let's say a proton) produces electric field. 2) A moving ...
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1answer
275 views

How many coils of a wire in an electromagnet would make 100 milliTeslas field? [closed]

This is a rather stupid question. I'm making a basic electromagnet using copper wire and an iron nail and need to make an electromagnetic feild roughly about 100 milliTeslas . Ill be using a gauge 6 ...
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0answers
110 views

What is the cause of the time-varying electric and magnetic near-fields around a wire with a 60Hz AC current?

I know that it's the accelerating electrons in an AC current that are the cause of far field effects. But, what about the reactive near field close to a wire that has a 60Hz AC current? What is the ...
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2answers
167 views

Is there any other momentum besides the Poynting momentum stored in an electromagnetic field?

I am having some conceptual difficulties with energy and momentum stored in the EM field. The force density at a point is $\rho E + j\times B$ Because of conservation of momentum, and because the ...
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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: $$ ...
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1answer
126 views

Poincaré' lemma and EM potential $A^{\mu}$

My lecturer said that given the sourceless Maxwell's equations $$ \partial_{\mu}\, ^ *F^{\mu\nu} = 0 $$, we can find a solution $$ F^{\mu\nu} = \partial_{\mu}A_{\nu} - \partial_{\nu}A_{\mu},$$ that ...
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2answers
107 views

Why can't LIDAR be implemented with traditional antennae?

In robotics, LIDAR and RADAR are both used for obstacle detection. The data they produce are very similar, but they do have different qualities (LIDAR is more precise but less robust). There are many ...
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1answer
41 views

What is the intensity distribution behind the first beam splitter on an observation screen at the same distance like behind the second beam splitter?

Mach-Zehnder interferometer has been used, among other things, to measure phase shifts between the two beams caused by a sample or a change in length of one of the paths. My simple question is: What ...
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0answers
196 views

Confused on how to apply the volume integral form of the Biot-Savart law

In Viscous evolution of point vortex equilibria (Jing, 2011) the author explains that the velocity equation for the Lamb-Oseen vortex is found by applying the 2D Biot-Savart law to the vorticity ...
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2answers
1k views

How do I actually calculate the Lorentz transformation of a field strength tensor

Say now I have an arbitrary field strength tensor $F$, and I want to boost it according to a Lorentz transformation matrix $(\Lambda)$ The transformation is given by $$ F^{'\mu \nu} = ...
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0answers
30 views

Image charges for currents?

I am struggling to get my head around a problem involving creating a transmission line using a wire of radius $a$ in a vacuum a distance $d$ above an infinite grounded plate ($d \gg a$). I wish to ...
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1answer
2k views

Relationship between polarization density and electric field

Imagine I have an electric field $E$ created due to some free charges. Then I introduce a dielectric material somewhere. What confuses me is the polarization field $P$, which is now proportional to ...
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94 views

Calculating current on power line from induced voltage on nearby loop

The question I'm having trouble with is: A small circular loop of 5mm radius is placed 1 meter away from a 60Hz power line. The voltage induced on this loop is measured at 0.6 microvolts. What is the ...
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1answer
66 views

Infinite, straight, current-carrying wire uniformly charged to a negative electrostatic potential

I am working on a problem that states the following: Imagine an infinite straight wire carrying a current $I$ and uniformly charged to a negative electrostatic potential $\phi$ I know here ...
2
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2answers
108 views

Units of vector differential operator del ($\nabla$)

My book says that $\left[\nabla \cdot (\vec E \times \vec H)\right] = \mathrm{W/m^3}$. I see that $\vec E$ is in $\mathrm{V/m}$ and $\vec H$ is $\mathrm{A/m}$, so these multiplied is $\mathrm{W/m^2}$, ...
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2answers
1k views

Do all the things have a magnetic field?

I search for this but there is no answer for this question! Do all the things have a magnetic field? And also gravity ? Can you answer?
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2answers
202 views

Why does a magnetic field raise the ground state energy of an electrical particle?

I heard a statement that the ground state energy of a electrical particle in a magnetic field is larger than its ground state energy without the magnetic field. I just heard this statement. This ...
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2answers
62 views

Why does electrical impedance have as many parameters as it has?

The impedance of a circuit is written: $$ Z(R,L,C,f) = R + j ( 2 \pi f L - \frac{1}{2 \pi f C} ). $$ What is the line of thinking that frequency, resistance, capacitance, and inductance are the only ...
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0answers
67 views

Does the length of a bar magnet matter in producing EMF?

Consider the following diagram. There are 3 apparatuses to produce EMF. The coils are identical in cross section area, the number of winding, and length. The bar magnets have the same cross section ...
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1answer
298 views

Would a strong diamagnetic material exhibit inductance?

Do metals with the property of strong diamagnetism also exhibit inductance? Would a fluctuating magnetic field induce a weaker current in a strongly diamagnetic metal as compared against a ...
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1answer
80 views

Calculating the Magnetic Component of a Photon

I've been trying to figure this out for some time. I have found some formulae on other sites that claim to allow me to calculate the magnetic component of a photon, but I have seen so many variants of ...
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1answer
177 views

EM waves in conductors

On a recent test in my E&M class, we derived what happens to an EM wave propogating in a conductor of conductivity $\sigma$, but I'm having trouble understanding the results. We started from the ...
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47 views

Electron's Frame Aharonov-Bohm Effect

In the electron's inertial frame the solenoid moves past it in the Aharonov-Bohm Effect. That means the electron sees a time varying vector potential which, by: $\vec{E}$ = ...
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6answers
967 views

Why can't electrons fall into the nucleus?

I read a book on pop sci book on quantum mechanics and the author said that electrons do not fall into the nucleus due to quantum mechanics- which principles suggest this (I think it was Heisenberg's ...
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7answers
17k views

Does a current carrying wire produce electric field outside?

In the modern texts of electromagnetism in the presence of stationary currents the electric field is assumed conservative $\nabla \times E =0 $. Using this we get $E_{||}^{out}=E_{||}^{in}$ which ...
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1answer
62 views

Absorption from a classical to quantum

Today, I learned that using rubidium atoms at very low temperature in a Magneto-optical trap, one can experimentally show that the Lorentz classical derivation of absorption using dipole is valid. The ...
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2answers
225 views

Magnetic field by a solenoid?

I am really not able to understand why is it that the current,while finding the magnetic field in the solenoid (ideal one), is not $I$ the current flowing throught the solenoid but $nI$, where $n$ is ...
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1answer
79 views

Would the Lorentz force of the two wires be different?

From the following diagram: Lorentz force can be calculated from the following formula: $F$ = $IL\times B$ What about this wire in the following diagram? Assuming that all the variables are the ...
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2answers
1k 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 ...