Tagged Questions

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
votes
1answer
107 views

Properties of the Faraday tensor for constant fields

I'm doing a special relativity past exam paper and have got caught up with something that I hope someone can help me with! I have to show that for constant fields, the magnitude of A, the ...
3
votes
1answer
268 views

Fermi level with Landau levels

So my question is regarding where the Fermi energy is when you have 2D electron gas in an applied magnetic field. My book explains that, using the Landau gauge, you find that the 2D density of states ...
7
votes
0answers
141 views

How can I find the position of an image charge when the boundary is parabolic or hyperbolic?

If the position of some charge Q is known, the boundary condition is u=0 on some parabolic surface, and we know the image charge has its electric volume of Q', then how can I determine the position of ...
2
votes
1answer
45 views

Minimum atomic clearance permitting motion

Suppose you were to build the piston and cylinder in a car engine atom-by-atom. Let's just say carbon, since you can make a lot of different shapes due to it's high valence. So assuming you make the ...
2
votes
1answer
43 views

Does an Ising lattice that returns to equilibrium create a current by induction?

Consider you have an Ising lattice with a dominant up component out of thermal equilibrium, that's your initial state. The down spins want to flip up and align with the ups, and they'll do so until a ...
0
votes
0answers
300 views

covarient derivative of electromagnetic field tensor

I'm trying to prove the energy momentum tensor in curved spacetime for Electromagnetic field is Divergence-less directly(Without using general lie derivative method which can prove any energy momentum ...
14
votes
1answer
2k views

Equation describing magnetic hysteresis

So when you're looking at B-H curves for ferromagnetic substances, you often see these magnetic hysteresis curves, which occur, I gather, largely because of domain formation which has some reversible ...
0
votes
3answers
4k views

Does an electromagnet increase in strength more by voltage or wire turns?

I'm working on a science project and need to know what makes a strong electromagnet - more wire turns or more voltage? I understand energy can be lost to heat with too many wire turns.
0
votes
1answer
62 views

Is there a material that changes local conductivity

I hope this is the right forum to ask this question. Is there a material (preferably thin, like a membrane) that changes its local conductivity (by that I mean the permeability for an electric field; ...
0
votes
1answer
265 views

Electric field at a point $P$

Problem Charge is distributed uniformly over a large square of side $l$, as shown in the figure. The charge per unit area ($C/m^2$) is $\sigma$. Determine the electric field at a point $P$ a ...
3
votes
1answer
631 views

Bohr-van Leeuwen theorem and quantum mechanics

Preamble: If one considers an ideal gas of non interacting charged particles of charge $q$ in a uniform magnetic field $\mathbf{B} = \mathbf{\nabla} \wedge \mathbf{A}$, then the classical partition ...
5
votes
1answer
168 views

What is the action for an electromagnetic field if including magnetic charge

Recently, I try to write an action of an electromagnetic field with magnetic charge and quantize it. But it seems not as easy as it seems to be. Does anyone know anything or think of anything like ...
1
vote
1answer
523 views

Showing Lorenz gauge is satisfied in retarded potential - vector calculus

I am trying to show that $\nabla\cdot \vec{A}=-\mu_0 \epsilon_0 \frac{\partial V}{\partial t}$ $V=\frac{1}{4\pi\epsilon_0}\int \frac{\rho(\vec{r}',t_r)}{r}d\tau'$ $\vec{A}=\frac{\mu_0}{4\pi}\int ...
4
votes
2answers
630 views

Relativistic Lorentz force law

If we consider the the relativistic Lorentz force law: $$\frac{d}{dt} (m\gamma \vec{u})=e(\vec{E}+\vec{u} \times \vec{B})$$ How can we deduce: $$\frac{d}{dt} (m\gamma c^2)=e \vec{E} \cdot \vec{u}$$ ...
2
votes
0answers
151 views

Electromagnetic inertia due to advanced radiation?

The scalar potential $\phi$ and vector potential $A$ at a distance $r$ from a charge $q$ are given approximately by $$\phi = \frac{q}{r}$$ $$\mathbf{A} = \frac{q\mathbf v}{r}$$ where the constants ...
1
vote
1answer
147 views

B-field and Magnetic forces, speed of a particle

Let's say I have a particle moving perpendicular to a uniform magnetic field of magnitude $x \ T$, and it moves in circle with a fixed radius. How do I find the speed of this electron? Initially I ...
1
vote
1answer
121 views

Quantum Mechanics of Lenz's Law?

I've searched the internet and two famous QM books (Sakurai and Messiah) for Lenz's Law, but haven't found anything. So my question is what the quantum mechanical explanation to Lenz's law is? Can ...
0
votes
1answer
2k views

How electrons act under rotating magnetic field?

I study Power Engineering in University. Today I asked my lecturer to explain me exactly how atom's electrons act under spinning rotor's magnetic field, that generated dynamic electricity. But he even ...
1
vote
0answers
36 views

Energy in the electric field of a translating charged sphere

The energy stored in the electric field of a uniformly charged non-conducting shell moving with a uniform velocity is more than the energy stored in the electric field of the same shell at rest. What ...
1
vote
3answers
1k views

Direction of the Area Vector (with regards to magnetic dipole)

I'm learning about torque on a conductive coil in a magnetic field. I have been taught that $\vec\tau = \vec\mu \times \vec{B}$, where $\vec\mu$ is the magnetic dipole moment. Also, $\mu = ...
2
votes
2answers
103 views

Where is the magnetic self energy term in $L$ for a charged particle in an electromagnetic field?

In the Lagrangian for a charged particle in an electromagnetic field $$L = \frac{1}{2}mu^2 - q(\phi - \frac{\vec{A}}{c}\cdot \vec{u})$$ the energy of the particle is contained in the kinetic term, ...
1
vote
1answer
421 views

Do magnets lose their magnetism/force when they attract/repel each other?

So yea, if two magnets attract/repel multiple times(A LOT) will they lose their magnetism? If they don't break during the whole process?
2
votes
1answer
1k views

Polarizability and the Clausius-Mossotti Relation

There seems to be a fairly large inconsistency in various textbooks (and some assorted papers that I went through) about how to define the Clausius-Mossotti relationship (also called the ...
1
vote
2answers
411 views

Lorentz force in homopolar motor

There are a lot of videos showing the simple homopolar motors in action. Let's look at this one: http://www.youtube.com/watch?v=6jyraFLqfqE It is supposed to be working due to the Lorentz force. ...
0
votes
0answers
108 views

cgs Gauss' system of units

I had never seen this system until today, and I'm really confused. I've read the wikipedia article about it but I still don't know how to change between this and the international system. For example, ...
2
votes
2answers
627 views

A charged sphere with pulsing radius

Radius increases and decreases periodically (as a pulse).And so does the charges on the surface of sphere. I can't get what is gonna happen.the EM waves are produced perpendicularly to motion of ...
11
votes
2answers
201 views

What mechanisms allow conductors to be transparent?

An electric field in a conductor causes charges to redistribute so as to cancel out the original field, bringing the field to zero. This is, I think, a common argument for why conductors are generally ...
0
votes
1answer
332 views

Eddy current losses in electric steel by harmonics of a magnetic field

I am working on an model of a permanent magnet synchronous machine. Right now I am stuck with calculating the eddy current losses caused by the harmonics of the stator magnetic field in the electrical ...
3
votes
2answers
338 views

Lorenz gauge fixing

Is it always possible to define function $\psi$ satisfying the Lorenz gauge equation $$ \partial_{\mu}\partial^{\mu} \psi + \partial_{\mu}A^{\mu} = 0? $$
4
votes
1answer
966 views

Will molten iron stick to a magnet?

I've known for a long time that if you heat a magnet, there is a point it loses its magnetism (the Curie temperature). It isn't clear to me if this applies to induced magnetism like iron sticking to ...
3
votes
3answers
5k views

Why is copper diamagnetic?

Cu has an unpaired electron in 4s, but it is diamagnetic. I thought that it has to be paramagnetic. What am I missing?
0
votes
2answers
108 views

Faraday's law in a ring

What role does the induced emf in Faraday's law play in generating current in a ring in which the magnetic flux is changing ?
2
votes
2answers
197 views

What is the maximum power available from a magnetic field?

I just want to validate something I inferred from studying Griffiths (1999). The instantaneous magnetic field $\vec{b}(t)$ at a distance $r$ from a long infinite conductor carrying a current $i(t)$ ...
3
votes
2answers
107 views

Why is $\vec j\cdot \vec e$ the joule dissipation?

I always see $\vec j\cdot \vec e$ as Joule's dissipation and I don't understand why. For example, if we have a uniform electric field $\vec e=e_o\vec u_x$ and we release an electron in it, it will ...
3
votes
2answers
2k views

Is it possible to get energy from electrical wire using induction?

Pardon me if this is a stupid question but my physics courses are from a long long time ago and I was a teenager back then. ;) Electricity in a wire generates electromagnetism, right? Would it be ...
1
vote
1answer
167 views

Derivative of covariant EM tensor

I cannot seem to prove that the derivative of the duel tensor = 0. $$ \frac{1}{2}\partial_{\alpha}\epsilon^{\alpha \beta \gamma \delta} F_{\gamma \delta} = 0. $$ Writing this out I get (for some ...
1
vote
1answer
176 views

Phase shift in electromagnetic potential

In Aharonov-Bohm effect, how to derive that the wave function of a electric charge $q$ acquires a phase shift $\phi=\frac{q}{\hbar}\int \mathbf{A} \cdot d\mathbf{x}$ after travelling in the non-zero ...
4
votes
2answers
1k views

Do protons exchange photons with electrons?

I'm sorry for this question but, I just don't get it. According to the electromagnetic field theory, electrons repel each other by exchanging photons. How do protons attract electrons, by photon ...
9
votes
4answers
3k views

How do permanent magnets manage to focus field on one side?

The actuator of a hard drive head consists of two very strong neodymium magnets, with an electromagnetic coil between them. If you take that apart, the magnets attract each other very strongly. ...
1
vote
5answers
459 views

Why isn't perpetual motion possible, even though we are so technologically advanced?

Why perpetual motion wouldn't be possible if we are so technological advanced? It is just a thing that I was wondering for too long. I mean, we are able to create so powerful permanent magnets, like ...
2
votes
3answers
550 views

continuity of the electric potential due to a surface charge

The Electric potential due to a charge distribution on a surface is : $\Phi \left ( x \right )=\int \frac{\sigma \left ( {x^{}}' \right )dx{}'}{\left \| x-x{}' \right \|}da$ I want to show that it's ...
0
votes
3answers
562 views

Advanced Heaviside-Feynman formula implies electromagnetic inertia?

The Heaviside-Feynman formula (see Feynman Lectures vol I Ch.28, vol II Ch. 21) gives the electric and magnetic fields measured at an observation point $P$ due to an arbitrarily moving charge $q$ $$ ...
3
votes
1answer
341 views

What breaks the symmetry between the electromagnetic and weak nuclear force?

I know the electromagnetic force is mediated by a photon and the weak nuclear force is mediated by two massive bosons. Are there any other insights into why the masses are so different?
0
votes
1answer
122 views

Possible to generate an electromagnetic field from moving an object through a magnetic field?

Is it possible to generate an electromagnetic field from a stationary rare earth magnet with a hole in it by passing an object through the hole? By definition, a Lorentz Force is the force on a ...
0
votes
2answers
100 views

Where's the energy in coupled magnetic fields?

Suppose I have two identical current loops superimposed upon one another, but with their currents in opposite directions. The coefficient of coupling $K=1$, and therefore the mutual inductance $M ...
2
votes
4answers
255 views

Why in $F = iLB$, $L$ is a vector but $i$ is not?

I learned $F = iLB$ recently. However, I don't understand why $L$ is marked as a vector but $i$ is not. For a normal rod, how should I define the direction of length vector $L$? And if I reverse the ...
-2
votes
1answer
26 views

Why coercivity increase in after depositing organic on top of Ferromagnet [closed]

I had deposited Fullerene on top of Cobalt thin film which result in increase of coercivity.
2
votes
0answers
442 views

How does a Fresnel rhomb work (half and quarter wave plate)?

How does a Fresnel rhomb work (half and quarter wave plate)? I am aware of birefringence, which creates a phase shift of $\Delta\phi=\dfrac{2\pi\Delta nL}{\lambda_0}$. But this doesn't explain how a ...
4
votes
1answer
321 views

Monochromatic wave

If we have an EM wave like this one: $$E=\begin{pmatrix}1\\i\\0\end{pmatrix}e^{-i(\omega t-kz)}$$ The wave has clearly only one frequency $\omega$, but is it monochromatic? My doubt is that it's ...
3
votes
3answers
492 views

Magnetostatic energy density — derivation without introducing inductance?

I was looking for a derivation of the expression for the energy density at any point in a static magnetic field. I do know that it is $$u_B=\dfrac {1}{2 \mu_0}\left|\mathbf{B}\right|^2,$$ I was just ...