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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1answer
42 views
Electromagnet, ideal turns depending on ohm
More turns -> stronger field
more turns -> longer copper wire
longer copper wire -> more resistance(ohm)
at what turn does the resistance make the electromagnet weaker? - I want to make an ideal ...
0
votes
0answers
35 views
Can you find the potential between capacitor plates with liquid dielectric in the capacitor's frame? [closed]
A magnetic field B is applied perpendicular to the plates of a capacitor whose parallel plates are separated by d. An electrically neutral liquid of relative dielectric constant k flows between the ...
1
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0answers
42 views
Book suggestion : geometric approach to electromagnetism [duplicate]
I´m looking for a book on electromagnetism that is introducing the topic from a geometric point of view, focusing more on the theoretical structure than on the application.
7
votes
1answer
3k views
Kubo Formula for Quantum Hall Effect
I'm trying to understand the Kubo Formula for the electrical conductivity in the context of the Quantum Hall Effect.
My problem is that several papers, for instance the famous TKNN (1982) paper, or ...
0
votes
1answer
55 views
If I have a rectangular loop moving through a magnetic field, which way will it go?
If I have a rectangular loop moving through a magnetic field, which way will it go?
A uniform magnetic field (say it's pointing into the page)
A square/rectangular loop of wire moving into the field ...
0
votes
1answer
105 views
Repulsive Magnetic Hammering Experiment
I have been thinking about something thing...
I want to attach four or any number of magnets with arms to an axle..
Blue dots be magnets.
Its is not a wheel configuration, all arms can move ...
4
votes
4answers
2k views
Understanding the diagrams of electromagnetic waves
I'm having trouble understanding the diagrams of elctromagnetic waves.
I have no problem with any concept in classical mechanics, and I think this can be answered without any relativity (which I ...
2
votes
1answer
52 views
Given expectation values for E and B, can you find an associated state?
When we quantize the electromagnetic field, we develop the concept of the field operator $A(\vec{r},t)$ and the simultaneous eigenstates of momentum and the free field Hamiltonian (i.e., each ...
2
votes
2answers
59 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, ...
3
votes
1answer
96 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 ...
2
votes
1answer
70 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 ...
0
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0answers
39 views
Magnetic field at a point [closed]
Find the magnetic field near a straight current carrying conductor of Length $L$ and current $I$, at point located along the perpendicular bisector of the wire a distance $r$ from the wire. Show all ...
2
votes
2answers
131 views
Should I use Coulombs law when magnets attract/repel?
When magnets attract to each other or repel.
Should I use Coulombs law? If not, why not?
Some would say that I shouldn't because: "Coulomb's law deals with static charges and force due to them. ...
3
votes
0answers
62 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
37 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 ...
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0answers
34 views
How do capacitance field fluxes change matter?
I asked my teacher at college how a field flux affects matter and he said nobody knows. Surely someone must. How does it change it?
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0answers
89 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 ...
2
votes
1answer
225 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 ...
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votes
1answer
58 views
A ring placed along $y^2 + z^2 = 4$, $x = 0$ carries a uniform charge of $5 \mu\ C/m$. Find $D$ at $P(3,0,0)$ [closed]
A ring placed along $y^2 + z^2 = 4$, $x = 0$ carries a uniform charge of $5 \mu\ C/m$.
Find $D$ at $P(3,0,0)$
How do I solve this using Coulomb's Law? I used $dE=\dfrac{dQ}{4\pi\epsilon_0 ...
2
votes
3answers
4k views
In electrostatics, why the electric field inside a conductor is zero?
In electromagnetism books, such as Griffiths or the like, when they talk about the properties of conductors in case of electrostatics they say that the electric field inside a conductor is zero.
I ...
0
votes
2answers
128 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
33 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; ...
4
votes
1answer
86 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 ...
0
votes
1answer
49 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 ...
-4
votes
4answers
919 views
What is the difference between gravitation and magnetism?
If you compress a large mass, on the order of a star or the Earth, into a very small space, you get a black hole. Even for very large masses, it is possible in principle for it to occupy a very small ...
4
votes
2answers
129 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:
$$\frac{d}{dt} (m\gamma c^2)=e \vec{E} \cdot \vec{u}$$
...
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vote
1answer
118 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 ...
2
votes
0answers
46 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 ...
3
votes
2answers
363 views
Is photon emission possible without electrons changing energy levels?
Does molecular vibrational transition and consequent emission of infrared radiation involve electrons changing energy level? In wikipedia, about vibronic transitions it says "Most processes leading to ...
1
vote
1answer
51 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 ...
0
votes
2answers
133 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 $q$
$$ ...
1
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1answer
87 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 ...
2
votes
2answers
159 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 ...
0
votes
1answer
48 views
Find the voltage between two points [closed]
Given two point charges $+2\text{ nC}$ and $-3\text{ nC}$ are located at the points $P_1$ $(2,2,-1)\text{ m}$ and $P_2$ $(1,-2,1)\text{ m}$, find the Voltage $V_{ab}$ between points $P_a$ ...
0
votes
1answer
110 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 ...
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3answers
59 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 = ...
1
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0answers
31 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 ...
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0answers
42 views
magnetic field due to circular coil [closed]
A circular coil has 5 turns and another coil has 8 turns.Both the coils havethe same diameter and both the coils are made up of same material with sameradius.Let 'n' be the strength of magnetic ...
1
vote
1answer
120 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?
1
vote
1answer
159 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 ...
0
votes
0answers
63 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
52 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, ...
10
votes
2answers
113 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 ...
3
votes
3answers
265 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?
2
votes
1answer
188 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 ...
2
votes
4answers
110 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 ...
1
vote
1answer
67 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 ...
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0answers
66 views
An electron has an initial velocity in B and E field [closed]
An electron has an initial velocity of (14.5j + 18.5k) km/s and a constant acceleration of (1.60×1012 m/s2)i in a region in which uniform electric and magnetic fields are present. If B =(400μT)i, find ...
6
votes
4answers
308 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. ...
0
votes
2answers
59 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 ?
