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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Physical meaning of magnetic length

What is the physical meaning of magnetic length $\ell_B=\frac{\hbar c}{e B}$ in 2D electron system under magnetic field? When $\ell_B \longrightarrow a$, where $a$ is the lattice constant, does that ...
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1answer
166 views

magnetic field of a rotating ring magnet

Can the magnetic field of a rotating ring magnet induce current in a conductor? Is there a flux linkage between magnetic field of a rotating ring magnet and a conductor?
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2answers
361 views

Are there analogs to resistance, inductance, capacitance, and memristance connecting the weak force to electromagnetism?

A question was asked over at EE.SE recently which I tried to answer, but much of my answer was speculative. I'm hoping someone here can help my ignorance. In electronics design, there are four ...
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1answer
34 views

Electron beam in magnetic field - increase or decrease the field?

When I fire an electron beam into homogenous magnetic field ( eg. inside Helmholtz coil ) it will bend into a circle => it will form a coil ( = current loop ) itself. The magnetic field created by ...
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0answers
34 views

Question about defining potential energy of a magnetic dipole placed in a magnetic field

We know that magnetic field is a non-conservative field, since it exists in closed loops. Then how can we define the potential energy of a magnetic dipole placed in a magnetic field ?
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2answers
351 views

Why does the dimension of the electric charge depend on the number of spacetime dimensions?

We can find via dimensional analysis that the dimension of the electric charge varies with the dimension of space-time $(D+1)$: $$[\text{charge}] = (\text{eV})^{(3-D)/2}.$$ It is dimensionless if ...
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5answers
14k views

Difference between electric field and electric displacement field

$$\mathbf D = \varepsilon \mathbf E$$ I don't understand the difference between $\mathbf D$ and $\mathbf E$. When I have a plate capacitor, a different medium inside will change $\mathbf D$, right? ...
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2answers
50 views

Solenoid cannon (coil gun) possibility and recoil?

Similar to my other question If you are to pass a magnet through a solenoid a current is produced in the solenoid due to the magnetic flux. However, if you were to have a magnet sit inside a ...
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1answer
2k views

Why is the magnetic field stronger at the edges of a bar magnet?

I have been performing an experiment at school in which I test the force on an iron surface from the magnetic field of an electromagnet. The electromagnet has a rectangular iron core. The theory ...
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1answer
58 views

Vector Potential As Electron Momentum Amplifier

Konopinski's "What the electromagnetic vector potential describes" (Gaussian dimension) equation (3): $\frac{d}{dt}[M\vec{v}+(q/c)\vec{A}]=-\nabla q[\phi-(\vec{v}/c)\cdot\vec{A}]$ contains a dot ...
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1answer
70 views

Is the photon's wave function the same as an electromagnetic wave(light)? [duplicate]

The first that i have been taught in Quantum Mechanics is the photoelectric phenomenon. Without analyzing it, it concludes that when we shine light at the circuit(roughly speaking), the work required ...
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1answer
35 views

Magnetic field inside parallel plate capacitor

Inside a parallel plate capacitor, we know that the electric field due to the static charge $E= \frac{\epsilon_0 A}{d} $ Now if we want to find the magnetic field inside the parallel plate capacitor, ...
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1answer
133 views

Opacity/Transparency of Conductive Meshes to Charged Particles (Electrons/Ions)

When using a conductive (metal) mesh, effectively a metallic woven fabric, in vacuum applications as a "grid" for charged particle optics, how does one calculate (or at least estimate) the opacity or ...
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3answers
74 views

doubts regarding the classical electron radius

When estimating the classical electron radius, people normally equate the energy needed to assemble a charged sphere to $mc^2$, due to the so-called mass-energy relation. However, personally I don't ...
3
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1answer
132 views

Motion in a Paul trap: $2n^{\text{th}}$ harmonic with larger amplitude than $n$th harmonic

Using a Paul trap, we captured the motion of a light charged particle (based on a rotating potential applied by AC current). Our rotational frequency was 50 Hz, and so when used FFT on the data, we ...
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1answer
212 views

Surely force on shell can't be balanced by field momentum?

Imagine a particle with charge $q$ at rest at the origin. It is surrounded by a concentric spherical insulating shell, also at rest, with charge $Q$ and radius $R$. At time $t=0$ I apply a constant ...
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0answers
28 views

Torque on a current carrying loop in non-uniform magnetic field

Does the formula $\mathbf{\tau = \mu\times B}$ work for a non-uniform magnetic field? Why? If not, how to calculate the torque on a current carrying loop placed in x-y plane, supposing the non ...
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1answer
11 views

Clarification needed:Projection Or Whole Length to be considered during integration

Sometimes in magnetism,electrostatics,friction problems when a force is acting over a curved we usually take the net projection of the curved path as the distance(to avoid integration).But it certain ...
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0answers
40 views

Finding the magnetic field due to a current carrying field at a point within the circumference

This is a question me and my friend were wondering about. How can one calculate the magnetic field due to a current carrying loop at a point in the area enclosed by the loop. For example, at point P ...
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0answers
11 views

Definition of self inductance of a coil

I saw a definition in a textbook for the term self-inductance of a coil as emf induced in a coil when rate of change of current is 1 ampere per second. But I think the following is correct: ratio of ...
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0answers
30 views

Wave propagation in micro strip waveguide

I am trying to understand wave propagation in micro-strip waveguide, the way i came to understand rectangular waveguide is this way (in brief): EM waves can be understood as reflecting from the ...
0
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1answer
48 views

Two Current Carrying Loops: Multiple Choice Question

I'm studying some simple multiple choice questions from an old test in electrostatics and I have some trouble intuitively grasping this question. I hope someone can explain the solution! Two current ...
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0answers
29 views

Is there a Lagrangian that can lead to the Rayleigh-Jeans law?

Is there a way to derive the Rayleigh-Jean's law using classical statistical mechanics only? On the internet there is a common way to arrive at the equation by using concepts in electrodynamics. This ...
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2answers
82 views

Electromagnetic wave and quantum mechanics [duplicate]

I'm very new to physics. I studied and read about quantum mechanics and what the assumptions are (wave particle duality, uncertainty principle, observation, wave function collapse, etc.), but I also ...
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2answers
131 views

Far Field Diffraction of EM waves: what does the zero frequency signify?

If you have a system of independently radiating electrons/point-charges, the far field distribution of the EM waves can be approximated by the fraunhoffer diffraction integral, or simply by the ...
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1answer
33 views

Electric field a distance z above the midpoint of a straight line segment

In Griffiths there's an example to evaluate the Electric field a distance z above the midpoint of a straight line segment of length 2L. Which carries a uniform charge $\lambda$. In that calculation, ...
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1answer
19 views

What is the magnetic property of an alloy?

If an alloy is made between a diamagnetic and a paramagnetic substance or paramagnetic and a ferromagnetic substance or a diamagnetic and a ferromagnetic substance, what will be the resulting magnetic ...
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1answer
222 views

Right hand solenoid example

I am just studying for my physics final exam. I came across this example and am not satisfied with the answer. My answer is that Z is South and the current would flow from Y to X because of the ...
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2answers
144 views

Energy conservation in electrodynamic system?

Consider two charged particles initially at rest in the configuration below. Let us assume the following: Starting at time $t=0$, we apply a constant force $f$ to the the bottom particle so that ...
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1answer
36 views

Voltage of a quadrupole magnet

I have a simple question and it's my first one in this community. :) Does the voltage of a quadrupole magnet depend on the power of the electron beam in a synchrotron? Perhaps someone has a good ...
2
votes
1answer
2k 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 ...
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0answers
41 views

Boundary conditions for Maxwell's equations at the interface between two media

Consider the following simple Maxwell's equations: $$ \nabla\cdot\mathrm{D}=\rho $$ $$ \nabla\times\mathrm{E}+i\omega\mathrm{B}=0 $$ $$ \nabla\cdot\mathrm{B}=0 $$ $$ ...
7
votes
1answer
106 views

Why do I hear voices when I touch my turntable needle?

So I was trying to figure out the reason why my old (and probably sufficiently damaged) needle on my phonograph (turntable) was not working like it was a little while ago. With my headphones on, I ...
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2answers
252 views

Induced emf in stationary conductor lying in varying magnetic field

We explain that motional emf is induced because charges in a conductor move along with the conductor and as a result a magnetic force pushes them to one side of the conductor. But in case of a ...
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2answers
152 views

Uniformity in a solenoid

I know the magnetic field strength increases as the number of turns in the solenoid increases. However, I've learnt the field inside the solenoid is usually nearly uniform. So, does the number of ...
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0answers
48 views

Are photons just electric waves in an electron's frame of reference?

They say that electrons emit photons when they jump to a lower orbit. But the way electrons should see it, there's no any emission, really. There's just rapid change in electric field due to a rapid ...
2
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1answer
122 views

Electromagnetic braking hypothetical situation?

Let's say there is a conducting rod in space - with a constant external magnetic field. If you spin the rod - will electromagnetic braking make the rod stop eventually? I feel like there is a change ...
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1answer
240 views

What are the boundary conditions for EM waves normally incident on the interface between two dielectric media?

An EM wave, amplitude $E_0$, frequency $\omega_0$, is incident upon a material with relative permittivity (dielectric function) $$\varepsilon \left( z \right) = \left\{ \begin{gathered}{\varepsilon ...
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votes
2answers
233 views

Currents induced by rotation of a coil about magnetic field lines

If the coil on the left is rotated about the magnetic field lines, will there be an induced current? I know that the flux concerns only the component of the magnetic field perpendicular to the ...
4
votes
1answer
118 views

Earnshaw Theorem for ionic solids

Is a single molecule of sodium chloride (say) or a cluster of molecules of NaCl unstable, although macroscopically NaCl is in fact, stable? How can I reason this based on Earnshaw's theorem?
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0answers
7 views

Enough high&configured field disabling electron avalanche?

The electron avalanche starts after sufficient high electric field through the hole where one electron is, often >300keV. However, I seem to be getting results, that at some energy instances of the ...
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1answer
82 views

Magnetic force paradox

Suppose a current carrying wire Is placed in vertical direction. A charged rod is placed is placed nearly horizontally.It is clear that magnetic field due to current carrying wire is inside the ...
2
votes
2answers
123 views

Is the wobbly rope depiction of a radio wave inherently wrong? And how do vectors of parallel waves align with each other?

I don't have a scientific education, yet I'm scientifically curious. Among other things, I'm struggling to understand the nature of electromagnetic waves. What I have recently realized is that the ...
1
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1answer
97 views

Can electromagnetic fields be used to deconstruct and reconstruct molecular bonds?

I was thinking one day and came up with a theory after reading about how scientists were studying anti-matter by using electro magnetic fields to separate matter from the anti-matter they made. It got ...
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0answers
9 views

Net electric field of a nanowire in the vicinity of a quantumdot

I know how to find the electric field when both nanostructures are same and in the vicinity of each other. However, when it comes to nanostructures of different dimentionality (ie.nanowire and a ...
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1answer
17 views

In magnetostatics, is there any relation between flux and current?

I have noted while trying to find analogy between electrostatics and magnetostatics, for the equation, flux = charge/epsilon, is there any corresponding equation in magnetostatics, relating magnetic ...
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0answers
10 views

Is the shear force required to separate two magnets different than the pull force?

I'm building something for a side project for fun =) Essentially, (simplified) Ill have a user standing on a metal platform. There is a robot placed beneath the floor. The robot moves the metal ...
3
votes
3answers
349 views

Why is the inner product between divergence-free current $\vec{J}$, and a gradient field$\nabla \varphi$ zero?

I read an article saying that the inner product between divergence-free current and a gradient field is zero. A divergence-free surface current is $\nabla\cdot\vec{J}=0$, and $\vec{J}$ could be ...
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1answer
333 views

Classical Mechanics - Equation of motion, Lagrangian, Newtons 2nd Law [closed]

I really don't even know where to start with this question. A particle with charge $q$ moving in an electromagnetic field is described by the Lagrangian $$L=\frac{m\mathrm v^2}2+\frac qc\mathrm ...
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1answer
42 views

Eddy Currents – Tubes with slits

When a magnet falls down a tube, eddy currents form and flow around the tube, perpendicular to the direction in which the magnet falls. However, when there is a vertical slit in the tube, are ...