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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What is the direction of the magnetic vector potential in an electromagnetic plane wave?

The vector potential $A$ is perpendicular to $B = \nabla \times A$, by definition, and hence, in a plane wave, it is either in the direction of $E$ or the direction of propagation. I suspect it is in ...
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6answers
660 views

Momentum of stationary electron in a curl free vector potential

The essence of this question is simplicity itself: There is an electron in a curl-free $\vec{A}$ field. The electron is stationary so its m$\vec{v}$ momentum is 0. However, it has "momentum" from ...
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0answers
46 views

Perpendicular Current Carrying Wires Forces?

Hi guys, I'm having trouble understanding the effect of sides BA and CD on the straight wire. With an amperian loop around the straight wire, the magnetic field on BA and CD is into the page, and then ...
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0answers
45 views

Magnetic field of cylinder equal to two current loops?

I was reading a text about the magnetic field of a uniformly magnetized cylinder when I came across this. The author states that since \begin{equation} A_{\phi}(\rho,z) = \frac{\mu_0 a M}{4\pi} \int_{...
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1answer
49 views

What is the difference in the behaviour of the commutator in a motor between DC and AC?

Is it correct to say that in DC, the split ring commutator produces direct current flow and therefore stronger voltage, as opposed to AC which produces alternating current and therefore weaker voltage?...
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1answer
148 views

I want to know why light even moves at all.

Never mind in a vacuum, why and how does light even move at all? What propels it?
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1answer
158 views

The Electric Quadrupole

I've read the following sentence: "Every electric circuit with two pairs of accessible terminals is called a quadrupole." I was wondering why does it happen that the multipole expansion gives us a ...
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1answer
26 views

squared position dependent electric field [closed]

Here's an asignment problem that got me puzzled. The thing is that I have to solve a second order non linear differential euqation, the course is an urdergraduate mechanics course. A particle of mass ...
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6answers
1k views

why is the magnetic field circular

According to relativity, If magnetic field is just an electric field viewed from a different frame of reference, why is the magnetic field around the wire is circular?
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1answer
219 views

magnetic field due to current in a wire using Biot-Savart's law

I am learning about Biot-Savart's law to calculate the magnetic field due to the electric current in a wire. $\displaystyle\mathbf{B}=\frac{\mu_0I}{4\pi}\int\frac{d\mathbf{s}\times\mathbf{\hat{r}}}{r^...
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4answers
2k views

How can I physically interpret voltage and current?

My understanding of an electrical current is that it is the flow of electrons through a material. The only magnitude I can fathom for this process is the number of electrons which are flowing. I know ...
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0answers
46 views

Does photon polarization take place without scattering processes?

Behind two perpendicular crossed polarisation filters (for visible light) the intensity of light will be zero. After installing a third filter between the two others non parallel to each of them some ...
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2answers
1k views

Does a nonzero Poynting vector mean that there is propagation of energy?

I don't know how this "paradox" can be solved. I'm given the following system: A permanent magnet with a magnetic field given by ($\hat{a}$ are unit vectors in the x and y directions) $$\vec{H}=H_0\...
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1answer
141 views

When (or what is the meaning of) $I \propto E^2$?

For a monochromatic plane wave: $$\mathbf E = \mathbf E _0e^{i(\mathbf k \cdot \mathbf r -\omega t)},\qquad \mathbf H = \dfrac{\mathbf B}{\mu _0}= \mathbf H_0e^{i(\mathbf k \cdot \mathbf r-\omega t)},$...
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5answers
4k views

Can you put a magnetic ball into a hollow magnetic sphere?

if all magnets have to have two poles(one north one south), is it possible to construct a hollow sphere where the inside face of the sphere was one pole, and the outside face another pole? is it also ...
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3answers
540 views

How can be effect of electric and magnetic fields on the motion of a particle be distinguished?

It's a conceptual question about magnetic field: Suppose a charged particle moves under the influence of an electric and a magnetic field, how do we distinguish the effect of the fields on the motion ...
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0answers
39 views

Where can I learn about Motors and Generators?

` Where can I learn about the functionality of motors and generators, so I can answer questions such as these? I need a good textbook that starts from the basics (eg faradays) then moves up with lots ...
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3answers
79 views

movement of particles in electric field

I am confused about a homework problem. Let's assume we have two electrically charged particles of which we know the charge and mass respectively. Let's say that at first they are fixed at some ...
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1answer
263 views

Basic question about Magnetism and Right hand rule? [closed]

In our physics class we had a brief introduction to magnetism - the very basics, but I don't understand the right hand grip rule. Here is the question I'm stuck on: "a negatively charged object ...
2
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1answer
178 views

How to build a MOKE (magneto-optic Kerr effect) magnetometer at home?

I'd like to build a magnetometer at home. The type does not matter actually that much but it should be doable at home. The MOKE (magneto-optic Kerr effect) magnetometer at least appears to use ...
3
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1answer
135 views

Biot-Savart Law Clarification

I have a question concerning Biot-Savart's law. It pertains to problems where you must find the total magnetic field at some point P, $x$ meters away from a current-carrying wire. I am a bit confused, ...
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1answer
823 views

Electrical eddy current visualization or simulation

Eddy currents are induced in a metal plate when it experiences a changing magnetic flux. Is there a realistic visualization or simulation of eddy currents available? The only picture I found, on ...
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0answers
26 views

Galerkin-type weak formulation for electrokinetics

I am currently working on finite element simulations about electrokinetics. My solver (getdp) accepts directly galerkin-type weak formulation of equations. I am thus trying to write my equations in ...
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0answers
45 views

Error in radius squared

I am performing an experiment to determine the ratio e/m using a Helmholtz coil. I am using a chi squared minimization routine to find e/m. The data is supposed to fit, $$r^2=kV$$ Where k is ...
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1answer
129 views

Why does magnetic flux linkage become negative when a coil is turned through 180degrees?

Taking a rectangular coil, in a constant magnetic field, it is understandable that BANcos(theta) should apply for the magnitude of the flux linkage. However if the coil is turned through 180 degrees ...
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0answers
43 views

What happens to the emf generated due to a change in magnetic flux in a coil?

When we move a magnet into a coil of wire (open circuit) and then out of it, we create a emf across the ends of the coil and another emf in the opposite direction. My question is what would happen to ...
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0answers
22 views

Why must the deviations from equilibrium be small for zero difference in energy in this solenoid?

A long solenoid with a square cross-section is partially immersed in a magnetic, non-conductive liquid of permeability $\mu$. The solenoid's axis is parallel to the liquid's surface. Task: ...
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1answer
86 views

How to calculate the Hamiltonian from the Lagrangian for a non-relativistic charged point particle in an EM field?

I was given the equation of the Lagrangian: \begin{equation} L~=~\frac{1}{2}m \dot{x}^2+\frac{e}{c}\vec{\dot{x}}\cdot \vec{A}(\vec{x},t)-e\phi (\vec{x},t). \end{equation} I proceeded to use the ...
4
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3answers
282 views

Magnetic field in a wire with constant current

I assume I have a wire parellel to the $z$ axis and with radius $R$. A constant current $I$ flows through it in the $z$ direction. I want to know the magnetic field inside the wire at distance $r<R$...
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1answer
78 views

Need help understanding a simple circuit in-depth

I would like to know more in detail what's really going on in a simple circuit like this. I think I understand it, but I have a lot of things mixed up and others aren't clear to me. I'm going to write ...
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1answer
91 views

Explaining induced emf: directions of lorentz force and electrostatic field in two reference frames

First question, what is the direction of electrostatic field? Here is the problem: Consider a conductor rod CD moving at a speed v in a magnetic field. The three are in right angles to each other. In ...
4
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1answer
4k views

How much power would it take to stop a bullet with a magnet?

If a 7.5 gram copper-jacketed lead bullet (say, a 9x19mm Parabellum) was travelling at 360 m/s, how much power would it take to diamagnetically stop it in the space of one meter? This question comes ...
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4answers
8k views

Does a magnetic field arise from a moving charge or from its spin, or both?

I learned that a moving charge creates a magnetic field perpendicular to its direction of motion. I also learned that charged particles like electrons have spin and they also create a magnetic field ...
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6answers
10k views

How does a spinning electron produce a magnetic field?

I learned in my undergraduate physics class that atoms have magnetic fields produced by the orbit of electrons and the spin of electrons. I understand how an orbit can induce a magnetic field because ...
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1answer
179 views

Dielectric constant of Transition Metal Dichalcogenides (TMD)

Why is the in-plane dielectric constant of transition metal dichalcogenides larger than the out-of-plane dielectric constant? Is this because of the spacing between monolayers of the TMD? Why do these ...
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1answer
68 views

Arcing through an inductor

When a current is passed through an inductor coil, potential difference across it is given as $$V=L\frac{dI}{dt} \, .$$ Now, if we suddenly disconnect the coil from the circuit, the current will ...
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1answer
110 views

Magnetic Field Along the Axis of the Current Ring - Alternative way to compute

This is Example 5.6 in Griffith's Introduction to Electrodynamics (4th Edition): Find the magnetic field a distance $z$ above the centre of a circular loop of radius $\ R$, which carries a steady ...
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1answer
408 views

Biot-Savart and Amperes law inconsistentcy

Imagine the situation below where a steady current $I$ goes through the line. The radius of the semicircle is $r$. How do I compute the magnetic flux in the point $P$, marked by a black dot? ...
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1answer
55 views

Field strength is always closed?

In electromagnetism, we can write the homogenous Maxwell equations succinctly as $$dF=0$$ Where $F=E\wedge dt+B$ is the field-strength 2-form. However, it is a well-known fact that the field-strength ...
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0answers
33 views

Does a superconducting coil need a minimum amount of electric current to be a superconducting magnet?

I'm not about that for a tiny amount of Cooper electrons perhaps we can't measure the magnetic field. I suppose that it needs some influental electric current below which the coil isn't a superfluid ...
0
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2answers
346 views

Origin of the Lorentz force from the point view of relativity?

I'm a physicist, when I'm working on the quantum spin hall effect, I recollected the high-school knowledge on Lorentz force and try to explain the origin of it, but didn't get it in the first glance. ...
0
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1answer
343 views

Magnetic field due to sheet current [closed]

Suppose we have current flowing along very long thin sheet with current per unit length as $\lambda$ so what would would be magnetic field at distance $r$ from this sheet? Using Ampere Law $$\int_C^{}...
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1answer
22 views

Magnetic field sensor readings varying depending on polarity

I was playing around with one of these Hall effect magnetic field sensors when I noticed something I thought was strange. I held a cylindrical magnet several centimeters from the tip of the sensor and ...
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1answer
534 views

Potential Difference and Electric field due to changing magnetic fields

Suppose above Frame is kept in time dependent magnetic field $B=\frac{B_0t}{T}$ where $T,B_0$ are constant , then What will be potential difference between Point $O , A$ and $A , B$ . I know that ...
3
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1answer
124 views

What does it physically mean to take the curl of the curl of a field (wave equation derivation)?

What does it physically mean to take the curl of the curl of a field in the derivation of the electromagnetic wave equation from Maxwell's equations, as presented here, on Wikipedia? Why was it a ...
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2answers
1k views

Does electric field cause changing magnetic field also?

According to Faraday's law, changing magnetic field causes eletric field (imprecise wording, but generally accurate). My question is, can Faraday's law be interpreted in opposite way - that is, ...
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0answers
84 views

Weizsäcker–Williams approximation

I'm having some trouble understanding the Weizsäcker-Williams approximation What I think it is is the following: I have a charged particle at high speed, close to the speed of light, at this speed ...
2
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1answer
64 views

Ampere's law, do I include the electric field causing the current?

Let's say I have a long, straight wire with a time varying current, $I$ through it. Now if I take a circular Amperian path around this loop wire (and concentric with it) there is both a current $I$ ...
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3answers
1k views

The propagation of electric field

In case of a charged particle which is travelling at a uniform velocity, the electric field due to it at a given point doesn't change instantaneously . The reason for this delay in change of electric ...
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2answers
320 views

How does electromotive force (emf) influence electrostatic force in a circuit?

I am learning about emf and I am using university physics of Hugh D. Young which states that when the emf source is not part of a circuit the non-electrostatic force of the source moves charge form ...