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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286 views

Huggins Displacement Theory and Retrocausality

I was looking at the Wikipedia entries on Time Travel and the Grandfather paradox and noticed a paragraph on the so-called Huggins Displacement Theory. I haven't been able to find the source although ...
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0answers
24 views

Vector potential $ \vec A$ of two parallel conductors

Let's say we have two parallel conductors of length $l$ parallel to the x-axis at $y=y_0$ and $y=-y_0$. Let's say the upper conductor has the current density $j$ and the lower one $-j$. Now I want ...
3
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2answers
107 views

Any quadrupole approximation? Any example?

In atomic and molecular physics we quite often encounter with electric dipole approximation. The dipole approximation we do when the wave-length of the type of electromagnetic radiation which induces, ...
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2answers
72 views

Can an electric field be there without a charged partical?

How does a photon have an electric field around itself without any charge particle inside it.
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20 views
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2answers
53 views

Do a receiving antenna interfers with an emitting antenna?

As I understand: Accelerating electrons generate electromagnetic waves. An emitting antenna have an alternating current (electrons are moving) which generates an electromagnetic waves. The ...
8
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2answers
4k views

Is it possible to kill a human with a powerful magnet?

I'm asking in terms of physics. Can powerful magnetic induction rearrange spins of my body in such way I will die? How? Or maybe it can rip all iron from me, which would make my blood cells useless? ...
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1answer
38 views

How does current increase in an LR circuit in order to reach a steady-state?

The equation of voltage of an LR circuit is given by: $$\boldsymbol{\varepsilon_0} - L\frac{dI}{dt}= RI\;.$$ Now, before the circuit is closed, $i= 0\;$ As is written in books, the current then ...
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35 views

Given magnetic field, calculate electric field

The B field depends on z and t so that $B = j B_{0}cos(kz-\omega t)$ where k is the wavenumber, $\omega$ is angular frequency and $B_{0}$ is constant. The electric field is $E = E_{x}i + E_{y}j ...
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0answers
34 views

Meaning of components of Maxwell's stress tensor $T_{ij}$

I am learning about Maxwell's stress tensor and what I understood is that the components, say $T_{ij}$ is something like a force parallel to the $j$th-direction acting on the surface with its normal ...
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0answers
24 views

magnetic field and permeability

Doubt on magnetism. The the problem with Magnetic Flux Density($B$) and Magnetic field intensity($H$). I can Imagine magnetic flux($\Phi$) lines and flux density($B$) Physically. But magnetic field ...
1
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1answer
58 views

Self induction in a circular coil

There's a formula for self inductance : $$ L=\frac{n\Phi}{i}$$ where n is the number of loops. But the book also says self inductance is directly proportional to $ n^2 $ I totally agree with the ...
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19 views

Does inductors oppose the same change in current that it itself produces in case of DC Current?

We know that the inductor is a solenoid. In that solenoid, due to self induction it produces its own emf. That causes a change in the initial current. To counter this change the inductor produces ...
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2answers
133 views

George Green's derivation of Poisson's equation

I was reading George Green's An Essay on the Application of Mathematical Analysis to the Theories of Electricity and Magnetism, and I got confused on one step in his derivation of Poisson's Equation. ...
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0answers
16 views

Are magnets attracted to mu-metal?

Are magnets attracted to mu-metal? Be aware that when watching videos of mu-metal they may have metals that are attached to the mu-metal and are attacted to magnets. Please don't get them confused.
2
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1answer
47 views

Time dependent electric field: Mathematical expansion for local electric field

In many articles and books I see that local electric field is expanded as $$\vec E_0(\vec r(t)) = \vec E_0(\vec R_0) − (\vec a(t) \cdot \nabla) \vec E_0(\vec R_0) \cos(\Omega t) + \ldots $$ For ...
1
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1answer
954 views

Interacting magnetic fields

Is there a reason why two magnetic fields perpendicular to each other do not interact? If they are parallel or at a non-90 degree angle they interact. Is it because magnetic field lines can be viwed ...
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1answer
58 views

Wightman function for massless vector fields in Coulomb gauge

I've been looking for quite some time an expression for the Wightman functions for a massless vector field in the Coulomb gauge $\nabla\cdot\mathbf{A}=0$ (I think it is equivalent to the Feynman gauge ...
0
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1answer
22 views

Why does emf of a cell equal to $\frac{F_b d}{q}\;?$

In Concepts of Physics by H.C.Verma, it is written: Some internal mechanism exerts forces on the charges of the battery material. ... We show the force on a positive charge $q$ as $\vec F_b\; .$ ...
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2answers
80 views

Why can the divergence of vector potential be anything?

Purcell in his book was deriving the vector potential $\bf A$ using $\text{curl}\;(\text{curl}\; \mathbf A)= \mu_0 \mathbf J\; .$ After some algebra, he came to this: $$-\frac{\partial^2 ...
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1answer
61 views

Is there a way to write the Lorentz force in terms of one field, $L$, and one charge, $X$?

I have heard that physicists like to write electromagnetism as one force (the Lorentz force) and define it as $\vec{F_L}\left(q, \vec{v}, \vec{E}, \vec{B}\right) = q\left(\vec{E} + \vec{v} \times ...
2
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0answers
23 views

Why are only some materials ferromagnetic? [duplicate]

I've been told in my solid state class, that ferromagnetism occurs when electron spins are alligned. This happens, as I understand it, when it is "energitically favorable" and when the wavefunction is ...
0
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4answers
119 views

Why does the frequency of a Transformer not change?

I was reading about the transformer, when I came across a note by the author that stated that the Frequency is not changed as the flux linked to the secondary coil in a transformer changes. Is there ...
0
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1answer
47 views

Does the alternating current “move” such that there can be Lorentz force present on the wire?

As in question, does the alternating current "move" such that there can be Lorentz force present the wire? Becuase the current alternates on a fixed place there can be no Lorentz force?
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18 views

ECG depolarization potential

On page 12 of 'ECG Interpretation: From Pathophysiology to Clinical Application' there is a diagram showing the potential difference between the terminals of different leads of an ECG machine when a ...
4
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1answer
47 views

Force on an electric or magnetic dipole

The formulae for the force on an electric dipole and the force on a magnetic dipole seem to be closely related: $$\mathbf{F}_{e} = (\mathbf{p} \cdot \mathbf{\nabla}) \mathbf{E}, $$ $$ \mathbf{F}_{m} ...
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2answers
113 views

What happens when a field turns on or off?

Short Setup I am curious about the the mechanics of fields, whether electromagnetic, gravitational, etc. So as a specific example in order to simplify (hopefully) how to ask this question, consider ...
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1answer
93 views

What is the current of a single charged particle moving at a constant velocity?

Disregarding quantum and relativistic effects what is the current of a single electrically charged particle with charge $q$ moving at a constant velocity, $\vec{v}$? I am looking into to this to make ...
2
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1answer
486 views

Can solids behave like liquids?

Basically, the more I look at things the more it seems like there's entirely no difference between a liquid and gas, it all just depends on the relative density of what's around it. This being said, ...
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2answers
39 views

When two coils carrying opposing currents are placed coaxially

The current flowing through these coils increases in this case according to the book. But it does not clearly state how. This is what it says: "When brought closer induced effects should produce ...
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0answers
14 views

Books about non-uniform magnetisation

I need to study non-uniform magnetisation and the only book where I found this was Berkeley Physics Course Vol.2. Is there other book's options to this issue?
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0answers
39 views

Crest in Light Doppler Effect

The way it is explained in wiki Doppler effect in sound happens because waves of particles with biggest density called crest hits our eardrums more often if object is moving towards us (increased ...
0
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1answer
54 views

Can a magnetic ball “diffract” around a metal object?

Consider a magnetic ball falling through a copper pipe. It falls slower than it would if it wasn't near any copper. We can use this to determine that the closer a magnetic ball is to copper, the ...
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1answer
72 views

Does the direction of propagation of the natural light is perpendicular to the direction of electric and magnetic field making up natural light? [closed]

I know that the direction of propagation of the wave(light) is perpendicular to the direction of electric and magnetic field in the situation of plane waves. And I want to know the relation between ...
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0answers
64 views

Disc Eddy current brake Torque calculation [duplicate]

I have built a rotational Eddy brake in a small car, where I use gears to make the metal disc in the brake rotate faster or slower at will. I know that the braking torque is dependant on the rate of ...
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0answers
61 views

Magnetic hysteresis energy loss derivation

we have that the magnetic energy stored in a field is : $$W_m = \frac{1}{2}\int_v B.H dv$$ From here im unsure how to get to the expression that the power dissapated in a hysteretic cycle is given by ...
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2answers
54 views

What does it means to say an electric charged person has a higher potential voltage compared to the door?

I'm studying electric potential energy and its defined as the energy needed to move a charge inside an electric field from a point $a$ to $b$. Then, it defines electric potential energy per charge, ...
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1answer
168 views

Is the number of turns in a loop part of the magnetic flux?

In all both my physics textbooks, the number of loops $N$ is left out of the equation for magnetic flux $\phi_B = \int_s \vec B \cdotp\hat n dA$ and only when calculating the Emf induced by an ...
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2answers
188 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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0answers
39 views

What is ferromagnetism?

Simple question. As far as I've understood it; a ferromagnetic material is one where all the electron spins are alligned parallely - that is, the wavefunction has an antisymmetric spacial part and a ...
3
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4answers
1k views

Why can't a magnet change a charged particle's speed?

I know that magnetic force acts perpendicular to the direction of the original velocity, so the velocity in that original direction is unchanged, but once the magnet starts acting, the particle's ...
0
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1answer
36 views

TM modes expression for a cylindrical cavity, Jackson's book

Consider a cylindrical cavity with circular cross section of radius R. The height of the cylinder is d; the walls are considered as being a perfect conductor. I'm facing a difficulty findind the TM ...
0
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1answer
58 views

Back Emf in Motor

Why does back emf tend to decrease as the rate of doing work increases. When load increases it reduces the angular speed of motor as a result induce current due to back emf also decreases because the ...
2
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3answers
325 views

What can change a photon's frequency?

I found this question: Is it possible to apply force to a light particle? As it states, gravity can change the frequency of light by changing its momentum. My question regards other phenomena that ...
2
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0answers
94 views

Can a Set of “Maxwell's Equations” for Newtonian Gravitation be Derived from Newton's Force + Special Relativity?

When I learned about electromagnetism in my first year of undergraduate school, Maxwell's equations were derived roughly in the following way (see also here or in [1]): Gauss's law for a static ...
3
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1answer
719 views

Biot-Savart Law from linear to volume current distribution

Biot-Savart law for a linear current distribution is: $\displaystyle \vec{B}=\frac{\mu I}{4\pi}\int\frac{\vec{dl}\times \vec{r}}{r^{3}}$. In the book that my professor uses says that if we have ...
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0answers
11 views

Why does magnetic resonance frequency lower when electron density rises? [closed]

I would like to know why magnetic resonance frequency lowers when elecron density rises. It would be good if you have a formula discribing the relation and could share it.
2
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3answers
2k views

Shielding magnetic field without mu-metal

I need to shield my device from magnetic interference, including earth magnetic field (if you move device around, it might be enough to cause slight currents i guess) and magnetic field caused by ...
4
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2answers
107 views

Can wifi signal reception be improved by opening a door? [closed]

Use Case A wifi user is in a different room than the router. The computer is having a hard time connecting and receiving the wifi signal. Engineering Question Can the wifi signal from the router to ...