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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On the atomic level how do permanent magnets work? [duplicate]

How do permanent magnets work on the atomic level? Is there current inside this type magnet? How do electromagnets work on the atomic level? Is there a current inside this type magnet?
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1answer
86 views

Why don't we consider the electromagnetic field in the equations of Bohr's theory of atoms? [duplicate]

hi,i know these, my mean that the magnitude magnetic field B formule that i find here my question is that we consider Coulomb force in Bohr's theory but don't consider the electromagnetic field? An ...
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0answers
37 views

equation for total inductance of a filament in composite conductor

here in book after equation 4.38 total flux linkage for filament is divided by current through it. Though i think the flux linkage due to other on that filament should be divide separately by their ...
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2answers
212 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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4answers
512 views

What is an electromagnetic pulse?

Is an electromagnetic pulse a plain electromagnetic wave with one peak?
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1answer
55 views

Current density from amperes law

I wanted to find the current density ${\bf J}$ in a wire that gives rise to a magnetic field ${\bf B}$ along the closed curve that encloses the area of some disk with radius $r$ with normal vector ...
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0answers
208 views

What is problematic about the Moving magnet and conductor problem?

The problem is posed as follows: There is a conductor and a magnet in relative motion. This motion induces emf in the conductor. The value of the induced emf is independent of whether it's the ...
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3answers
6k views

How do I calculate the power consumed by a lightbulb?

I'm studying a lightbulb and its variable resistance, given by the expression: $R(T) = Ro[1 + α(T-T_0)]$, where $R_0$ is the resistance of the lamp at $T_0$. In this case, $R$ is not given by Ohm's ...
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5answers
2k views

Resistance between two points on a conducting surface

Suppose we have a cylindrical resistor, with resistance given by $R=\rho\cdot l/(\pi r^2)$ Let $d$ be the distance between two points in the interior of the resistor and let $r\gg d\gg l$. Ie. it is ...
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1answer
77 views

What is the physical explanation of Fresnel's equation?

When EM radiation is travelling between the media of different refractive indices, parts of it are reflected and transmitted according to Fresnel's equations. The amount of reflectivity and ...
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1answer
185 views

Constant magnetic field attenuation by µ-metal (mu-metal)

I am interested in the magnetic field attenuation by µ-metal. Specifically shielding from earth's magnetic field $\boldsymbol{H}_E$ through closed cylindrical layers of such a metal, yielding an ...
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0answers
381 views

What is “spin-orbit torque?”

I am trying really hard to understand the concept of spin-orbit torque. It is a new-ish discovery in the field of spintronics and has many applications for magnetic devices. The information that has ...
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2answers
350 views

Can metal rods detect underground wires?

Until not long ago, I was working in the drilling and infrastructure business. Some people I was working with used metal rods as a "quick and dirty" method of detecting underground wiring, before ...
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2answers
67 views

Why does the displacment vector change in cavity?

Consider the situation where we have a dielectric, in which there is an electric field $\vec E_0$ we then hollow out a cavity in the dielectric. These notes: Physics 217 Lecture Notes: Electricity ...
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1answer
39 views

What is happening from electro-magnetic point of view when a proton and an anti-proton annihilate?

Purely electro-magnetically before a proton (+) and an anti-proton (-) annihilate each other /assume they do/, they form a dipole. What happens after that? In other words, what does an external ...
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31 views

Does an electron's orbital speed affect the potential for a bond to form or break?

Many experiments and text books confirm that (in the contemporary fashion) the orbit of an electron will speed up or slow down due to the orientation of a magnetic field. "Effect of a Magnetic Field ...
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1answer
656 views

How could two current flow in the opposite direction?

Could someone explain what the green underlined part means? It doesn't make sense to me. It says the current (induced) flows CCW (that means towards the right). Then it says it flows to the left?
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0answers
85 views

Electric Potential for concentric spheres and charged surface

A few conceptual questions about a charged sphere inside an initially neutral spherical shell and about uniform E for a charged plane. (A) How would I go about calculating potential at a certain point ...
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0answers
26 views

Formation of vortices in superconductors

So I'm trying to understand how mixed state of a type II superconductor becomes the energetically favourable state. I've been through a simple approximate calculation considering a S-N boundary to ...
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1answer
54 views

Why does a coil retain a magnetic field after unplugging the power?

I remember that if you connect a coil with AC and then turn off the current, the coil will retain a magnetic field for a (short?) period of time. How or why does it retain the field after unplugging? ...
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16 views

Sign Of Magnetic Flux :Clarification Needed- YouTube Physics Video

In this problem on youtube why is the flux taken as positive even in the rotated position of the coil.The area vector is outward while the magnetic field is inward.Shouldnt it be negative in the ...
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1answer
67 views

Can an electric field help generate a current?

What i know or am guessing:- A moving/changing magnetic field creates an electric field. So if we take a rotating magnet near a coil of copper wire, it will induce a current right. So it means an ...
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1answer
47 views

Are permanent magnets depleted when they induce electric currents in conductors by movement

Are permanent magnets depleted when they induce electric currents in conductors by movement. If a permanent magnet is moved so that it induces a current in an electrical conductor that is connected ...
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0answers
101 views

Magnetic Field on Curved wire

by using the Biot-Savart law we can easily find the magnetic field at the center of the semicircle, where the straight wire will not contribute any field on the semicircle. But what If we want ...
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49 views
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30 views

Fields with source property != influenced property?

As far as I know, a field (gravitational, electric, magnetic) is created by something (mass, charge, moving charge) and can exert a force on something (a mass, charge, moving charge). Is there a ...
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1answer
155 views

Does a laser beam converge and diverge without lens?

In the case of $TEM_{00}$ mode laser produces gaussian beam. I read in wikipedia that it converges till some point called 'beam waist'and then it diverges to infinity. If there is a lens placed along ...
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2answers
98 views

Sphere of uniform charge density with a cavity problem

Suppose we have a sphere of radius $R$ with a uniform charge density $\rho$ that has a cavity of radius $R/2$, the surface of which touches the outer surface of the sphere. The question was to ...
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1answer
141 views

Region of most and least intense magnetic field

It's a unmagnetized iron screw placed in the north pole of a U shaped magnet. I believe the region of least intense magnetic field is at the far left of the board. From what I understand the screw ...
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1answer
20 views

What will happen to reactance of an inductor with some value, if a permanent magnet is brought near it ?

Change in value of inductor : Change in value of reactance : Formula to calculate it theoritically :
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3answers
73 views

How to evaluate energy conservation for magnetization?

A permanent magnet can attract a certain magnetic material mass, giving a certain amount of energy $U_{mag}$. If we first use this magnet to magnetize $n$ ferromagnetic pieces, we would have $n$ ...
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3answers
2k views

Saturation of an iron core?

Lets assume we have a 1 kg iron core, and a 100 kg iron core. Now saturation is defined as how much that core can absorb the magnetic field, since they are different sizes, don't they saturate at ...
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1answer
80 views

Why can't we see better with an antenna?

Or alternatively: Why don't antennas look like telescopes? If visible light and radio transmissions are the same thing, why is there such a difference in the equipment we use for augmenting it?
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3answers
97 views

Why do electrons have to fall on the nucleus in the Rutherford atomic model?

As I read on Wikipedia, the Rutherford atomic model is not correct according to classical electrodynamics, as it states that electron must radiate electromagnetic waves, lose energy and fall onto the ...
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3answers
5k views

Physical meaning of the energy density of an electrostatic field

I understand the physical meaning of electrostatic energy of a system of charges (or a distribution with given density) as the energy stored in the system while working to carry the charges from ...
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2answers
45 views

Faraday's law and potential along a closed curve

$$\int(\nabla \times \vec{E}) \cdot d\vec{S} = \oint \vec{E}\cdot d\vec{l} = -\frac{\mathrm{d} \phi_b}{\mathrm{d} t}$$ The second expression is the potential difference along a closed curve. Isn't ...
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1answer
57 views

Finding the limit for $\theta$

To find out the magnetic field for infinite wire, we can have an equation $$B= \frac{\mu_0 I}{4\pi s}(\sin\theta_2 - \sin\theta_1) $$ But how the limit of theta could be $\theta_1 = -\pi/2$ ...
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27 views

How does a dynamic electric field change a semiconductors carrier mobility?

Can an electric field that is spatially and/or temporally dynamic affect the carrier mobility in or on a semiconductor and, if so, how and to what degree? I appreciate any help or thoughts you have ...
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1answer
42 views

magnetic field strength paradox-where is my error in reasoning?

Under the assumption that I understood the following circumstances correct: Magnetism occurs when a charged particle moves with a certain velocity through space. Faster velocity equals a stronger ...
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0answers
38 views

Analytic solution for AC current density in a wire? (skin effect)

As I was reading about skin effect , I realized that the skin depth $\delta$ is defined assuming a plane wave incidence with a flat media and then the surface current is given by: ...
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2answers
67 views

What are the consequences of the angular momentum of the electron in an atom?

Well, since the electron is going around the nucleus, it must have an angular momentum (plus its spin). And since it is charged it must create an electric field. If I have understood it correctly it ...
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5answers
2k views

How does electricity produce magnetisim? [duplicate]

I am in class 10 and I have just read the electromagnetism chapter. They always keep on saying that electricity produces magnetism! but how!? We know electrons move, but how do they produce something ...
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0answers
294 views

Hamiltonian for Electron in Magnetic Field with Symmetric Gauge in Polar Coordinates

I am new on the board and have a question about how to write the Hamiltonian for an electron in a magnetic field rotating at a fixed radius. I would like to write the hamiltonian using the symmetric ...
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0answers
36 views

Eddy current brake with superconductor

There are eddy current brakes used with metal plates to stop trains. It converts kinetic energy of the metal plate to heat via eddy current on it. So what if we have a superconducting metal plate and ...
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1answer
47 views

Demonstration for the existence of longitudinal electrostatic oscillations

How could I demonstrate that in a linear, homogeneous and isotropic medium without losses but electrically charged, Maxwell's equation admit as solutions longitudinal electromagnetic waves, beyond ...
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1answer
93 views

Feynman propagator for arbitrary values of the gauge parameter $\zeta$

For the choice $\zeta = 1$ the Lagrangian can be brought into a particularly simple form upon integration by parts in the action integral. Equation$$\mathcal{L}' = -{1\over4}F_{\mu\nu}F^{\mu\nu} - ...
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1answer
66 views

What is the effect of circularly polarized light on a charged particle?

The force on an electron due to electric field is given by $F=qE$. For a circularly polarized light, $E=E_x+iE_y$. So $F=q(E_x+iE_y)$ which means particle travels in a circle. As there is magnetic ...
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0answers
35 views

Bar magnet moving perpendicular to solenoid

Suppose i have a solenoid of finite length and i take a bar magnet and move the magnet's North pole along the solenoid. The magnet end/pole is essentially against the outside of the solenoid forming a ...
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1answer
38 views

Relaxation time for moving charges in a medium

This will be a short question: How could I show that the quotient between permittivity and conductivity $\left(\frac{\varepsilon}{\sigma_c}\right)$ is the relaxation time for moving charges in a ...
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1answer
42 views

What is general strategy to find the resistance of medium?

What is general strategy to find the resistance of medium? These are some of the examples Two metal balls of the same radius $a$ are located in a homogeneous poorly conducting medium with ...