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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Magnetic field due to long current carrying conductor at a point near its end

How can one show that the magnetic field due to an infinitely long current carrying conductor at a point near its end is given by $B = \frac{\mu_0I}{4\pi a}$ using Ampere's Circuital Law ...
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2answers
33 views

why positive-poistive , negative-negative repel and positive-negative attract?

Why two positive charge and two negative charge repel but two positive and negative charge attracts
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20 views

Why is emf 0 for coil rotating in field?

Say you have a coil (that can carry a current) rotating in the field between two magnets. When the area of the coil is perpendicular to the field lines the emf induced in the coil is 0, why is this?
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1answer
54 views

What is the electric field inside a hollow sphere?

If you have a conducting hollow sphere with a uniform charge on its surface, then will the electric field at every point inside the shell be 0. The reason the electric field is 0 at the center is ...
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21 views

Phase added on reflection at a beam splitter?

If we have light of a particular phase that is indecent on a beam splitter, I assume the transmitted beam undergo's no phase change. But I thought that the reflected beam would undergo a phase change ...
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11 views

What subtopics should I read in order to understand practical electromagnetism? [on hold]

I apologize if this question might be not exactly on-topic here. But it seems related enough, and I found no better place to ask. I'm a student that is fairly behind in his course. I have already ...
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0answers
10 views

Density of polarisation charge is zero always for linear isotropic homogeneous dielectrics?

It seems to me that the volumetric density of polarisation charge in a linear homogeneous isotropic dielectric in an external field is always zero, but I find this rather surprising. Consider such ...
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0answers
9 views

Flux due to a charge placed just outside the vertex of a cube [duplicate]

There was a solved example in my book on Gauss's Law. The question says that there is a charge q placed at a distance d outside one of the vertices of a cube such that d tends to 0.My objective is to ...
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19 views

About the non-physicallity of the scalar derivatives of potentials in Lorenz qauge and longitudinal EM waves [on hold]

First, I'm quoting from a 13 page article: "Within the framework of Classical Electrodynamics (CED) it is common practice to choose freely an arbitrary gauge condition with respect to a gauge ...
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1answer
48 views

How long does it take a magnet to lose its magnetization? [duplicate]

As we all know, a magnet has some magnetization that lets it attract other ferromagnetic materials but I was wondering how long it takes to lose this magnetization? Is it even possible or not?
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23 views

How do I calculate the energy flux for this toroid?

My question is how do i calculate the total energy flux through the walls of the torus in this situation? The Poynting flux is given by $\vec{N} = \frac{1}{\mu} ( \vec{E} \times \vec{B})$ but the ...
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23 views

What about the Electric energy density of the toroid?

But what about $U_{E}$, the electric field energy density?
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28 views

Current Density Boundary Conditions and its Implications

According to Ohm's Law, one can say $ \overline{J} =\sigma \overline{E} $ if the field is in a conductor, and $ \overline{J} =0 $ if it's in empty space. Now if we take the surface of a conductor and ...
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1answer
43 views

Why is the Meissner effect not used for levitation in MagLev trains?

The Meissner effect does not seem to be used as primary source of levitation in MagLev trains, instead, electromagnetic and electrodynamic syspension are mainly used. Why is this? Could it be that the ...
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0answers
18 views

Are the EM waves that result from each of these processes distinguishable? Phase of light upon emission

In case 1, you have a single source of light that you pass through a diagonally oriented linear polarizer and then a half waveplate, such that the horizontal and vertical components become $\pi$ out ...
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15 views

Can a magnet lose its strength over time? Especially when repulsive magnets are around? [duplicate]

I was thinking of hanging delicate art glass pieces separated by opposing magnets on walls simply using the power of magnets. I was wondering will the opposite poles have any effect on the strength of ...
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0answers
20 views

Is it possible to focus an electromagnetic field such as an aura into a stream of pure energy? [on hold]

Is it possible to focus an electromagnetic field such as an aura into a stream of pure energy? http://www.biofieldglobal.org/what-is-human-aura.html
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1answer
31 views

Is it possible for two polarizable bodies to induce dipoles in each other in the absence of an external electric field?

If there exist two initially neutral bodies (say atoms) some distance apart, with no external electric field applied, can they induce dipoles within each other?
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3answers
50 views

Does the equation CV = q break down when V is small or a wire is very long?

I'm trying to understand how the equation $CV=q$ relating the voltage difference and charge stored on a charged capacitor breaks down under extreme circumstances. In most problem statements that I'm ...
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0answers
19 views

Magnetic moment of a paramagnetic crystal

I've been having some trouble with a paramagnetism problem from my Statistical Mechanics class textbook (F. Mandl, Statistical Physics, 2nd edition, p. 25). The problem is as follows Since the ...
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2answers
59 views

Proof that work is the same whaever the path of approach for two charges

I have a quick question on a figure that outlines a proof. There is a stationary charge $q_1$ and is kept fixed. $q_2$ is moved to the same position along two different paths at a fixed distance ...
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1answer
83 views

Real life physics problem: Why is my desktop computer affected by my plasma ball?

Note: this is strictly a physics question, not meant to be an advertisement I was running my desktop with a plasma ball on the side. The desktop has a touch screen enabled. I started to notice that ...
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2answers
56 views

Does a moving charge have additional (magnetic) energy

My understanding so far: A moving charge produces a magnetic field, B, in an analogous way to a current produces one. A magnetic field has an energy density which is proportional to B squared. My ...
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1answer
20 views

where does the term half in the formula of electrostatic potential energy comes from?(system of point charges)

Electrostatic potential energy stored in a system of point charges (from wikipedia) The electrostatic potential energy $U_E$ stored in a system of N charges q1, q2, ..., qN at positions r1, r2, ...
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3answers
131 views

Are magnetism and electricity the same thing? [on hold]

I have read at certain place electricity and magnetism are the same thing, bit in reality we see both have different properties.
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51 views

What is the origin of the “polarization catastophe”?

According to the Clausius-Mossotti relation, $$ \frac{\epsilon_r - 1}{\epsilon_r + 2}=\frac{n\alpha}{3\epsilon_0} $$ So when $n\alpha = 3\epsilon_0$, the relative permittivity $\epsilon_r$ ...
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1answer
39 views

What is the acceleration of a charged particle in uniform electric field [on hold]

I would guess its $kC/r^2$, just like the in the context of gravitation, as one arrives at the acceleration due to gravity $g = GM/r^2$; but I am not sure.
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35 views

Maxwell Equation in dispersive media Griffiths

I am wondering where is the following equation (9.162) of Griffiths Electrodynamics 4e from? $$ \nabla^2\tilde{E} = \tilde{\epsilon}\mu_0{\partial^2\tilde{E}\over\partial t} \tag{9.162}$$ I know ...
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1answer
27 views

Series magnetic circuit [on hold]

Consider the above setup: I know: $$ R=\frac{l}{\mu A} $$ Where, $R$ is the reluctance, $l$ the length of the wire, $A$ the area and $\mu$ the permeability. $$\Phi_m=\frac{F_m}{R}$$ Where, $\Phi$ ...
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1answer
28 views

Why does the inductance of a coaxial cable not depend on the number $N$ of coils?

Suppose we have a long coaxial cable like in the following figure: I want to compute the self-inductance $L$ of the cable. As I understand, given a coil with $N$ closely winded turns, through which ...
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0answers
39 views

Does Lorentz force affect the magnetic field?

When a charged particle moves thru magnetic field, it is affected by Lorentz force. But does the magnetic field surrounding the particle experience a force opposite to the particles Lorentz force ? ...
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0answers
18 views

Is polarization a meaningful notion for an EM mode in a close cavity?

For a generic close cavity, I suspect that all the modes are non-degenerate? Therefore, in each mode, at each point, the E field has a definite direction, and consequently, it is meaningless to say a ...
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1answer
39 views

How can equipotential planes differ in potential in an uniform field?

I'm trying to solve an exercise that asks me the field strength of an uniform electric field where equipotential planes differ by $1\ \mathrm{V}$ and are $2.5\ \mathrm{cm}$ apart. First of all, I ...
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0answers
24 views

How can we calculate the spherical wave vectors L,M,N [on hold]

if we have a vector wave function in spherical coordinates ae^ikz. then how can we calculate its spherical wave vectors M,Nand L
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0answers
9 views

what is the difference between mictomagnetism and spin glass?

What is the difference between mictomagnetism and spin glasses? I mean what are the distinguishing characteristics of them which makes them separate?
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0answers
30 views

How does the “computer fan with magnets” generator “work”? [duplicate]

This question is in relation to the following YouTube video: https://www.youtube.com/watch?v=jiAhiu6UqXQ In it, an electrical generator is shown consisting of a CPU fan with thin magnets attached to ...
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1answer
23 views

Induced current from moving coil through non-uniform magnetic field

A rectangular coil with dimensions $L_x$ and $W_y$ , mass $m$, $n$ turns , and resistance $R$ is moving at an initial velocity of $v_i$ when a non-uniform magnetic field pointing directly up ...
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1answer
10 views

Damping constant and length of the pendulum [closed]

Is there any relationship between damping constant and length of a mechanical pendulum and electromagnetic pendulum?
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1answer
26 views

What about a material makes it diamagnetic

For a material to be magnetic, all the magnetic atoms (atoms with half filled electron shells) need to be aligned, but what about a material gives it the property of diamagnetism?
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12 views

Can a material be diamagnetic to electromagnetic waves, such as the visible spectrum

Diamagnetism is shown to repel a magnetic field and therefore the material counterpart of that field. Can an object be diamagnetic to electromagnetic waves, and repel them? I understand that there is ...
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0answers
14 views

Electric potential of a wire [closed]

A wire having a uniform linear charge density "h" is bent into the shape of semicircle. Find the electric potential at point O.
2
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2answers
35 views

Is the photon truly not absorbed in Raman scattering?

In reading about Raman Scattering, I was thinking while reading it "okay, incident photo absorbed by molecule, molecule goes to higher energy vibrational state, molecule re-emits photon with either ...
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1answer
51 views

Why can't I measure electric potential from source to point according to this formula

The formula for electric potential of points A and B in the presence of an electric field due to a point charge where $R_a$ and $R_b$ are the distances from source to point A and B respectively is: ...
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0answers
21 views

Can magnets be the possible cause for this card spin? [closed]

so I've ran into these 3 videos of people spinning a card on their fingers. Now I know this is possible with the flick of the finger to make it spin for a few seconds but definitely not like how they ...
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1answer
42 views

If EM waves are made of oscillating electric and magnetic fields, how come photons have no charge?

I just recently learned that electromagnetic waves are made of oscillating electric and magnetic fields, and I have a question. In the standard model, photons are described as having zero charge, and ...
2
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1answer
26 views

Is this a standing or moving wave?

The electric field of a uniform plane wave traveling in a source free region of free space is given by: $ \vec E= (0.5j \vec x + \vec y)(e^{j \beta z} - e^{-j \beta z})$. Is this a traveling wave or ...
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1answer
28 views

Is the magnetic field calculated by Ampere's Law only because of the currents crossing the loop? [duplicate]

In Ampere's Law, $$\oint \vec B \cdot \mathrm d \vec l = \mu I$$ the current outside the curve is not included on the right hand side of the equation.* Does it mean that the magnetic ...
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47 views

What will be the motion of an electron kept at rest in the vicinity of a magnetic monopole?

Consider that both an electron and a magnetic monopole are kept at rest close to each other. Will this result any motion of the electron? If yes, please give a derivation of the equation of motion.
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2answers
56 views

Relation between magnetic moment and angular momentum — classic theory

How do I prove the relation between the vectors of magnetic moment $\vec\mu$ and angular momentum $\vec L$, $$\vec\mu=\gamma\vec L$$ ? Many text books and lecture notes about the principles of ...
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1answer
49 views

Two objects with mass m with charge q connected to spring [closed]

I am taking Physics 201 on Yale Open Courses. I having trouble with the 5th problem in the problem set 1, which says as follows: Two sphere of mass m and negligible size are connected to two ...