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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26 views

Imaginary component in capacitive reactance

This is a trivial question in first year but, unfortunately, this popped up in an elementary yet compulsory lab experiment. The capacitive reactance is defined as $$X_{c}=\frac{1}{\omega c}$$ The ...
5
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1answer
769 views

Magnetic Levitation : Confusion with Lenz's Law

Considering the above diagram, I am failing to see why the aluminium ring floats upwards rather than oscillates. I understand Faraday's law, but I am struggling with Lenz's law - as there is an ...
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4answers
159 views

How can we prove charge invariance under Lorentz Transformation?

We have gravitational force between two massive particles and we have electromagnetic force between two charged particles. When special relativity suggests that mass is not an invariant quantity why ...
2
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2answers
129 views

Reference frequency

In my experiment, I have to obtain a relation between magnetic field of a solenoid and the applied Vp-p. The setup is as follows. http://www.teachspin.com/instruments/faraday/ Does my choice of ...
3
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2answers
251 views

Why is infrared radiation associated with heat?

I am little confused with infrared radiation. I understand that when an object is hotter, it radiates electromagnetic waves of a bigger frequency and this waves are also more energetic, that is why ...
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2answers
2k views

Why is a paramagnetic material attracted into a magnetic field?

I want to start by saying I've seen this topic: Attraction and repulsion of Magnetic materials and its supposed duplicates and it hasn't helped me very much. In the book Introduction to ...
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1answer
699 views

How does the magnetic field get induced in car's tire?

I was amazed to see how my car's tire started attracting the nuts and bolts, how in the world did it induce the magnetic field? Never heard of such phenomena!!!
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1answer
24 views

C60 Macro Ojects

What would happen if you could make a ball or hollowed geometric shaped say the size of a basketball made out of C60.. due to its superconductivity could it have exotic or anti- effects on gravity and ...
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7answers
4k views

Do Maxwell's Equations overdetermine the electric and magnetic fields?

Maxwell's equations specify two vector and two scalar (differential) equations. That implies 8 components in the equations. But between vector fields $\vec{E}=(E_x,E_y,E_z)$ and ...
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0answers
18 views

Clarification of why pressure must be constant at a boundary between two fluids/gases?

In section 13.2.1 of these notes on acoustic waves it says that the pressure must be continuous at the boundary of two fluids/gases in equilibrium. If they were not it says the following - If the ...
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1answer
34 views

Period of oscillation of magnet levitated over another magnet

The situation is similar to what we used to do as kids, take a vertical wood dowel, with a ring magnet placed at the bottom, and another ring magnet opposing it, floating on top. More precisely, it ...
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2answers
213 views

Dielectric boundary

I am trying to determine why electric field may be confined to a certain region if there is a large difference in the permitivity for example if electric field flows through water and then reaches a ...
10
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5answers
4k views

Why does a ceiling fan start up slowly? [closed]

I think it probably has something to do with the capacitor inside but I don't get it why doesn't it just Start spinning instantly when we push the button why does it slowly start to spin and gradually ...
10
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3answers
423 views

Why are electrons treated classically in cyclotron measurements?

As I understand , systems having large angular momenta relative to the planck constant (limit of large quantum numbers, e.g. $J/\hbar \to \infty$), can be treated as classical systems. Now in the case ...
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0answers
31 views

Why should we use good conductors as electrodes of a capacitor?

I was asked if and why why should the electrodes of a capacitor should be good electrical conductors. It seemed obvious to me that if I want to form a Helmholtz double layer (because that's what ...
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0answers
21 views

Plot current-voltage curve

I have been given the following question as part of an investigation for my Applied Maths course: "[...] Then construct the current-voltage curve, i.e. the dependence of $\left< \dot{\theta} ...
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2answers
824 views

Why is the “canonical momentum” for the Dirac equation not defined in terms of the “gauge covariant derivative”?

The canonical momentum is always used to add an EM field to the Schrödinger/Pauli/Dirac equations. Why does one not use the gauge covariant derivative? As far as I can see, the difference is a factor ...
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2answers
1k views

How to get the magnetic field strength in space near a solenoid

I am trying to find the magnetic field strength along the axis of a solenoid. If I know the strength of the field at the center of the solenoid, and I know the distance from the center of the ...
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1answer
38 views

How can current remain same when potential difference decreases?

According to Ohm's law, $$\text{potential difference} = \text{resistance}\times\text{current}$$ Also, current remains the same everywhere. Now when current passes through a bulb, the electric ...
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1answer
17 views

Reflection coefficient of surface with sources

While solving the maxwell equation for surface where both surface charge and current density are present to calculate its reflection coefficient, i.e. $$\nabla \times E=-\frac{\partial}{\partial ...
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1answer
37 views

Can the magnetic field of the earth be utilized in a manmade craft? [closed]

What if there was a magnetic ring around the craft on a gimbal that could freely rotate from positive to negative, then could the craft utilize the magnetic field of the earth without flipping the ...
1
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1answer
115 views

Do magnets also attract super conductors?

If a superconductor can be repelled by a magnet (or, well, locked in a certain position by the magnet) can a magnet be attracted to a superconductor. What would happen if you take a peace of metal, ...
9
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1answer
243 views

How can I find the position of an image charge when the boundary is parabolic or hyperbolic?

If the position of some charge $Q$ is known, the boundary condition is $u=0$ on some parabolic surface, and we know the image charge has its electric volume of $Q'$, then how can I determine the ...
2
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1answer
144 views

polarization of a transverse wave travelling in ionosphere with polarization direction perpendicular to earths magnetic field

Assume a transverse electromagnetic wave entering ionosphere such that its Electric field of wave is perpendicular to earths magnetic field. Now, i read that as it will enter plasma, the wave will ...
4
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1answer
328 views

Boundary conditions on current carrying wire

I'm trying to simulate by finite elements method Maxwell equations for a current carrying wire. My 3d geometry consists of a cylinder and a box containing it. I will use a mixed formulation and ...
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0answers
24 views

Would a spinning carbon steel (high magnetic permeability) sphere become magnetized?

Would a spinning carbon steel (high magnetic permeability) sphere "falling" through Earth's magnetic field become magnetized? Would a spinning, evenly charged sphere generate a magnetic field?
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1answer
370 views

Force of a solenoid on a ferromagnetic material

As I was trying to find an answer to another problem, I was informed that the equation I was using would not work. The equation I was using was $$F = (NI)^2\mu_0\frac{\text{Area}}{2g^2}$$ If this ...
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2answers
693 views

What happens when you pass a current through a coil made of a ferromagnetic substance?

I know when you pass a current through a length of copper coil or a solenoid, there is the induction of a magnetic flux. But what of the coil made of the ferromagnetic material, the permanent type ...
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1answer
118 views

Does a proton oscillating at RF produce an EM field with a -E vector

to clarify - FACT: oscillating charges of sufficient frequency produce a propagating EM field. All communications is based on electron oscillation. Question: IF a proton was oscillated at RF would ...
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0answers
21 views

Effective resistance of a plate with varying conductance

I'm trying to find the resistance across a plate made up of different materials. Do my formulas and method make sense - will they yield the correct answer? Reference I'm using: ...
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1answer
202 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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0answers
16 views

Wigner-Eckart theorem and Van Vleck paramagnetism

Using the Wigner-Eckart theorem, we can express the matrix elements of Langevin's paramagnetic Hamiltonian $L_z + g_S S_z$ using only the quantum numbers of the total angular momentum, $J$ and $m_J$, ...
0
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1answer
25 views

I want to use the complex susceptibility to prove that E dot J is independent of the real part

As stated above, I want to use $\chi$=$\chi'$-i$\chi''$ to prove that E$\cdot$J is related to $\chi''$ and independent of $\chi'$. I need to do this using a monochromatic field ...
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0answers
16 views

Relation between relative dielectric constant, complex permittivty and complex conductivity

To complete Maxwell's equations in a natural medium we need a set of constitutive relations: $$ \vec{D} = \epsilon_0 \vec{E} + \vec{P} $$ with $\vec{D}$ the electric displacement, $\vec{E}$ the ...
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1answer
38 views

Relation between the electromagnetic wave and quantum wavefunction

I have been thinking about this for a while. I think I misunderstood something about the basics of quantum waves. Let's look at light diffracted in conditions similar to the double slit experiment. ...
4
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1answer
22 views

Characterisation of passivity

I'm a mathematician trying to work on some problems involving metamaterials. I have some trouble to understand the notion of passive media. Informally, this means that (without sources), a medium ...
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2answers
35 views

Electromagnetism on a compass [closed]

A current $2\ \mathrm A$ flows in a vertical wire. The value of the horizontal component of earths magnetic field in the region is $2*10^{-5}\ \mathrm T$. A small compass is placed $5\ \mathrm{cm}$ ...
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0answers
15 views

compass with current in a wire [duplicate]

How would you calculate the angle of deflection of a compass placed near an electric current? would you use b fields and the earths bfield to work out the arc length of the compass' movement? thanks
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2answers
42 views

Quantization of the Hamiltonian of a particle in a uniform magnetic field

If a particle of mass $m$ and charge $q$ is subject to a uniform magnetic field and if we have a vector potential $\mathbf{A}$ then we know that classically the dynamics of the particle will be ...
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2answers
266 views

What kinds of behavioural anomalies can a zero-field-cooled (ZFC) / field-cooled (FC) split indicate?

If a material shows a spiltting in the ZFC and FC curves, is it necessarily superparamagnetic, or could there be any other reason for the irreversibility? I have heard spin glasses also show ZFC-FC ...
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4answers
2k views

Apparent violation of Newton's 3rd law and the conservation of angular momentum for a pair of charged particles interacting magnetically

Consider a system of the two identical point positive charges situated in free space (isolated from influence of any other external fields) as shown in the figure below. Particle 1 is at $(a,a,0)$ and ...
0
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1answer
507 views

Why do we always consider the area of inner solenoid while calculating the mutual inductance of two coaxial solenoids?

While we know that flux linked to an area is the dot product of the area vector of the loop(and not the actual area encountering the field lines) and the magnetic field linked to it, why then do we ...
2
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2answers
212 views

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

Consider the following simple Maxwell's equations: $$ \nabla\cdot\mathrm{D}=\rho $$ $$ \nabla\times\mathrm{E}+i\omega\mathrm{B}=0 $$ $$ \nabla\cdot\mathrm{B}=0 $$ $$ ...
3
votes
2answers
27 views

Current loop and direction ambiguity of the magnetic moment

Consider a circular loop in the XY-plane which carries a current $I$. Then it behaves as a magnetic dipole with moment $\textbf{m}=I\int d\textbf{S}$ where $\int d\textbf{S}$ is the area of the loop ...
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0answers
25 views

For two electron that moving parallel,can we using this equation for Electric Field and Magnetic Field?

i suppose we have two electron and they are moving parallel,please see picture: my question is can we use this this equation for it? Griffiths' Introduction to electrodynamics): $$\vec E(\vec ...
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3answers
135 views

How to find the parts across which emf is induced?

Is there a general rule that will allow one to find the parts where the emf is induced? Consider for example the following two cases: Case 1: A rod is made to move normally to the magnetic field at ...
0
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3answers
27 views

Flux - Scalar Multiplication in Integral?

No textbook and website seems to answer this so here is my question: When we have a scalar flux: I understand that you take the scalar product of the vectors. And I understand the need for using an ...
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2answers
1k views

Meaning of boundary condition for steady current density?

Although I understand the derivation of boundary condition in case of steady electric current but I did not understand, that the electric field which is in direction of $J$ current density that is ...
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0answers
19 views

What is a simple model to find the jump height for the Thomson's Jumping Ring experiment?

I'm trying to devise a basic formula that could give a rough estimate for the jump height reached in the Thomson's Jumping Ring experiment, as detailed here. However, it's pretty difficult as most ...
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2answers
656 views

How is the current flow perpendicular to the wire?

This answer gives a great explanation of how surface charge builds up to force the current to move perpendicular to the wire: http://physics.stackexchange.com/a/102936/41086 However, it fails to ...