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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Is it possible to detect fake Tungsten aka Wolfram gold bars with a strong magnet?

Tungsten aka Wolfram is paramagnetic so it is weakly attracted to magnets. A guy devised the following to test for Tungsten in gold bars: http://www.youtube.com/watch?v=foELQ7T8_90 But he is using ...
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652 views

Does there exist a single plate capacitor(conductor)?

Does there exist a single plate capacitor(conductor)? if yes How will you define the capacitance and potential(difference) of such conductor?
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447 views

Electrons on Stern–Gerlach experiment

My questions about spin and negative charge of electrons. Stern-Gerlach experiment is very famous in order to find spin of electron. this video created by paris-sud university really well-explained on ...
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192 views

Rubbing a rod with silk?

Rubbing a glass rod with silk causes charges to be exchanged and consequently both objects get charged. Why do the objects have to be "rubbed"? I get that one has a stronger pull on the electrons ...
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555 views

Why do waves diffract?

There have already been a lot of questions on this site on diffraction but I still believe this one might be slightly different. In electromagnetic waves, diffraction and any other phenomenon of wave ...
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333 views

Derivation of Lagrangian density for an infinite classical dielectric in interaction with the EM field

I am tasked with reading and reproducing all the steps in J.J. Hopfield's 1958 paper "Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals". Embarrassingly I am stuck ...
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68 views

Coefficients of capitance

In my syllabus about electromagnetism, they introduce the coefficients of capacitance by stating that, if we have $n$ conductors enclosed by linear dielectrics, then we can write 'because of ...
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5k views

Generator using Earths's Magnetic field?

In the past I've been shown Internet sources claiming a specific generator that uses nothing but the earth's magnetic field to generate power, I'm looking for the name of the generator? I realize ...
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526 views

Gauge theory in classical electromagnetism

I understand gauge theory as the theory of continuous transformation group which keeps Lagrangian (or dynamics) invariant. So some integral invariants could be found. In terms of classical ...
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3k views

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

How do magnets work on the atomic level? Is there current inside the magnet?
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139 views

How is momentum conserved when a magnet attracts a metal?

Suppose your have any magnetic object and no external force acts upon it, and the object comes near a metal which causes an impulse (think that will happen). However, the magnetic force is internal to ...
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4answers
6k views

Transformer: primary side & secondary side current 180 degree out of phase

I am a novice in electrical engineering. I notice that in transformer the secondary side current & current referred to as primary are 180 degree out of phase from each other. But why it is so, I ...
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169 views

Accelerated charged particles produce electromagnetic radiation, but holes (the charge carriers) do not. Is this correct?

Holes are treated as particles in solid-state physics, so I've had some trouble with reasoning through this properly.
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2k views

Electromagnetic wave propagation through two lossless dielectrics

In Elements of Electromagnetics (Sadiku, 3rd edition, Section 10.8), the author says to consider two lossless dielectric materials joined at an interface $z=0$. Here two lossless dielectric materials ...
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2k views

Direction of the Area Vector (with regards to magnetic dipole)

I'm learning about torque on a conductive coil in a magnetic field. I have been taught that $\vec\tau = \vec\mu \times \vec{B}$, where $\vec\mu$ is the magnetic dipole moment. Also, $\mu = ...
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595 views

Do magnets lose their magnetism/force when they attract/repel each other?

So yea, if two magnets attract/repel multiple times(A LOT) will they lose their magnetism? If they don't break during the whole process?
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164 views

Faraday's law in a ring

What role does the induced emf in Faraday's law play in generating current in a ring in which the magnetic flux is changing ?
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133 views

Are electrons moving? If so relative to whom? and why is there no magnetic field?

electrons on earth moving with us, due to rotation of earth, revolution of earth, sun and our galaxy right? Then, why is there no magnetic field around a piece of copper wire?
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213 views

Field energy of/from virtual Photons

I have a slightly out-of line question: Consider a single electron (or it's current if you please) The STATIC ELECTROMAGNETIC field surrounding it will (no doubt) have a field energy (T) to go with. ...
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927 views

In which of the above figures will the light bulb be glowing?

What is principle of solution behind this induction problem (problem 29)? The problem can be find in here (problem 29). Problem is: The five separate figures below involve a cylindrical magnet and ...
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371 views

So do I use this Lorentz's law or which law do I use?

I have difficulty understanding exercise 24 in this document: Two parallel wires I and II that are near each other carry currents i and 3i both in the same direction. Compare the forces that the ...
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1answer
325 views

Would someone who is only interested in string theory benefit from working out the problems in Jackson electrodynamics?

This is a soft question. I'm not sure if it is appropriate for this site. Would someone who is only interested in string theory benefit from working out the problems in Jackson electrodynamics? I ...
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4answers
177 views

physics , magnetic lines of a magnetic

Why magnetic lines comes from north to south out side of the magnet is any magnetic lines comes from south to north if so in which direction What is the reason of magnetic lineS
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699 views

Could a solar flare cause the Earth's magnetic poles to reverse?

With all the hype of the impending "2012 Mayan doomsday" I was thinking it might be interesting to see what principles of physics prevent the theories of doomsday from occurring. One overarching ...
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1answer
453 views

How is the operation of a Goldleaf Electroscope explained in terms of virtual particles?

If an electroscope is charged negatively the electrons on the leaves will repell each other and stand apart. It is clear than there is a steady force between the leaves that counters gravity. How is ...
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277 views

Gravititonal fields compared to electromagnetic fields - are they infinite in range?

me and my friend has a discussion last night, and he argued that both an electromagnetic field and gravititonal field are infinite in their area of effect, but with diminishing effects as you get ...
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4k 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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2k views

How can I tell which end of a wire will have a higher potential?

I have the following setup: A <---- wire ----> B $V_b - V_a = \Delta v = \text{a positive value}$ I have two questions: Which end of the wire has a ...
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820 views

How is electrical energy transmitted?

My original question was in an effort to understand the electrical analogy to Markov chains, which is explained in Snell's article. There are some neat parallels that involve taking a Markov chain ...
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1answer
53 views

Could a magnetic field repel electromagnetic waves?

If I understand magnetism correctly, then two magnetic fields with opposing directions would repel each other. Could this same affect happen to electromagnetic waves? And if so, could it be reproduced ...
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3answers
102 views

Is this a Lorentz-scalar? How do I tell?

I'm struggling to identify whether a scalar is a Lorentz-scalar. E.g: $$\partial_i A^i \quad i \in {1,2,3}.$$ How do I determine if this is a Lorentz-scalar or not? If got the same problem with ...
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1answer
59 views

The truest/most general Maxwell's equations in isotropic, linear, inhomogeneous media with sources

Sources use $\mu H=B$ and $\epsilon E= D$, assuming homogeneous media. Obviously if $\mu$ is space varying, $\nabla . (\mu H)$ need not be equal to $\nabla . B$ What is the most general form for ...
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2answers
74 views

Larmor Precession - What does precession actually means?

Larmor Precession - What does precession actually means? Is it change in the orientation of the axis with which electron revolves around the orbit or what. But, shouldn't the radius of the orbit ...
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2answers
79 views

Total time derivative of magnetic vector potential $A$

I am looking at this document, which tries to establish the Lagrangian of the Lorentz force. Everything is fine, but I don't see why: $$\frac{dA_i}{dt}=\frac{\partial A_i}{\partial t}+\frac{\partial ...
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34 views

How to tell apart a permanent and induced magnet which are otherwise identical?

In his educational series "Demonstrations in Physics" the late Prof. Julius Sumner Miller posed the following riddle: Given two identical bars of iron, one magnetized and one not, how can you ...
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1answer
79 views

Motional emf with rod

We know that $\mathcal{E} = -N \frac{\mathrm{d}\phi}{\mathrm{d}t}$. When we have a rod such as the one on the left moving through a constant magnetic field, how is it the case that the flux is ...
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46 views

Electron propagation

How can electrons travel in these beams if they repel?
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1answer
110 views

How does the magnetic field generated from a rectangular cross-sectional current-carrying conductor differ from a circular cross-sectional conductor?

I can find much information of cylindrical conductors (ie. regular wires), where $B=\frac{\mu_0 i}{2 \pi r}$ and $r$ represents the radius (or distance) from the centre of the conductor, however I ...
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2answers
144 views

Why particles don't lose mass when they radiate

Charged particles radiate when accelerated: in the rest frame of the particle moving with acceleration $\textbf{a}$ the amount $dW$ of radiated energy over time $dt$ is $$ dW \propto \textbf{a}^2 ...
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1answer
106 views

Why is gravity viewed as a curvature of spacetime and not the electromagnetic force? [duplicate]

There are four known forces in the universe. Two of these forces are the force of gravity and the force of electromagnetism. The first is the result of the mass of the object that has the gravity. The ...
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1answer
48 views

Signs in derivation of capacitor discharge differential equation

In deriving the discharge current for a capacitor I have seen two different approaches: By Kirchhoff's law we have: $$ \begin{align} 0 &= I R + \frac{Q}{C}\\ \implies 0 &= \dot I R + ...
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1answer
45 views

Electro-magnetic radiation

What happens when a free electron and an electromagnetic wave interacts? Does it vibrate or move along the direction of the electromagnetic wave?
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1answer
86 views

Is lasering some natural phenomenon soemwhere?

Laser is actually the simplest kind of light, from a physical point of view. Many people think it is some rocket science simply because it is not natural but takes some human efforts to create. So, ...
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1answer
148 views

Derivative of the magnetic field to the vector potential

So the magnetic field is defined with the vector potential A as: $$\mathbf{B}=\nabla\times\mathbf{A}.$$ How would I calculate the derivative: $$\frac{\delta}{\delta\mathbf{A}}|\mathbf{B}|^2$$ I ...
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1answer
102 views

What are the electromagnetic fields of a photon?

I'm looking for expressions for the electromagnetic fields (preferably $E$ and $B$) of a typical photon which is localised in space to some extent (i.e. I'm not interested in the infinite plane wave ...
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1answer
67 views

How can we define the energy stored in a (conservative) force field?

I have come to know from my textbook that energy is stored in the E-field of a capacitor, in the B-field of an inductor and so on. Take the example of an inductor. The derivation bewilders me ...
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2answers
47 views

selection of p substrate as wafer in typical cmos process flow

why is p-substrate typically used as wafer in the typical cmos process flow? why not n substrate?with respect to memories, Has it got anything to do with the aplha-paritcle radiation induced errors ...
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3answers
144 views

Do electromagnetic fields are already present all over the space? [closed]

Consider a region $R$ in space without any source of electromagnetic field. Now put a source $S$ of electromagnetic wave in the vicinity of $R$ so that at time $t=0$, $S$ starts radiating ...
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1answer
93 views

Electric force vs. Magnetic force

$E$, electric field $\to$ $H$, magnetic field $D$, electric flux density $\to$ $B$, magnetic flux density $D=\epsilon E$ $\to$ $B= \mu H$ Electric force calculation is based on electric field: ...
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54 views

Is there a material that could “convert” near infrared frequency to infrared?

I have a laser with a wavelength of 650 nm (visible red light) and was wondering if there is some sort of material that could be used to absorb and disperse a different frequency of light around (900 ...