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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How the lagrangian density is found?

In Classical Mechanics one usually considers the Lagrangian as $L = K - U$ where $K$ is the kinetic energy of the system and $U$ is the potential energy. One then gets the Euler-Lagrange equations and ...
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1answer
24 views

Far Field Diffraction of EM waves: what does the zero frequency signify?

If you have a system of independently radiating electrons/point-charges, the far field distribution of the EM waves can be approximated by the fraunhoffer diffraction integral, or simply by the ...
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5 views

why field cooling is necessary in Exchange bias?

Why is field cooling necessary for exchange bias? For instance, if we take an example of Ni/NiO, in this sample NiO is already anti-ferromagnetic in nature at room temperature. And as we know that ...
3
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1answer
79 views

Motion in a Paul trap: $2n$th harmonic with larger amplitude than $n$th harmonic

Using a Paul trap, we captured the motion of a light charged particle (based on a rotating potential applied by AC current). Our rotational frequency was 50 Hz, and so when used FFT on the data, we ...
2
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1answer
66 views

What is the intensity of this light?

I am struggling with a derivation that calculates the cross sections for Mie scattering and since the incident light is considered to be a x-polarized plane wave I thought that we would have $$I_i = ...
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1answer
58 views

Electric field intensity of spherical shell (with cut out cap)

Consider a charged spherical shell of radius $R$ and surface charge density $\sigma$. Choose a point on the surface of the shell and cut a spherical cap of radius $a \ll R$. What is the electric field ...
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2answers
325 views

Electrostatics/ magnetostatics: why is $\int_{all space} d\vec r \; \nabla \cdot(\vec A \times \vec B)$ equal to 0?

I'm reading electrodynamics notes and come across that: $$\int_{all space} d\vec r \; \nabla \cdot(\vec A \times \vec B)=0$$ in case of magnetostatics and : $$\int_{all space} d\vec r \; \nabla ...
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2answers
31 views

EMF in an Open circuit?

If a wire that is connected to a circuit passes a magnetic field, it would induce $EMF$ that would allow current to flow, and generate an opposing force. Now what if the wire is not connected to a ...
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2answers
56 views

Work producing current = energy stored in the magnetic field?

It is stated that "the formula for the energy stored in the magnetic field is: $$E = \left(\frac{1}{2}\right)(LI)^2$$ and the energy stored in the magnetic field is equal to the work done to produce ...
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18 views

Can energy be harnessed and collected safely from a magnetic/electric field within a tube with the following scenario?

Scenario: Is it possible to safely collect energy from the following: Friction of a metallic object moving around a circular tube that is filled with water Inside the tube, magnetic forces are at ...
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45 views

Can a magnet or a magnetic field push gravity? [closed]

I have been asking around at my school and at the high school and at EWU but no one can answer this question: can a magnet or a magnetic field push gravity?
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1answer
53 views

Magnetization force on domains after alignment?

When a domain is aligned to an exterior uniform magnetic field. There are no forces acting on it? Magnetization force = 0, and all other force?
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1answer
237 views

Deriving the group velocity of a wave produced by some basic cosine waves with unequal amplitudes

Consider some basic cosine waves of the form ${E_i} = {E_0}\cos ({\omega _i}t - {k_i}z)$ with different amplitudes, frequencies and phases. We know a combination of such waves could result in a wave ...
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1answer
49 views

How fast does it take to create a magnetic field in a solenoid?

If a solenoid/electromagnet has current flowing, it creates a magnetic field. Electricity is very very fast, I believe close to the speed of light? So, when power is given to a solenoid with $n$ ...
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2answers
40 views

Magnetic field by a solenoid?

I am really not able to understand why is it that the current,while finding the magnetic field in the solenoid (ideal one), is not I the current flowing throught the solenoid but nI,where n is the ...
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0answers
24 views

Derivation of Boundary Conditions

Source: http://my.ece.ucsb.edu/York/Bobsclass/201C/Handouts/Chap1.pdf (page 6). I am trying to make sense of the derivation on the right side of these two integrals. The first one which says ...
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2answers
900 views

Noticing that Newtonian gravity and electrostatics are equivalent, is there also a relationship between the general relativity and electrodynamics?

In classical mechanics, we had Newton's law of gravity $F \propto \frac{Mm}{r^2}$. Because of this, all laws of classical electrostatics applied to classical gravity if we assumed that all charges ...
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318 views

Geometric interpretation of Electromagnetism

For gravity, we have General Relativity, which is a geometric theory for gravitation. Is there a similar analog for Electromagnetism?
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113 views

Classical electrodynamics formulated in terms of forces

The Newton's law of universal gravitation is described in terms of a force, which is produced by an action at a distance. It also can be described using the concept of a field, and that would be an ...
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2answers
132 views

Do electromagnetic fields gravitate?

It's well known that electromagnetic fields contains energy but do they gravitate ? When we talk about the composition of the universe it's now accepted that the 74 % is dark energy , the 22 % is ...
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0answers
29 views

Photomagnetic effect

I just saw an article on wikipedia: http://en.wikipedia.org/wiki/Photomagnetic_effect "This article appears to contain unverifiable speculation and unjustified claims. Information must be verifiable ...
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3answers
419 views

Do we need waves for fields?

I was pondering about EM Waves and fields and felt that there is an inconsistency in the physical picture of EM waves that I have in my mind. For example let us consider a charge at rest . Now lets ...
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1answer
190 views

How to calculate flux density in a toroidal CT that has an electrical line pass through the center?

I have seen the Biot-Savart equation but I don't know how to apply it. Also does the conductor length need to be considered finite and equal to the core length or will the calculation be for infinite ...
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3answers
37 views

What is the purpose of the factor $N/L$ in the magnetic field of a solenoid?

I am supposed to find the expression of the magnetic field on the axis of a solenoid of radius $a$, length $L$, and with $N$ the number of turns. So in order to find $dB$ (to integrate over dx), I ...
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9answers
4k views

Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?

As an exercise I sat down and derived the magnetic field produced by moving charges for a few contrived situations. I started out with Coulomb's Law and Special Relativity. For example, I derived the ...
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Is it possible to project a magnetic field at a location in space?

A magnetic field strength drops-off quickly as the distance from a magnet increases. Is there any way to use electromagnetic fields to create a magnetic field at a location. For example, if there are ...
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74 views

Is the existence of electromagnetic standing waves dependent on the observers reference frame?

If I take two plane EM waves travelling in opposite direction e.g. $E = E_0 \sin(kx-\omega t)$ and $E=E_o \sin (kx + \omega t)$, they sum to give a standing wave with a time-averaged Poynting vector ...
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2answers
95 views

Can the $\vec H$-field have non-zero divergence? Does an $\vec H$-field monopole exist?

We know that, $\vec \nabla \cdot \vec B=0$ but $\vec \nabla \cdot \vec H\neq 0$, if $\vec \nabla \cdot \vec M \neq 0$. Does it mean that, in those cases $\vec H$-field has poles although $\vec ...
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24 views

Does back emf occur in transformers?

Does back emf occur in transformers and if so does it reduce the efficiency of the transformer and can it be reduced?
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19 views

What equation should be used when this conductor passes a magnetic field?

A large thick copper wire(or bar if you like,) passes a magnetic field. What formula is best to calculate the $EMF$ here? $$\epsilon = \frac{\Delta \Phi \ }{\Delta t} $$ or, $$\epsilon = ...
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5answers
1k views

Can you put a magnetic ball into a hollow magnetic sphere?

if all magnets have to have two poles(one north one south), is it possible to construct a hollow sphere where the inside face of the sphere was one pole, and the outside face another pole? is it also ...
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0answers
18 views

A conductor passing a magnetic field?

A large thick copper wire(or bar if you like,) passes a magnetic field. What formula is best to calculate the EMF here? $$EMF = BA / Delta(t)$$ or, $$EMF = vBL$$ When the conductor passes the ...
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3answers
85 views

Explanation of Lorentz-Force

In high-school level books (for example the german standard text: "Dorn-Bader") I have often seen an explanation of the Lorentz force as on the following picture: The textbooks consider the ...
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20 views

An EM problem of polarized sphere

Today, I'm doing an EM problem and my question is exactly the same as this site shows: http://www.physicsforums.com/showthread.php?t=133501 To reiterate my confusion, why we can't use that integral ...
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3answers
45 views

question on dipole moment of water molecule

I read a line today and don't get it: "Molecules with mirror symmetry like oxygen, nitrogen, carbon dioxide, and carbon tetrachloride have no permanent dipole moments." ...
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1answer
51 views

Why does a magnetic field raise the ground state energy of an electrical particle?

I heard a statement that the ground state energy of a electrical particle in a magnetic field is larger than its ground state energy without the magnetic field. I just heard this statement. This ...
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1answer
77 views

The relation between the movement of electrons and energy

So, I've been enjoying reading a lot of helpful posts, but now, I found myself in the need of asking something. I have a hard time grasping the general concept of electricity / how the relation ...
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1answer
41 views

Lorentz force in rotating frame of reference?

This is the common problem of a charged particle moving in a static electric and magnetic field. Say $\textbf{E}=(E_x,0,0)$ and $\textbf{B}=(0,0,B_z)$. In the inertial frame of reference, the ...
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23 views

Magnetic flux and transformers

I have two linked quesitons, 1. what is magnetic flux linkage's formal definitaiton and 2. when people say the flux flows better thru a soft iron core then the air what do they mean by flux flowing ...
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2answers
228 views

Power transfer in a transformer

Can it be proved using the concept of induced emf that power supplied at the primary coil equals power consumed at the secondary. I tried following. Let primary coil be called 1 and secondary be ...
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3answers
216 views

magnetic scalar potential far above a magnetic film

The situation I am looking at is a magneto-static problem of a finite magnetic film with magnetization $\bf{M}$. I would like to find the the magnetic field far above the plate. My expectation is that ...
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2answers
86 views

Electromagnetism: Conductors

Even though the thermal velocity of electron in a conductor is comparatively high, the thermal velocity is not responsible for current flow? Why is this the case?
2
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1answer
69 views

Motional EMF and EMF?

What is the difference between motional EMF = $-vBL$ , and Faraday's law of induction $\displaystyle\mathcal{E} = \left|\frac{d\Phi_B}{dt}\right|$? Aren't they the same? What is the relation of ...
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1answer
53 views

Current induced in coil when magnet falls through it?

So I know that magnetism can be obtained from the combination of electric fields and special relativity. I am familiar with the way one can derive the magnetic field of a current carrying wire felt by ...
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1answer
54 views

The position of center of mass of electron cloud in an atom

I read Griffiths EM today and it says something very interesting but a little bothering to me. It states for an atom, the position of center of mass of an electron cloud lies in the center of the ...
4
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1answer
113 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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1answer
180 views

Magnetic field, alternated current and EMF

If I apply an alternated current to a solenoid and insert into it a smaller solenoid, I could measure the induced EMF (electromotive force) and study how it changes in relation to the frequency of ...
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1answer
79 views

Faradays law for a falling magnet

If experimentally it can be proven that the velocity of a falling magnet through a coil is proportional to the emf induced, has it also been proven that dB/dt is proportional to the emf induced and ...
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3answers
801 views

Which form of Maxwell's equations is fundamental, in integral form or differential form?

I am not sure which form of Maxwell's equations is fundamental, integral form or differential form. Imagine an ideal infinitely long solenoid. When a current is changing in time, can we detect ...
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2answers
77 views

Magnetism due to relativity?

So I have been reading in some books that magnetism does not have to be assumed a priori, but can be obtained from the electric field + special relativity. And I have seen how this leads to the common ...