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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Sign convention for EMF

When we define the field generate by EMF, why there is not negative sign in $\mathcal{E} = \oint \vec{E} \cdot d\vec{l}$? Usually we talk about potential, there should be a negative sign, right?
3
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5answers
2k views

How to find the poles on a spherical magnet?

To any nonmagnet, the whole sphere is a magnet. To another spherical magnet though, there is a rough area on the surface where it is strongly repelled. Given a spherical magnet, how should the poles ...
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2answers
47 views

force on a moving charge in magnetic field

Need help in understanding the direction of magnetic force in the magnetic field!Totally confused by directions. Why is it that magnetic force is perpendicular to the direction of magnetic field and ...
3
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2answers
526 views

Effective aperture of isotropic antenna

I have always taken for granted that 'the aperture of a loss-less isotropic antenna is $\dfrac {\lambda^2} {4\pi}$'. On a whim, I tried to look up how this expression was derived, but so far I have ...
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2answers
264 views

Work done by Lorentz Force in case of motional emf

In the classical example of the slidewire generator where the rod slides on a U-shaped conductor in a magnetic field, we get a charge separation due to the Lorentz force. The way the induced emf is ...
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0answers
17 views

When a pole of magnet comes near another pole possessing the same charge does it generate electricity?

That's all there is to it. Well... I was daydreaming about making gloves that had the ability to shock people when you punched them. I thought how it would be better if they didn't need a battery and ...
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1answer
55 views

Understanding fields and their correlation to forces

I seem to be confused between the concept of a "force", and a field. Now let's assume there is a magnetic field of $1$ $\mathrm{Tesla}$, what does that mean in relation to force? Finally, if field is ...
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1answer
71 views

Why is an optical magnon with k=0 not an eigenenergy state?

I found in a paper the following explanation. Unfortunately, I can't understand it. Can anyone help me on this? In the limit of equal spins an optical magnon with k=0 gets an acoustical one at the ...
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1answer
28 views

Quantifying the current drawn by a motor at half speed

I have a h/w question regarding the current in the armature of a motor when it is operating at half it's normal speed. The question is; A motor is designed to operate at 118V and draws a current ...
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0answers
53 views

Electric field in a sphere with a cylindrical hole drilled through it

Suppose that you have a sphere of radius $R$ and uniform charge density $\rho$; a cylindrical hole with radius $a$ ($a\ll R$) is drilled through the center of the sphere, leaving it like a "necklace ...
0
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1answer
17 views

Current when the slide wire attains a constant velocity

In a slide wire generator, when the slide wire attains a final constant velocity, is the current in the circuit 0? It does not make sense if it is 0 as the area keeps increasing, allowing more flux ...
2
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3answers
104 views

Force on a point charge q inside a cavity in an uncharged conductor

This is problem 2.40 from Introduction to Electrodynamics by D. J. Griffiths: A point charge $q$ is inside a cavity (not necessarily spherical or anything similarly regular) in an uncharged ...
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0answers
86 views

Electromagnetism - Proof of the Uniqueness theorem for an external problem

In the electromagnetic Uniqueness theorem, we consider a volume $V$ enclosed by a surface $S$. It is initially assumed that two different fields are valid solutions for the Maxwell's equations with ...
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0answers
48 views

Sommerfeld radiation conditions for an electromagnetic field

There is some confusion in the definition of Sommerfeld radiation conditions for an electromagnetic field, which are related to the asymptotic behaviour of the field for a distance $r \to \infty$ ...
3
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2answers
57 views

Maxwell's Equations: Induction

What is the reason for some writing Faraday's Induction Law as $$ \nabla \times E= -\frac{1}{c}\frac{\partial B}{\partial t} $$ versus $$ \nabla \times E= -\frac{\partial B}{\partial t} ?$$
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1answer
28 views

Would electromagnetic radiation impart a pressure on a surface of neutrons?

In my physics textbook, it says that a qualitative way to envision pressure from EM waves is as follows: the electric field drives charges in the $x$ direction, and the magnetic field then exerts on ...
2
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2answers
2k views

Magnetic field in a cavity

We are given an infinitely long cylinder of radius $b$ with an empty cylinder (not coaxial) cut out of it, of radius $a$. The system carries a steady current (direction along the cylinders) of size ...
2
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1answer
103 views

Contradicting forces on a circular loop under current in magnetic field?

I have the following general conceptual concern. Think of a thin conducting loop of radius $R$ placed in the $x$-$y$-plane at $z=0$. There is a homogeneous current density $\vec{j}$ running through ...
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2answers
58 views

What is supposition of equilibrium? How do Rayleigh, Jean know the electromagnetic wave in equilibrium behave?

In a cavity of dimension L, the wave must give zero amplitude at the wall, means wave equation has zero amplitude. Why? Answer from hyperphysics "since a non-zero value would dissipate energy and ...
2
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0answers
28 views

What are the parameters for Pauli's repulsion pseudo-force?

I have found the following formula for the repulsion potential due to the overlap of the electron clouds arising from Pauli's exclusion principle: $$V = A\exp(-r/\phi)$$ where r is the distance ...
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1answer
78 views

How DC and AC relays works?

I was told long time ago that DC relay had a coil. There was a switch (2 wires, one is stable, the other one is flexible) inside the coil. The switch was parallel to the axial direction of the coil. ...
1
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1answer
119 views

Frequency of rotating coil

Given a coil initially in the x-y plane, rotating at angular frequency $ \omega $ about the x-axis in a magnetic field in the z-direction. This uniform time varying magnetic field is given by $B_z ...
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0answers
19 views

Question regarding 2 conducting spheres connected by a long fine line (electric potentional)

I'm having a bit of trouble understanding a pretty simple issue. Assuming I have 2 conducting spheres uniformly charged connected by a long fine line (as shows in the added photo) and im being asked ...
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1answer
38 views

Closed circuit integral, magnetic vector potential for straight line segment

Looking at a worked example in Chengs Field and Wave Electromagnetics (newest ed, p 236) about Biot-Savarts law. 'A conducting wire of length $2L$ is carrying the current $I$. Find $\textbf{B}$ by ...
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2answers
67 views

Why doesn't the magnetic field polarize when polarizing light?

If the magnetic field doesn't polarize does it follow the electric field path of propagation? or does it vanish?
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1answer
17 views

Back EMF & Current, how can we make I stable?

Lets assume we have a wire that has $10V$ across and $1$ $Amp$ flowing, now if this conductor is introduced to a changing magnetic field, $-EMF$ is induced, can we control our voltage to increase it ...
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2answers
56 views

Does electric field cause changing magnetic field also?

According to Faraday's law, changing magnetic field causes eletric field (imprecise wording, but generally accurate). My question is, can Faraday's law be interpreted in opposite way - that is, ...
4
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1answer
68 views

In a strong field, how long would it take for domains to align?

How fast is the rate of "switch" for the domains to align with an exterior field? Possibly in milliseconds? I assume it won't take much time since domains are very small, and they only turn from a ...
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1answer
79 views

Heating effects of microwaves

I understand the heating effect(dielectric heating) of microwaves. This heating is caused by using a frequency of 2.45 GHz and this is the same frequency at which Bluetooth works and the L and S ...
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2answers
59 views

In calculating resistivity of skin do I use the body's surface area?

Most of the resistance of the human body comes from the skin, as the interior of the body contains aqueous solutions that are good electrical conductors. For dry skin, the resistance between a ...
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2answers
53 views

Condition for the magnetic field

Let $B$ be the magnetic field. If $$\nabla \times B = 0$$ and of course $$\nabla \cdot B= 0$$ Can we conclude that $B=0$? For a general field it is wrong because every constant vector will ...
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0answers
27 views

Is this expression of electric field correct?

I know that the electric field component of electromagnetic wave should be written as the first equation shown, but some times it can be expressed in terms of complex amplitude of electric field as in ...
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0answers
28 views

Magnetic forces and generators

i'm really struggling to break this question down into manageable chunks and identify which quantities are relevant. I'm also unsure of some of the wording in the question. A bar of length ...
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2answers
110 views

How electromagnetic fields travel through vacuum?

How electromagnetic fields travel through vacuum is it right to say that they propagate or travel? do they alternately form each other by charged particles?
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3answers
4k views

Does a current carrying wire produce electric field outside?

In the modern texts of electromagnetism in the presence of stationary currents the electric field is assumed conservative $\nabla \times E =0 $. Using this we get $E_{||}^{out}=E_{||}^{in}$ which ...
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1answer
37 views

Is the charge of a current carrying wire real or hypothetical?

Realist753 Theory Relativistic Electrodynamics assert that a current carrying wire being neutral in a laboratory frame of reference appears electrostatically charged to an observer in relative ...
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1answer
144 views

Electric field from current without Maxwell's law of induction

A long, straight wire carries a current that decreases linearly with time. What is the direction of the induced electric field outside the wire? I would interpret this as follows: A current induces ...
0
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1answer
57 views

What determines the magnitude of force of an electromagnet?

Let's assume that there is an electromagnet with $B = 1 Tesla$ , what is the magnitude of the force it could generate on other dipoles equal to it's surface area? Can the magnitude of force (in ...
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4answers
2k views

How does inductor store energy in the magnetic field?

I learnt from book that magnetic field does no work because the force is proportional to $\vec{v}\times\vec{B}$ where $\vec{v}$ is the particle velocity. That vector cross product is always at right ...
4
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3answers
132 views

Integral form of Gauss's law for magnetism from Stokes' theorem?

How can the integral form of Gauss's law for magnetism be described as a version of general Stokes' theorem? How does it follow?
2
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1answer
192 views

Electromagnetic duality

A key aspect of modern physics is the mapping of theories or different descriptions of a theory into a one-to-one correspondence. As I am trying to further understand the electromagnetic field tensor, ...
0
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1answer
59 views

Thomson's jumping ring experiment

For the jumping ring experiment (I think it is also called Thomson ring experiment) why does the ring float and not move up and down the ring as would be expected, I think by Lenz's law, since would ...
0
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1answer
61 views

Force from solenoid

I'd like to approximate the force from a solenoid, or at the very least find a formula which is proportional to the force so that I can experimentally find the constant for my particular case. ...
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2answers
75 views

Lorentz force, must there be two magnets?

The diagram above demonstrates the Lorentz force, Yet I wonder... must there be two magnets/electromagnets? If the two magnets form a magnetic field of $1$ $Tesla$ acting on the wire, why not use 1 ...
0
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1answer
53 views

Calculting the magnetic field from rectangular conductor?

I want to apply the Biot-Savart law to calculate the magnetic field at a point created by current flowing through a square/rectangular conductor. More specifically, a trace on a printed circuit board. ...
0
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2answers
56 views

Is there an analogue of a geodesic for the evolution of the electromagnetic field? [duplicate]

For a charged particle moving in free space, we can say from the homogeneity of space-time, that it moves along a geodesic. Is there an analogous principle for the evolution of the electromagnetic ...
3
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2answers
68 views

Locations of destructive interference for two spherical waves

I have looked at this, but it did not help with locations. Really this just comes down to mathematical manipulation, which for some reason I fail to see. Here is my paraphrased setup: Consider two ...
0
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1answer
78 views

H field of this magnet?

How can I know the magnetization force that this magnet applies to other ferromagnets? What is H(in Oe or A/m) of this strong magnet? From the site: ...
5
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1answer
68 views

Is it possible to see light intensity fluctuate?

Solutions to Maxwell's equations shows that the $\vec{E}$ and $\vec{H}$ component are of the same phase, which means they go to maximum and zero together, therefore the intensity of the ...
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1answer
334 views

Induction and Halbach Array

I have recently studied and tried to understand the concept of the Inductrack ( http://en.wikipedia.org/wiki/Inductrack ), a form a magnetic levitation. In the following link they present three ...