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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2answers
256 views

Motor Speed depends on voltage or number of turns

What limit the size of motor speed? Does the speed of motor depend on the voltage applied or magnetic field strenght or frequency or what?
-3
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0answers
11 views

Determining pole strength experimentally

Could anyone give me a quick explanation of what magnetic pole strength is or a website that does so? Also, how can i determine this value experimentally? Thanks in advance
2
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3answers
565 views

Helical motion of charged particle in external magnetic field

When an electron moves in uniform external magnetic field, with velocity not perpendicular, I understand that the magnitude of force is only due to the perpendicular component of velocity and is ...
0
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2answers
49 views

Gauss Theorem and Faraday's Law

I'm studying magnetic fields and I have a big problem. The Gauss theorem stats that the magnetic flux through a closed surface is null, since the line of flux enter and exit from it in the same amount....
0
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0answers
23 views

Bowl Shaped magnetic fields responsible to keep galaxies together?

David LaPoint released some videos on youtube. In part 2 he claims that because galaxies look similar to the shape of plasma created in his experiment they must be shaped also by external magnetic ...
0
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2answers
354 views

Domains of electromagnets

I know that magnetism is due to the alignment of domains of a magnetic substance. Due to this reason, an iron bar may be magnetized by stroking it with a permanent magnet such that its domains are ...
0
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2answers
45 views

Find the current enclosed in the loop

Magnetic field at point $(x,y,z)$ is given by $\overrightarrow B = \dfrac{B_0y}{\sqrt{ x^2+y^2}}\hat i - \dfrac{B_0x}{\sqrt{ x^2+y^2}}\hat j $. Find current enclosed in the loop $x^2+y^2=r^2$. My ...
9
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9answers
652 views

How do electrons know when a circuit is closed?

I was told that electrons do not begin flowing unless the circuit is closed. The electrons from the battery are not in the ends of wire when it is open, apparently, as there is no reason for them to ...
1
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1answer
66 views

is the sun going to make an electromagnetic pulse in the next 50 years?

EMP is very damaging and can cause blackouts and other bad results specially on electronic devices , so i heard that our sun is going to make an electromagnetic pulse during the next 50 years ..is ...
7
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1answer
68 views

How can a radiocontrolled watch receive waves of a couple of km long without a (large) antenna?

Some watches has the ability to make contact with DCF77. The DCF77 is a German longwave time signal and standard-frequency radio station. The highly accurate 77.5 kHz (approximately 3871 metres ...
2
votes
1answer
117 views

Maxwell's equations in covariant form

Maxwell's equations of electrodynamics in vector calculus form are \begin{align} \nabla \times \mathbf{B} - \partial_t \mathbf{E} & = \mathbf{J} \\ \nabla \cdot \mathbf{E} & = \rho \\ \nabla ...
0
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2answers
114 views

Why doesn't current induced by changes in flux affect the flux, while current induced by a battery does?

Faraday's law states $V=-\frac{\mathrm{d}F}{\mathrm{d}t}$, where $F$ is flux. Since $V=IR$ for passive circuits, $I = V/R = -\frac{1}{R}\frac{\mathrm{d}F}{\mathrm{d}t}.$ But by the Biot-Savart law, ...
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2answers
24 views

How to compute work needed to build a configuration of charges

Suppose an alignment of four charges in the vertices of a square. The first pair on one diagonal has positive charge and the other negative charge, all charges are of the same absolute value. length ...
0
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1answer
24 views

Eddy currents induced throughout the volume of the material or only the surface?

If a conductive slab has induced Eddy currents to oppose the change in magnetic flux, is the induced current uniformly distributed throughout the slab's volume or only at the surface? I might have ...
1
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1answer
244 views

How is Magnetic force on a current carrying conductor $Blb$

I was reading an answer about torque acting on a rectangular current carrying loop kept in a uniform magnitude field B. Force acting on each sides is $F_1$, $F_2$, $F_3$, $F_4$. It's written here : ...
0
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1answer
154 views

Issue with running a Van De Graaff generator

I am following this online video: http://www.youtube.com/watch?v=fsT69XnjxgU but my Van De Graaff generator is not working. I am using pvc pipes for the body, nylon and electrical tape as two rollers, ...
0
votes
1answer
491 views

Coercivity of a ferromagnetic material?

I understand that coercivity is the field/force required to demagnetize/magnetize a ferromagnetic material. What if we had two opposite magnetic fields of different strengths values H acting on the ...
1
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1answer
56 views

If a electrical current can induce a magnetic field, can the reverse be true?

From my GCSE studies, near to the end of a module, I was told that when an electrical current flows through a conductor, a magnetic field will be induced. Can a magnetic field create an electrical ...
6
votes
1answer
433 views

Biot-Savart law from Ampère's with multivariate calculus

Let us assume the validity of Ampère's circuital law $$\oint_{\gamma}\mathbf{B}\cdot d\mathbf{x}=\mu_0 I_{\text{linked}}$$where $\mathbf{B}$ is the magnetic field, $\gamma$ a closed path linking the ...
2
votes
3answers
222 views

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

In a cavity of size $L$, 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 violate ...
2
votes
1answer
138 views

Maxwell Stress Tensor at material boundaries

I am trying to grasp the meaning of the Maxwell Stress tensor $T_i^j$ at material boundaries. Concretely, I am trying to calculate the force between two waveguides. The results are given in an article ...
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1answer
36 views

Relationship between current and magnetism [closed]

If 2 pieces of iron are next to each other, there is no magnetic attraction. However if a current is run through them, they will behave as magnets. My question is is there a relationship between the ...
0
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2answers
225 views

Drift Speed and Current in Two Different Inertial Frames

We have a long, cylindrical wire carrying a constant current I in an inertial frame. At a distance of R from the center of the wire, the magnitude of magnetic field is $μI/2πR$. What is the magnitude ...
2
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0answers
40 views

Electromagnetic field transformations under rotations

If you have magnetization vector in vertical magnetic field, it starts precessing. If you do a transformation to rotating coordinate system you obtain a stationary magnetization. Is there a direct ...
4
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1answer
110 views

Why didn't Newton study electricity and magnetism?

This is a soft question about the history of physics. In Newton's time, electrical and magnetic phenomena (such as amber attracting things or the alignment of compass needles) were well known. Yet I ...
1
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3answers
109 views

Please help me with this doubt from electromagnetic induction

If motional emf and induced emf are the same, how to explain the emf induced in a conducting rod moving with a velocity perpendicular to length of rod and magnetic field(magnetic field is ...
3
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4answers
5k views

Artificial planetary magnetic field

I wonder how difficult it is to create an artificial planetary magnetic field with generators? What power they would need? The question is inspired by thinking about possible colonization of Jupiter'...
0
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0answers
36 views

How inversion layer shields electric field?

This question is somewhat relating to electronics but I think it fits better with physics. The leccture here (slide 13) shows that the gate to body capacitance (Cgb) in linear region is 0. However, ...
1
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1answer
11 views

Magnetizing a ferromagnetic material.

if i pass a DC current directly through iron or steel or something ferromagnetic, Would that magnetize that material? And would it be magnetized permanently? Another question, does anyone know what ...
2
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1answer
155 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 ...
1
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0answers
42 views

What started the magnetic field of Earth [duplicate]

The Dynamo theory gives a good explanation of the magnetic field of the Earth: the Coriolis force focus inner currents into current spirals generating magnetic(s) field(s). What I'm trying to ...
1
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1answer
107 views

Finding the force between parallel currents using the formula of magnetic pressure

Let there be two parallel current-carrying infinitely-long wires separated by $d$ carrying currents $i_1$ & $i_2$ respectively in the same direction. In order to find the force on an ...
1
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1answer
51 views

Name of electric force

during a lab experiment, i noticed that a metal ball has a much harder time rolling on metal rails when their is a current passing through it and the rails. I was wondering why and if there was a name ...
0
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0answers
26 views

Electro-magnetic fields and Faraday Cages

If you have an area that is affected by an electromagnetic field, would it be possible to build a Faraday cage and then build a device INSIDE the Faraday cage so that it would work (as opposed to ...
1
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1answer
38 views

Refractive index of plasma dependance on temperature

How does the refractive index of plasma changes with temperature? Temperature is not high enough for new ionization. Would it be like in gas p/T dependence ? http://farside.ph.utexas.edu/teaching/em/...
2
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1answer
67 views

Need clarity about relativity/magnetism explanation

Please bear with me - this is not a duplicate question.... From what limited knowledge I have gathered, it seems that when two wires in parallel are carrying equal current, the electrons moving in ...
0
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0answers
30 views

Domains in Ferromagnetism

I am currently reading the book "Introduction to electrodynamics" by David J. Griffiths. As far as I understood, a Ferromagnet when not applying external magnetic field, the magnetic dipoles inside a ...
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1answer
25 views

Could an infinite spinning windmill create endless electricity? [closed]

This is not so much a question about if this idea is physically possibly, but more a question of if it was possible. If a magnet could somehow push a windmill around at a constant speed continuously, ...
30
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4answers
4k views

What is momentum really?

Wikipedia defines momentum as in classical mechanics: In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object. However, an ...
0
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0answers
26 views

What is the difference between the Lorentz force in a cloud chamber and the repelling/attracting force between magnets?

In a cloud chamber charged particles are bend by a magnetic field where the Lorentz force is causing the bending. But when a ferromagnetic material is attracted or repelled by a magnet it is more or ...
1
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1answer
63 views

Differential equation in RL-circuit

I am self-studying electromagnetism right now (by reading University Physics 13th edition) and for some reason I always want to understand things in a crystalclear way and in depth. Now look at this ...
3
votes
1answer
110 views

How to evaluate possible values of spin of two photon system?

Photon hasn't well defined quantity such as spin. Instead of it, it is characterized by helicity $h$. Let's assume state of two photons in CM frame (with $\mathbf k$ being the momentum of one of ...
-1
votes
2answers
199 views

What would happen to a motor without a split-ring commutator?

I realise that a similar question has already been asked (Why does the coil in this apparatus reverse its direction of oscillation?), however after reading the answers, I am still unclear on certain ...
7
votes
1answer
165 views

From Liénard-Wiechert to Feynman potential expression

When studying the potential of an uniformly moving charge in vacuum, Feynman proposes to apply a Lorentz transformation on the Coulomb potential, which reads in the rest frame $ \phi'(\mathbf r',t') =...
2
votes
1answer
71 views

Why does the divergence of the Poynting vector have energy flux density?

The poynting vector is defined as $\vec{S}=\mu_{0}^{-1}\vec{E}\times \vec{B}$ Taking the divergence of the poynting vector, one arrives at $\vec{\nabla} \cdot \vec{S}=-\frac{\partial u}{\...
0
votes
1answer
105 views

Searl effect generator simulation

This is an scheme of SEG: https://www.youtube.com/watch?v=pUUvhuQtba4 https://www.youtube.com/watch?v=wAMrTxMzrds This mechanism change the electrons around the conversor. Would be really nice to ...
3
votes
1answer
61 views

Can Light Waves Be Irregular?

From what I understand, electromagnetic radiation produced by an antenna is of the frequency that corresponds to the motion of the electrons moving around in the antenna. And I assume that the ...
3
votes
1answer
78 views

Can Non-Magnetic Dust Particles become magnetic if their particle size is decreased (by grinding them probably)?

Problem: Take some dust particles whose size is big. Bring a magnet near them. They do not get attracted to the magnet and thus we conclude that they are non-magnetic. However, when you grind the ...
1
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1answer
148 views

Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
2
votes
1answer
267 views

Boltzmann Distribution of Electrons in Confining Potential

I have a particle simulation wherein many non-interacting electrons are trapped in a electric potential well. I would expect, and therefore I initialize according to this, that the electrons would ...