# Tagged Questions

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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### Why does the magnetic field of the induced current oppose the change in the external magnetic field? [closed]

Why does the magnetic field of the induced current oppose the change in the external magnetic field? please help me to solve this Link:http://d2vlcm61l7u1fs.cloudfront.net/media%2F4dc%2F4dc03c3b-...
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### How is there not an infinite regress in the emf of a motor?

Say we have a coil connected to a battery in a uniform magnetic field, perpendicular to the magnetic moment of the coil. This is a simple motor. There is a torque on the coil that varies with the ...
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### How much and what kind of sunlight passes through a cloud?

I've read that 80% of sunlight is passing through clouds. Now there a small clouds and big clouds so it should be some average. Is this true or is the amount of sunlight passing throug a cloud really ...
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### Find the electric field of a uniformly polarized sphere

The problem statement, all variables and given/known data We want to calculate the field of a uniformly polarized sphere of radius=R Relevant equations V(\vec{r}) = \frac{1}{4 \pi\epsilon_{0}} \...
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### 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
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### 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 ...
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### 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....
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### How did physicists know that there are two kind of charges?

Problems The question I am about to make is either too stupid or hasn't bothered anyone because its obvious because I can't really find the answer anywhere. I am currently studying electricity and ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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, ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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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### 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 ...
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### 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, ...
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### 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/...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### Is energy required to preserve information on a disk?

I was just wondering if the large files I have on my PC, which have remained there for years, take up some energy to remain preserved. Just curious. Maybe a silly question after all, but I would like ...
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### What is magnetic levitation? Can it be done with electromagnets?

I have done a bit of a research on magnetic levitation and maglev trains but I fail to understand the phenomena. I also saw some videos on YouTube , they were about superconductors and magnets. I ...
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### Create spinning / modulating plasma effect without spinning the whole appartus

I'm trying to create a variable spinning / modulating plasma effect without turning the whole apparatus as seen in the video. https://www.youtube.com/watch?v=Wis_HEa3X84 Is it possible to just use ...
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### 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 ...
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### 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 ...
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### Charged pendulum oscillations

Recently, a question was asked in our school, regarding the calculation of the time period of a charged pendulum bob suspended in an electric field with the help of an insulating thread(gravity also ...
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### Why does the Microwave disturb the Internet Signal?

I have no back round in physics but was wondering how come every time the microwave is turned on, the internet slows down dramatically?