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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28 views

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-...
1
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0answers
29 views

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 ...
0
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1answer
45 views

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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1answer
47 views

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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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
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24 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 ...
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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....
36
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5answers
5k views

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 ...
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 ...
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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 ...
2
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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 ...
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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 ...
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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 ...
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 ...
4
votes
1answer
111 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 ...
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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, ...
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2answers
46 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 ...
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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 ...
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 ...
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 ...
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 ...
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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, ...
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/...
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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 ...
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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
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
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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
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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
35 views

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 ...
0
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0answers
23 views

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 ...
0
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0answers
13 views

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 ...
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 ...
3
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1answer
111 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 ...
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0answers
40 views

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 ...
3
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0answers
82 views

Question on the Derivation of Lorentz Invariance of Electric Charge

On Zwiebach's "A First Course in String Theory" textbook, there is a problem 8.2 that derives the invariance of electric charge given a few assumptions: 1. Conservation of Maxwell 4-current $j = (c\...
0
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2answers
59 views

If photons mediate EM force? How can they tell to be attracting? [duplicate]

Like a photon is neutral, when a proton say feels a push from another proton, how does the proton know it was a proton pushing it and not an electron pulling it? Im not asking how the details work of ...
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3answers
47 views

Charged Particles: How is energy conserved?

I can seem to understand where the energy comes from when two particles accelerate. I tried searching through the forums and it seems like the answer is always potential energy was the source of ...
0
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0answers
25 views

Resistance increase with length because of more scattering?

I am referring to one of the answers to this old question. I can't add comment to it because I don't have enough reputation. So here I rephrase the question: does resistance increase with length ...
3
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1answer
162 views

Is Biot-Savart Law valid for time-varying currents unlike Ampere's law?

I have just finished learning the basics of magnetism, and it should be noted that I am not very familiar with Maxwell's equations. Note: In the question, when I say "Ampere's Law", I am referring ...
2
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0answers
66 views

Confusion in reaction force of Ampere's Force Law [closed]

I am reading Maxwell's "A Treatise on Electricity and Magnetism" and I have some confusion in the following pages: The element ds is resolved into its components $\alpha$ and $\beta$;and the ...
1
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2answers
45 views

Is the energy needed for a current through a straight and a coiled wire different?

When you add current to a straight piece of wire does it use less electricity than if it was coiled? Does the curve of the wire and proximity to itself make a difference in energy consumption?
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2answers
86 views

Why the reflection of randomly-polarized light is not random? [closed]

"Reflected light is polarized". The following photo demonstrates this: (source) When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore ...
2
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1answer
66 views

Clarifications on Ampere's Law [closed]

I have just learnt Ampere's Law, useful for calculating the magnetic field in situations having a high degree of symmetry. However, I have some conceptual doubts regarding it: Before I begin, I would ...
0
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2answers
75 views

Why is the magnetic force not (never) in the opposite direction

In this question I'm talking about the left-handed orientation for the direction of the $x$, $y$ and $z$-axis. This is the coordinate system at the left of the picture below. I learnt that when you ...
1
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1answer
63 views

Can we derive Biot Savart law from Coulomb's law?

Can we derive Biot Savart law from Coulomb's law by setting a current source at an instant in place of one of the charges? Let's say $dl$ is the length of the conductor having $dQ$ charge which ...
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0answers
51 views

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?
3
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1answer
47 views

A puzzler in induction and special relativity

Some reading I was doing jogged my memory about a puzzle in E&M that hit me back in my undergrad days, but I just let it go at the time and never found an answer. A pretty common undergrad E&...
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2answers
52 views

What is a magnetic line of force? [closed]

Do magnetic limes of force 'flow'? If so, what is it that is flowing? What is the 'line of force' composed of?