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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12 views

Origin of the magnetic field of Earth

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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1answer
35 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
37 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
60 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 ...
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1answer
31 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 ...
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0answers
19 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 ...
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0answers
10 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 ...
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0answers
23 views

Magnetic field transformation 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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1answer
94 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
36 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 ...
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1answer
40 views

use of earth magnetic field [on hold]

If we use earth magnetic field as a big magnet and take earth opposite charged electromagnet than it is possible that earth opposite charged electromagnet repel from earth surface.
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1answer
14 views

Electric vs permanent magnets for alternator (turbine generator) [on hold]

Here's what I know about rotating electric generators (alternators?): they spin permanent magnets on rotor alongside stationary wire coils or vice versa (sync vs async designs) produce 1 or 3 phase ...
3
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0answers
49 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\...
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2answers
56 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
39 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
22 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 ...
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1answer
50 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
63 views

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

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 ...
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2answers
40 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
78 views

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

"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
54 views

Clarifications on Ampere's Law [on hold]

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 ...
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2answers
63 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
55 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
45 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?
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0answers
17 views

Landau Diamagnetism and Pauli paramagnetism

In the conventional definition of Landau's diamagnetism we ignore the effect of spin-electron coupling and vice versa for the derivation of Pauli's paramagnetism. I want to know what would happen if ...
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0answers
29 views

Transformer primary and secondary coil current problem [on hold]

A step up transformer has 300 turns on its primary coil and 90,000 turns on its secondary coil. The potential difference of the generator to which the primary circuit is attached if 60 V. The ...
3
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1answer
41 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
47 views

What is a magnetic line of force? [on hold]

Do magnetic limes of force 'flow'? If so, what is it that is flowing? What is the 'line of force' composed of?
3
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0answers
43 views

Lagrangian of classical electromagnetism without $A_{\mu}$ field [duplicate]

Is there a Lagrangian reproducing Maxwell's equations without the use of the scalar and vector potential? Obviously $\mathcal{L} = -\frac14F_{\mu \nu}F^{\mu \nu}$ doesn't work since in terms of $E$ ...
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1answer
32 views

How can a sinusoid be a steady current? [duplicate]

As far as I understand it, a steady/stationary/constant current is defined to have $dJ/dt=0$ (i.e., no explicit time dependence). So I would say that sinusoids cannot produce steady currents, yet ...
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0answers
42 views

What one would observe on extremely high voltages?

I am interested which physical processes generate the highest electrical voltage and what the highest measured voltage is. In everyday life, electrostatic charges e.g. while walking with socks on a ...
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0answers
26 views

Newton's third law, weak and strong law of action and reaction [duplicate]

In the case of a system of moving charges forces between charges is obtained by Biot-Savart law. But it violates both action and reaction law. How can we prove that?
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1answer
35 views

Laser as an optical oscillator

How can Laser can be compared to a positive feedback amplifier? Does the two conditions for oscillator holds good for lasers too.
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1answer
74 views

How to find the magnetic field due to a revolving electron of hydrogen atom in first orbit

So, I was thinking about the Bohr model of atom and I started to wonder how we could find the magnetic field due to a revolving electron (produced at the location of proton) of hydrogen atom in first ...
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0answers
21 views

Circuit with capacitor and battery- why same voltage?

If we connect a capacitor and a battery in the same circuit, what makes the charge move until the voltage on the capacitor is the same as voltage on battery?
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3answers
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 ...
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1answer
41 views

Intuition differential ampere's law

Ampere's differential law states that - $$\nabla \times {\bf B} = \frac{4 \pi \, {\bf J}}{c}$$ I know to derive amperes integral form from special relativity, and to use stokes theorem in order to ...
2
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0answers
38 views

Consistency of atomic magnetic dipole spatial orientation with Stern-Gerlach picture of electrons

I've been recently trying to understand the concept of paramagnetism, but I feel like I'm running into 2 conflicting models. Stern–Gerlach seems to suggest that electron spins always point up ...
2
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1answer
34 views

How to properly apply Ampere's Law in this case?(Square Loop)

Suppose ABCD is square loop (conducting). There is a wire carrying a current $I$ into the plane of paper within the region of loop. So how to properly apply Ampere's Law in this case? The closed ...
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0answers
51 views

Does gravitational radiation have a formalism similar to Wheeler-Feynman electrodynamics?

Binary systems radiate energy away in gravitational waves as the orbits of the two masses spiral in towards each other. My understanding of gravity is that we think of it as a mediator of particle-...
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0answers
27 views

What difference does it make if the turns in a tesla coil overlap?

Would a tesla coil in which the primary coil is made by overlapping the turns of the wire work?
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1answer
55 views

Electricial impedance - why does it work?

I learn electricity from Purcell's book. Analyzing AC circuits is easier with complex numbers. I understand you have to replace the regular representations with those containing complex numbers ...
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0answers
29 views

research and development guidance [closed]

Ramaswami writes about a powerful electromagnet that he has made. He mentions that the copper wire needs to be thick. I have questions regarding the electromagnet he has made. How thick is the copper ...
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1answer
32 views

main cause of self charging of unshorted capacitors?

In the lab, we keep all of the high voltage capacitors shorted when not in use, and rightfully so. They tend to spontaneously charge when left for some time and become extremely dangerous. while the ...
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0answers
31 views

Plane wave propagation [closed]

Can anyone help me please understand how this is done? and whats epsilon ? many thanks
0
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2answers
35 views

Electric field of uniformly charged plane

Which are the null components (in a Cartesian coordinate system) of the Electric field of uniformly charged plane in $x=0$? I consider the coordinates : $(x,y,z)$ I think that $E_x = 0$, $E_y \ne 0$, ...
0
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1answer
38 views

Possible reasons of electrocution in a good earthed appliance during sudden power off [closed]

I was working in a industry. When I was using a machine, I felt slight electrical shock at the exact moment when the power supply went off. The machine is properly earthed. In spite of that I got ...
0
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0answers
24 views

Why soft iron 'plates' are used to hold the bar magnet pairs that are available in shops? [closed]

I know that retentivity and coercive force for hysteresis are smaller for soft iron, than steel. But still, only soft irons are found to be stuck together with the pair of bar magnets when unboxed. ...
0
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2answers
35 views

Need for pulsed laser [closed]

Pulsed laser emit light in non continuous mode.How this makes it superior to cw laser? Why there is need in reducing the time-lag between the pluses of pulsed laser.
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0answers
39 views

Confusion in the derivation of the potential of a magnetic shell

I am reading an old book on electromagnetism (THE MATHEMATICAL THEORY OF ELECTRICITY AND MAGNETISM) and I have some confusion in the following pages: First let me clarify what a "magnetic shell" ...