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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266 views

Gravititonal fields compared to electromagnetic fields - are they infinite in range?

me and my friend has a discussion last night, and he argued that both an electromagnetic field and gravititonal field are infinite in their area of effect, but with diminishing effects as you get ...
6
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4answers
8k views

Derivation of self-inductance of a long wire

Currently I am stuck, trying to derive the self-inductance of a long wire. According to literature it should be $$L=\frac{\mu_r\mu_0l}{8\pi}$$ and in literature its derived by looking at the energy ...
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0answers
112 views

Show that the electric field E in the system at rest is $E=\frac{Q}{4 \pi \epsilon_0 \sqrt{(x^2+y^2+z^2)^3}} (x,y,z)$ [closed]

A body point charge $Q$ moves in relation to the reference system $\Sigma$ according to the law of motion $x(t)=v_0 t$, $y(t)=0$, $z(t)=0$.
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2answers
3k views

How do I calculate the power consumed by a lightbulb?

I'm studying a lightbulb and its variable resistance, given by the expression: $R(T) = Ro[1 + α(T-T_0)]$, where $R_0$ is the resistance of the lamp at $T_0$. In this case, $R$ is not given by Ohm's ...
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2answers
231 views

How to apply Andreev reflection formalism to ferromagnet ,normal metal interface?

The traditional formalism for andreev reflection deals with what happens at normal metal, super conductor interface.http://en.wikipedia.org/wiki/Andreev_reflection (i.e when an electron from normal ...
4
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3answers
278 views

How does capacitance work?

I have a circuit whit a AC source a capacitor and a resistance all in series. I find that the difference of potential between the capacitor leads begin to change after some instants as it should. But ...
5
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1answer
447 views

What happens when two smooth surfaces touch?

I am wondering what will be the physics to explain how two neutral, chemically nonreactive objects stick. I know that using van der Waals formalism, we can treat neutral body electrodynamic forces and ...
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3answers
2k views

How to make a small tokamak?

$\require{mhchem}$I made a fusor once, like the easy science project: deuterium-deuterium ones, but they're really inefficient. I was wondering if it would be possible to make a small tokamak; not one ...
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1answer
1k views

What are the characteristics of the magnetic field surrounding a human brain?

The human brain is said to produce a magnetic field resulting from the action potentials released inside the brain. What's the nature of such a field in terms of size and strength, and what is the ...
2
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1answer
413 views

Does magnetised material lose weight?

I've had argument on a forum about this. People think, that: if the same object is magnetised, it has less energy. so by $E = mc^2$ it weighs less than an object that is not magnetised (hence spin of ...
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1answer
307 views

The acceleration of a particle in uniform electro-magnetic field

A particle of charge $q$ and mass $m_0$ is subject to the action of a uniform electro-magnetic field $\vec{E}=(0,E,0), \vec{B}=(0,0,B)$ and at $t = 0$ moves with velocity $\vec{v}=(v_1,v_2,v_3)$. ...
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0answers
60 views

Degenerated Anderson Model Simulation

I'm trying to simulate the degenerative Anderson model. So depending on an energy difference first orbital and afterwards spin magnetism occurs. First i try to solve an easier ansatz with a limitation ...
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3answers
5k views

Why and how exactly is electric motor torque limited?

Inspired by this question and specifically this answer to it. From my experience there's always some very specific limit to how much torque an electric motor can output. For example, an electric ...
2
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1answer
286 views

Using the Scalar Electrostatic Potential to Calculate Transition Probabilities

transition probabilites of atomic systems prone to some time-varying electromagnetic field are very often calculated using perturbation theory leading to expressions including the vector potential ...
3
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4answers
347 views

existence of other forces obeying inv square law

Is there any restriction in what we know of physics to the existence of other type of forces that obey the inverse square law in 3 dimensions. I mean other than electromagnetic and gravitational. ...
5
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2answers
144 views

Photon absorption

[sorry, this way below the level of this forum -- flames are most welcome] When a photon is absorbed by a piece of matter that does not reflect it -- where does the photon "go"? Eg, one shines light ...
5
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1answer
121 views

Sum of angular momentum of all electrons in a magnet

Can the sum of angular momentum of all rotating electrons in all the aligned atoms in a permanent magnet have a significant contribution to the macro angular momentum of the magnet? If yes, why does ...
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2answers
175 views

Optimum magnet layout for tripping a coil sensor

If I had say a dozen or so small bar magnets and I wanted to used them to trip the traffic light sensors (which are a coil embedded in the road where a vehicle would stop for the lights), what would ...
4
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1answer
303 views

Electric potential energy in curved space-time

In flat space-time the electric potential energy between two charges is $\frac{k Q_1 Q_2}{r_{12}}$, where $Q$'s are charges and $r_{12}$ is the distance between them. What would happen if the two ...
3
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3answers
158 views

Distant bodies emitting photons

This comes from a discussion forum, where a friend of mine asked the following: We can see objects in space billion of light years away, right? I started wondering about that. If you take 2 ...
2
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2answers
303 views

Electron gun - electron in cylindrical anode

In an electron gun, the heating filament heats the cathode, releasing electrons by thermionic emission. I've read that "electrons are negatively charged particles and the positively charged ...
3
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2answers
11k views

How do you magnetize an object?

How can I go about magnetizing something? Specifically, a whole bunch of BB's. I want to try this so that I could make a cheaper version of the Bucky Balls product. Even if I can't replicate Bucky ...
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2answers
2k views

How does the electric motor and generator vary?

what is the key difference between the windings of the electric motor and electric generator?
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3answers
337 views

Field created by varying Gravitational field

Changing Electric Field causes Magnetic filed and changing Magnetic Field causes Electric Field. Is there anything similar in relation to Gravitational Field? What sort of field is created by varying ...
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0answers
1k views

What properties of certain plastics make them x-ray transparent? [closed]

I was blown away by the abundance of good info here, and I thought maybe you could help me out with this query. I am in the process of developing a product, and I need to find a clear plastic resin ...
3
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2answers
502 views

Experimental evidence showing the kinetic energy of an electron changes in a static non-uniform magnetic field?

In a previous question, Does a magnetic field do work on an intrinsic magnetic dipole?, one highly rated answer suggested that static magnetic fields do work on intrinsic magnetic dipoles in a ...
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5answers
29k views

Is aluminium magnetic?

From high school, I remember that Aluminium has 13 electrons and thus has an unpaired electron in the 3p shell. This should make Aluminium magnetic. However, the wiki page of Aluminium says its ...
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4answers
3k views

Can heat be transfered via magnetic field in a vacuum?

Say you want to store hot coffee in a container surrounded by a vacuum. To remove all sources of conductive energy loss the container is suspended in the vacuum by a magnetic field and does not have a ...
4
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3answers
957 views

How can I physically interpret voltage and current?

My understanding of an electrical current is that it is the flow of electrons through a material. The only magnitude I can fathom for this process is the number of electrons which are flowing. I know ...
24
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6answers
5k views

Does a magnetic field do work on an intrinsic magnetic dipole?

When you release a magnetic dipole in a nonuniform magnetic field, it will accelerate. I understand that for current loops (and other such macroscopic objects) the magnetic moment comes from moving ...
3
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5answers
588 views

Particles, waves and parallel wire filters. Transmission formula?

If I think of a photon as a particle, I think a parallel wire filter should transmit proportionally to the uncovered area. (and reflect proportionally to the covered area). Obviously polarization ...
8
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2answers
391 views

Quantizing EM field

Why when we quantize EM field, whe quantize the vector potential $A^\mu$ obtaining vectorial particles (photons) like the elastic field (phonons) and we can't quantize directly the EM-field tensor ...
10
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6answers
2k views

Why don't waves erase out each other when looking onto a wall?

If I stand exactly in front of a colorful wall, I imagine the light waves they emit, and they receive should randomly double or erase out each other. So as a result, I imagine I should see a weird ...
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1answer
374 views

Light travels a straight line write down the principle in which this is manitested [closed]

Light traveles a straight line write down the principle in which this is manifested.
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2answers
4k views

How does reflection work?

In Newton's model of light as being composed of particles, it's easy to imagine reflection as being the rebounding of individual corpuscles off a surface. However, since light can also behave like a ...
0
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1answer
195 views

My computer turns on when it “sees” me [closed]

Before asking, I swear that, for how it may seem strange, this is all true. Very briefly, when my pc is off and I am near it, it turns on. I do not touch anything. Nobody else have the same effect. I ...
3
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1answer
659 views

Measuring the vacuum permittivity

So I was reading the EF experiment that's used at the MIT to measure the vacuum permittivity and I was thinking about trying it just to see how it works: ...
2
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1answer
248 views

What stochastic process is associated with spontaneous electromagnetic radiation?

Imagine one have an ideal sensor, which can convert the emission to some kinds of signal (typically, voltage, and suppose no noise at all), then what process can describe the measure data? Is it ...
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2answers
149 views

A question on a system of particles governed by laws of gravity and electromagnetic field

Consider a system of many point particles each having a certain mass and electric charge and certain initial velocity. This system is completely governed by the laws of gravitation and electromagnetic ...
2
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1answer
320 views

Are inductance and self-inductance synonyms?

Wikipedia mentions that the word self in the word "self-inductance" is to differentiate it from "mutual inductance". But it does not state whether the two things are the same thing. So do the both ...
6
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0answers
278 views

1-form formulation of quantized electromagnetism

In a perpetual round of reformulations, I've put quantized electromagnetism into a 1-form notation. I'm looking for references that do anything similar, both to avoid reinventing the wheel and perhaps ...
2
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1answer
372 views

What will happen when we cool the permanent magnet?

When we are cooling the permanent magnet what property of magnet changes? normally the superconductivity of magnets occurs at critical temperature(negative degrees).It is having a long life period and ...
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0answers
516 views

Can we increase the magnetic flux for a permanent magnet?

When we pass current to the permanent magnet does its magnetic flux, magnetic property of attraction and repulsion increases? if possible Is there any other way for increasing the magnetic flux?
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2answers
701 views

What is the difference between north and south magnetically?

Is there a reason that magnetic field lines are drawn from north to south or is this a purely arbitrarily defined protocol (for convenience)? Essentially what I am asking is: without a reference ...
3
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5answers
4k views

Figuring out North and South Magnetic Poles for Earth

I currently make my living as an electrophysicist, so am a bit embarassed at having not thought this one through, before. Magnetically, the North pole is.... North. We get out our compass, and wait ...
3
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3answers
358 views

Singularity-free stationary electro-vacuum solution

Let's say we have a spherically symmetric fluid: $$ T^{\alpha \beta} = \begin{bmatrix} \rho & 0 & 0 & 0 \\ 0 & p & 0 & 0 \\ 0 & 0 & p & 0 \\ 0 ...
3
votes
4answers
6k views

Does a magnetic field arise from a moving charge or from its spin, or both?

I learned that a moving charge creates a magnetic field perpendicular to its direction of motion. I also learned that charged particles like electrons have spin and they also create a magnetic field ...
0
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1answer
518 views

How is a magnetic field translated into physical force?

Related to this question Where do magnets get the energy to repel? If I have a magnet repelling another, eg one in my hand, the other being pushed along the desk, how do the each of the magnet's ...
2
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2answers
304 views

Phase Accumulation of Hankel-waves upon propagation

Hankel functions are solutions to the scalar Helmholtz-equation $$\Delta\psi + k_e^2\psi = 0$$ in cylindrical and spherical geometry (with respect to a separated angular dependence). Thus, they are ...
3
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1answer
2k views

Moving conductors in magnetic fields: is there electric field or not?

this is my first question on PhysicsSE (I'm already an user of MathSE). I'm a mathematics students trying to understand Faraday's law, that is $$\varepsilon= -\frac{d \Phi_B}{dt}$$ where ...