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.

learn more… | top users | synonyms (5)

10
votes
1answer
345 views

How many atoms per light year does light encounter when traversing interstellar space?

Interstellar space is pretty empty but there a small number of of atoms (mostly hydrogen?) floating around. How many atoms per light year would a photon encounter while traversing interstellar space?
10
votes
2answers
741 views

If you run an electric current through a wire loop, do the accelerated charges radiate?

Does an accelerated charge always radiate? For example the current electrons in an electric circuit when moving through a turn they are accelerated, do they radiate because of that acceleration? If ...
10
votes
3answers
12k views

Can magnets rotate infinitely?

There are many videos on youtube in which people arranged magnets in circle and rotated one placing in middle of that circle on a shaft, and the magnet (magnet motor) starts madly and continues its ...
10
votes
3answers
5k views

Where can I find simulation software for electricity and magnets?

Is there easily-available* software to simulate coils, solenoids, and other magnetic and electromagnetic devices? I'd like to play around with some design ideas, such as Halbach arrays, but physics ...
10
votes
2answers
378 views

Forcing quadrupole moments to vanish for a neutral system

For a system of electric charges $q_i$, at positions $\mathbf{r}_i$, with a nonzero net charge $Q=\sum_i q_i$, one can define a "centre of charge" in the obvious way as $$ ...
10
votes
2answers
15k views

Effects of a very large magnetic field on the human body

Ever since reading about the NHMFL I have always wondered about this and asked several people without getting a good satisfactory answer. My question is, considering the simplest case let's say a ...
10
votes
3answers
728 views

Can the path of a charged particle under the influence of a magnetic field be considered piecewise linear?

Ordinarily we consider the path of a charged particle under the influence of a magnetic field to be curved. However, in order for the trajectory of the particle to change, it must emit a photon. ...
10
votes
8answers
4k views

Why no longitudinal electromagnetic waves?

According to wikipedia and other sources, there are no longitudinal electromagnetic waves in free space. I'm wondering why not. Consider an oscillating charged particle as a source of EM waves. Say ...
10
votes
6answers
1k views

Why aren't there compression waves in electromagnetic fields?

I just started learning about optics, and in the book I'm reading they explain how the electrical field caused by a single charged particle could be described by a series of field lines, and compare ...
10
votes
1answer
564 views

Do we have magnetic monopole? Is charge, according to yesterdays paper on Nature by Ray-Roukokoski-Kandel-Möttönen-Hall (30.01.14) quantized?

As everybody here knows, Maxwell's equation would look more beautiful if a magnetig charge were present. Beyond the aesthetics question, if a single magnetic Dirac monopole would be found, the ...
10
votes
2answers
321 views

The problem of self-force on point charges

Allow me to preface this by stating that I am a high school student interested in physics and self-studying using a variety of resources, both on- and off-line, primarily GSU's HyperPhysics website, ...
10
votes
2answers
591 views

Charge on Sphere due to Contact with Capacitor

Question: Consider a parallel plate capacitor which is connected to potential difference $V$. Let there be a small spherical conductor, assume that its radius is much smaller than the distance ...
10
votes
1answer
366 views

How the inverse square law in electrodynamics is related to photon mass?

I have read somewhere that one of the tests of the inverse square law is to assume nonzero mass for photon and then, by finding a maximum limit for it , determine a maximum possible error in ...
10
votes
5answers
349 views

Are metals more heavy due to the Earth's magnetic field?

Non-metal objects are attracted to the Earth due to gravity. So the weight of non-metal objects can be only dependent on their mass. On the other hand metals can be attracted to the Earth's magnetic ...
10
votes
2answers
906 views

Is it possible to levitate a toy maglev train using only permanent magnets as long as the train stays in motion?

I am fascinated with magnets, and specifically the idea of magnetic levitation. I recently purchased a Levitron toy maglev train and have enjoyed finding the perfect sweet spot at which it levitates, ...
10
votes
1answer
289 views

Nuclear Magnetic Resonance (NMR) Conceptual Questions

Let $M$ be the magnetic moment of a system. Below are the Bloch equations, including the relaxation terms. $$\frac{\partial M_x}{\partial t}=({\bf M} \times \gamma {\bf H_0})_x-\frac{M_x}{T_2} $$ ...
10
votes
0answers
240 views

What different approximations yield Gravitoelectromagnetism and Weak Field Einstein Equations?

This question is inspired by this answer, which cites Gravitoelectromagnetism (GEM) as a valid approximation to the Einstein Field Equations (EFE). The wonted presentation of gravitational waves is ...
9
votes
2answers
412 views

What is the origin of the factor of $-1/4$ in the Maxwell Lagrangian?

I have seen numerous 'derivations' of the Maxwell Lagrangian, $$\mathcal{L} ~=~ -\frac{1}{4}F_{\mu \nu}F^{\mu \nu},$$ but every one has sneakily inserted a factor of $-1/4$ without explaining why. ...
9
votes
6answers
7k views

Electromagnetic fields vs electromagnetic radiation

As I understand, light is what is more generally called "electromagnetic radiation", right? The energy radiated by a star, by an antenna, by a light bulb, by your cell phone, etc.. are all the same ...
9
votes
3answers
1k views

Electromagnetic Field as a Connection in a Vector Bundle

I would like to know more about Ehresmann connections in vector bundles and how they relate to the electromagnetic field and the electron in quantum mechanics. Background: The Schrödinger equation ...
9
votes
4answers
2k views

How is a spherical electromagnetic wave emitted from an antenna described in terms of photons?

When an atenna transmits radiowaves isn't it true that the electromagnetic pulse is radiated away from the accelerating electron as a spherical wave in all directions simultaneously, and if so how can ...
9
votes
1answer
659 views

Why is there a photon every millimeter on earth coming from a star millions of light years away?

I've been having this confusing thought for so long now it would be amazing if someone could answer me. Imagine this asterisk * . As you see, from the center point, lines go outwards, just like a ...
9
votes
3answers
318 views

Historical analysis of light interference - difference frequencies

It is well-known that light of two different frequencies illuminating a detector will produce an output with a component at the difference frequency. While such considerations are eminently useful ...
9
votes
4answers
2k views

Can the Lorentz force expression be derived from Maxwell's equations?

The electromagnetic force on a charge $e$ is $$F=e(E+v\times B),$$ the Lorentz force. But, is this a separate assumption added to the full Maxwell's equations? (the result of some empirical ...
9
votes
2answers
1k views

Would a gauss rifle based on generated magnetic fields have any kickback?

In the case of currently developing Gauss rifles, in which a slug is pulled down a line of electromagnets, facilitated by a micro-controller to achieve great speed in managing the switching of the ...
9
votes
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 ...
9
votes
1answer
519 views

Is there a strong force analog to magnetic fields?

In special relativity, magnetism can be re-interpreted as an aspect of how electric charges interact when viewed from different inertial frames. Color charge is more complex than electric charge, but ...
9
votes
3answers
5k views

What is reason for electronic compass calibration

Most GPS receivers and smart phones contain an "electronic compass", which I understand is generally a Hall effect magnetometer. These devices generally require "calibration", which involves waving ...
9
votes
4answers
169 views

Is magnetic levitation a good way to vibrationally isolate a speaker from its support?

I recently heard about this company planning to sell a magnetically levitating bluetooth speaker. I understand that the levitation is partly motivated by visual aesthetics, but on the website the ...
9
votes
2answers
169 views

Which of these theories on why light slows in media are true?

This question is similar to previously asked questions, but the responses to them are confusing and I think it may be better covered by listing out all the potential answers for clarity. It's a ...
9
votes
5answers
2k views

Why Does Light Not Become Polarized In A Magnetic and/or Electric Field?

I am familiar with the Faraday Effect, but I remain confused as to why the electric and/or magnetic components of light do not naturally align themselves with a magnetic or electric field (in a ...
9
votes
1answer
637 views

If you are carrying a magnet, can you tell when you cross the event horizon of a black hole?

By the no hair theorem, a black hole is completely characterized by it's mass, charge and angular momentum. Therefore dipole and higher pole magnetic fields are completely expelled when a black hole ...
9
votes
2answers
352 views

Motivation for Potentials

This is a hypothetical question about "pedagogy". Let's say I am trying to take someone who has just a very small amount of knowledge about Newtonian mechanics and convince them that the Lagrangian ...
9
votes
2answers
213 views

Why don't black holes form from forces other than gravity?

Gravity is the weakest of the fundamental forces, so what is so special about gravity that it can form an inescapably strong field while a force like the EM force cannot? It seems to me that if there ...
9
votes
1answer
388 views

Magnets arranged in a sphere

If I was to take a bunch of magnets and arrange them in a sphere (And keep them there with glue or plastic or something) so that the north pole faces the outside of the sphere and the south pole faces ...
9
votes
3answers
7k views

How do you determine power flow direction in a transmission line?

Below is a picture of a typical transmission line(about 200 kV). Is there a simple physics experiment which can be performed safely near the line, to determine the power flow direction. Or in other ...
9
votes
4answers
646 views

fraction of magnetic energy stored outside a solenoid

If I have a long solenoid, e.g. length $l$ and radius $r$ with $l = kr$, where k >> 1, with a nonpermeable (e.g. air) core, how much of the magnetic energy is stored outside as compared to inside? If ...
9
votes
2answers
6k views

How & Why does accelerating charges radiate electromagnetic radiation?

Lets consider it case by case: Case 1: Charge particle is at rest. It has electric field around it. No problem. That is its property. Case 2: Charge particle started moving (its accelerating). We ...
9
votes
2answers
237 views

Ferromagnetism with mobile spins

How can electron spins in Iron at room temperature have ferromagnetic order even though they are travelling at very high speeds? One could argue that spin and motion are completely uncorrelated and ...
9
votes
5answers
857 views

Superposition of electromagnetic waves

The superposition of two waves is given by $$\sin(\omega_1 t)+\sin(\omega_2 t)=2\cos\left(\frac{\omega_1-\omega_2}{2}t\right)\sin\left(\frac{\omega_1+\omega_2}{2}t\right).$$ For sound waves, this ...
9
votes
1answer
3k views

What is the source of Earth's magnetic field?

I saw on Discovery channel that the source of Earth's magnetic field is the molten iron and metals in the earth's core. The spinning of these liquid metals produces the same effect as electric current ...
9
votes
5answers
1k views

What causes the permittivity and permeability of vacuum?

When light travels through a material, it gets "slowed down" (at least its net speed decreases). The atoms in the material "disturb" the light in some way which causes it to make stops on its path. ...
9
votes
3answers
320 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 ...
9
votes
1answer
291 views

Effect of introducing magnetic charge on use of vector potential

It is well known that Maxwell equations can be made symmetric w.r.t. $E$ and $B$ by introducing non-zero magnetic charge density/flux. In this case we have $div B = \rho_m$, where $\rho_m$ is a ...
8
votes
4answers
4k views

Derivation of Maxwell's equations from field tensor lagrangian

I've started reading Peskin and Schroeder on my own time, and I'm a bit confused about how to obtain Maxwell's equations from the (source-free) lagrangian density $L = ...
8
votes
2answers
7k views

What is the difference between an electric and a magnetic field? [closed]

This question is a consequence of another question of mine which is about spin. Here is my spin question. What is the difference between these two fields? How do they occur? Am I right if I say that ...
8
votes
3answers
1k views

Which form of Maxwell's equations is fundamental, in integral form or differential form?

I am not sure which form of Maxwell's equations is fundamental, integral form or differential form. Imagine an ideal infinitely long solenoid. When a current is changing in time, can we detect ...
8
votes
5answers
467 views

Is there a fourth component to the electric field and magnetic field?

The Question If the three vector electric and magnetic fields come from the four component four-potential, then is there a fourth component to the electric and magnetic field? Related Question I ...
8
votes
5answers
24k 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 ...
8
votes
2answers
840 views

What does it mean when physical theories are inconsistent?

I am hoping that someone can explain in layman terms why Newtonian mechanics and Maxwell's equations are inconsistent. Wikipedia says that this inconsistency is what led to the development of ...