Electrostatics is concerned with the field and potential of stationary electrical charges and electric charge distributions. Problems are this type are almost exclusively concerned with mathematics of geometries using the inverse-square law.
3
votes
4answers
557 views
is an electron/proton gun possible?
In the 1944 SF story "Off the Beam" by George O. Smith, an electron gun is constructed along the length of a spaceship. In order to avoid being constrained by a net charge imbalance, it is built to ...
6
votes
1answer
737 views
What is the penetration length of static electric field into conducting metals?
How large is the penetration length for static electric field into good conductors?
I have two versions: (1) few atomic spacings
$$a\sim n_{e}^{-1/3},$$
and (2) Debye length computed by Fermi ...
2
votes
1answer
223 views
Where does the smell of electrostatic charge come from?
Everybody knows you can produce electrostatic charge rubbing two different materials together. But have you ever smelt e.g. at the plastic after charging it? There actually is a distinct ...
2
votes
1answer
380 views
Electrostatics:Basic question on electric current
I've got two questions for you.
Electric current is the flow of electrons across a conductor.Why has it always got to do with electrons and not with protons? (I know neutrons are not eligible for ...
3
votes
1answer
137 views
Capacitance of two cocentric spheres, contradicting results
Suppose we are given two conducting, cocentric spheres of radius $a_1$ and $a_2$ respectively. The inner sphere with charge $q$, the outer sphere with charge $-q$.
I can calculate the capacitance of ...
1
vote
1answer
72 views
Creating the opposite of an optical lattice
Is it possible to create periodic potentials that instead of creating a well for an atom to be trapped in, repulsed by that specific location?
If yes, can we use this as a means to make artificial ...
6
votes
1answer
372 views
Boundary Conditions Invariant Under Conformal Transformations in Electrostatics?
in two dimensional electrostatics it is assumed that the whole physical system is translationally invariant in one direction. Here, the two-dimensional Laplace equation $$\Delta \phi(x,y) = ...
1
vote
1answer
927 views
Electric field between two conducting plates of different potential?
not sure if this question's been asked before, though I couldn't see one in my brief search on here... Well, the problem I am trying to solve is that I want to determine the electric field strength ...
3
votes
4answers
522 views
How beam focusing looks like in electron microscope?
I mean I know there are electrostatic/electromagnetic lenses which does focus the beam, but I am not sure how it is possible to foсus beam down to a few 10nm while emitter might be 1mm thick while ...
2
votes
2answers
297 views
How does electron gun accelerates electrons?
I know that in electron guns we see in TV's and lots of other places, we have electron emitter (cold/hot W needle in the simplest case) and electrons are accelerated using lattice with high-voltage ...
0
votes
2answers
676 views
Electric field of not-grounded conducting plate with a given potential?
I have been trying to find an equation (or some solution) of how to calculate the electric field strength (in N/C) of a conducting rectangular (nearly flat) plate which has non-zero potential to it, ...
3
votes
3answers
2k views
Electric field outside a capacitor
I know that the electric field outside of a capacitor is 0 and I know it is easy to calculate using Gauss's law. We create cylindrical envelope that holds the same amount of charges (of opposite ...
1
vote
1answer
302 views
capacitance between widely-separated parallel plates
There's a caveat, which is often ignored, to the "easy" equation for parallel plate capacitors C = epsilon * A / d, namely that d must be much smaller than the dimensions of the parallel plate.
Is ...
4
votes
2answers
295 views
Algorithm of Lightning Strikes?
Given an array of charge for a given area (2D or 3D), what algorithm would describe the path that lightning takes?
An example algorithm would be from the highest charge of the cloud, find the lowest ...
3
votes
1answer
326 views
How many electrons are displaced when combing hair?
Feynman talking about electricity: http://www.youtube.com/watch?v=qhh32JYkQPk
When brushing your hair, Feynman mentioned that a "few" electrons were transferred to the brush (or vice versa, can't ...
2
votes
2answers
236 views
Is it meaningful to imagine a sphere uniformly charged with 2e?
If the charge is large, considering charge density is usually meaningful despite the discrete nature of electrical charge. The following sentence is part of a problem in a textbook on ...
5
votes
2answers
850 views
Two spheres (A physics olympiad problem)
Browsing an archive of problems of a local physics olympiad, i stumbled upon a problem which seems not a very trivial.
Given two identical metal spheres in vacuum, with mass $m$ and radius $R$. One ...
0
votes
2answers
5k views
Determine the point at which the electric field is equal to zero?
Two point charges, -2.5 micro coulombs and 6 micro coulombs, are separated by a distance of 1m (with the -2.5 charge on the left and 6 on the right).
What is the point where the electric field is ...
7
votes
2answers
1k views
Tension in a curved charged wire (electrostatic force) - does wire thickness matter?
Consider a conducting wire bent in a circle (alternatively, a perfectly smooth metal ring) with a positive (or negative) electric charge on it. Technically, this shape constitutes a torus. Assume ...
5
votes
6answers
763 views
Laplacian of $1/r^2$ (context: electromagnetism and poisson equation)
We know that a point charge $q$ located at the origin $r=0$ produces a potential $\sim \frac{q}{r}$, and this is consistent with the fact that the Laplacian of $\frac{q}{r}$ is
...
0
votes
1answer
331 views
Electric field of a charged spherical surface
Figure
The dielectric shaped as on the figure has dielectric constant $\varepsilon=\varepsilon\left(r\right)$ and free charge density $\rho=\rho\left(r\right)$. What is the electric field and ...
1
vote
2answers
2k views
Determine Charge With Electroscope?
You have three separate glass rods, and you know one is positive, one is negative and one is neutral. You also have an electroscope that is positively charged.
How can you determine which rod is ...
7
votes
2answers
329 views
Torque of an electric engine
So I have some follow up questions from this thread:
Electric engine transmission
Basically, I am looking for a more precise mathematical statement that makes this true. Why is it that the electric ...
8
votes
5answers
1k views
Why isn't there a centre of charge?
When determining the gravitational attraction between 2 solid bodies, we can simplify computations by taking their masses to be concentrated at their respective centres of mass. However, had they been ...
4
votes
1answer
309 views
Balloon rubbing; where do the electrons go?
If you rubbed a balloon with a towel, where would the electrons go: the balloon or the towel? Why?
I'm guessing the electrons would go to the object with a larger mass, but it's just a guess. :)
6
votes
1answer
529 views
In Method of images type problems, why isn't the induced charge taken into account while solving it via Green's functions?
To take the simplest example,
$$\Phi(\vec{r}) = \begin{cases} V & r\leq a \\ 0 & r\geq a \end{cases}$$
For a plane conducting surface that extends all the way. Choosing an appropriate ...
2
votes
2answers
542 views
What is the effect of temperature on electrostatic-gravitational balance?
We have two identical massive metal spheres at the same temperature at rest in free space. Both have an identical charge and the Coulomb force [plus the black-body radiation pressure if the ...
7
votes
6answers
607 views
What prevents the accumulation of charge in a black hole?
What prevents a static black hole from accumulating more charge than its maximum? Is it just simple Coulomb repulsion?
Is the answer the same for rotating black holes?
Edit
What I understand from ...
5
votes
3answers
534 views
Paradoxical interaction between a massive charged sphere and a point charge
Suppose we have a sphere of radius $r$ and mass m and a negatively charged
test particle at distance d from its center, $d\gg r$. If the sphere is electrically neutral, the particle will fall toward ...
-1
votes
4answers
977 views
Resistance between two points on a conducting surface
Suppose we have a cylindrical resistor, with resistance given by $R=\rho\cdot l/(\pi r^2)$
Let $d$ be the distance between two points in the interior of the resistor and let $r\gg d\gg l$.
Ie. it is ...
11
votes
1answer
1k views
Why two objects get charged by rubbing
It is always told as a fact without explaining the reason. WHY two objects get charged by rubbing? Why one object get negative charge and other get positive charge?
0
votes
1answer
2k views
Electric Field for a hollow sphere with point charge
I think I am over thinking this question: A point charge of -2 microC is located in the center of a hollow sphere. The internal and external radius of the sphere is given by 6.5 and 9.5 cm, the charge ...
11
votes
2answers
3k views
What is the net charge of the Earth?
This question arose in a seminar today about the solar wind...
This is my vagueish understanding of the problem - please correct if you see errors!
The 'classical' picture of atmospheric electricity ...
5
votes
4answers
1k views
Method of Images
The method of image charges is a well-known and very useful tool for solving problems in electrostatics.
Unfortunately, when I was taught this method, it was presented simply as an algorithm. No real ...
6
votes
3answers
1k views
Charge density in concentric spheres
Question:
If there are two conducting spherical shells and the inner shell is grounded, what will be >the charge density in the inner shell if there is a charge Q placed on the outer shell?
...
10
votes
2answers
1k views
Virtual photon description of B and E fields
I continue to find it amazing that something as “bulky” and macroscopic as a static magnetic or electric field is actually a manifestation of virtual photons.
So putting on your QFT spectacles, look ...
0
votes
2answers
212 views
energy field of a newly created particle
The question is: How to calculate the energy of electrostatic field of a newly created particle.
The usual formula involves integration 'all over space'.
According to GR I believe that 'all over ...
4
votes
1answer
199 views
Couch shocks everything!
How do I get rid of the static charge I get from my microfiber couches? Every time I get up and touch something it zaps me...
I'm mostly worried about my laptop getting fried.
32
votes
10answers
4k views
Why does dust stick to rotating fan propeller?
Why does dust stick to rotating fan propeller?
Intuitively, most people (including I) think of the dust will not stick to rotating fan propellers.
EDIT 1:
Thank you for the great explanations. I am ...
31
votes
5answers
2k views
Does Coulomb's Law, with Gauss's Law, imply the existence of only three spatial dimensions?
Coulomb's Law states that the fall-off of the strength of the electrostatic force is inversely proportional to the distance squared of the charges.
Gauss's law implies that a the total flux through a ...
