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.
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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 ...
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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 ...
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 ...
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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?
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votes
3answers
532 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 ...
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vote
2answers
199 views
Point charge potential (sign problem)
I'm a bit embarrassed, but I'm not able to compute the electric potential at point $P$ (at a distance $R$ from the origin) generated by a positive unitary point charge in the origin with the right ...
9
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 ...
1
vote
2answers
462 views
Is there really no meaning in potential energy and potential?
I have been told all my physics life that potential energy between two mass/charge has no meaning and only their difference has meaning. The same goes for electric potential, only the difference ...
5
votes
5answers
443 views
Paradox with Gauss' law when space is uniformly charged everywhere
Consider that space is uniformly charged everywhere, i.e., filled with a uniform charge distribution, $\rho$, everywhere.
By symmetry, the electric field is zero everywhere. (If I take any point in ...
5
votes
6answers
758 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
...
3
votes
3answers
680 views
What are electromagnetic fields made of?
I am trying to understand electromagnetic fields so I have two question related to them.
What is a electromagnetic field made of? Is it made of photons / virtual photons?
How about a static electric ...
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vote
1answer
290 views
Finding the electric field for a shell of charge
Suppose we have charge density defined
$$
\rho(x,y,z) =
\begin{cases}
0 & 0 \leq r < a \\
K & a \leq r\leq b\\
0 & b< r
\end{cases}
$$
For some constants $K,a,b$
How would ...
5
votes
1answer
285 views
Are the Maxwell's equations enough to derive the law of Coulomb?
Are the 8 Maxwell's equations enough to derive the formula for the electromagnetic field created by a stationary point charge, which is the same as the law of Coulomb?
If I am not mistaken, due to ...
2
votes
2answers
704 views
Electric potential due to a point charge in Gaussian/CGS units
I learned electrostatics in SI units. In SI, the electrostatic potential due to a point charge $q$ located at $\textbf{r}$ is given by
$\Phi(\textbf{r}) = \frac{q}{4 \pi \epsilon_0 |\textbf{r}|}$.
...
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 ...
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votes
3answers
1k views
What is the answer to Feynman's Disc Paradox?
[This question is Certified Higgs Free!]
Richard Feynman in Lectures on Physics Vol. II Sec. 17-4, "A paradox," describes a problem in electromagnetic induction that did not originate with him, but ...
12
votes
4answers
544 views
Is there a deep reason why springs combine like capacitors?
I was solving a practice Physics GRE and there was a question about springs connected in series and parallel. I was too lazy to derive the way the spring constants add in each case. But I knew how ...
5
votes
3answers
228 views
A problem of missing energy when charging a second capacitor
A capacitor is charged. It is then connected to an identical uncharged capacitor using superconducting wires. Each capacitor has 1/2 the charge as the original, so 1/4 the energy - so we only have 1/2 ...
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votes
4answers
2k views
Will a charged capacitor discharge if one lead is connected to ground?
If I charge a capacitor and connect one lead to ground keeping the other lead floating, will the capacitor discharge ?
...
2
votes
2answers
203 views
Electric field of a negative charge
How was it discovered that the electric field of a negative charge points towards the charge itself? Is it true?
(Courtesy of wikipedia)
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votes
1answer
113 views
Why doesn't a gaussian surface pass through discrete charges?
I have read that Gaussian surface cannot pass through discrete charges. Why is it so?
I have even seen in application of Gauss' Law when we imagine a Gaussian Surface passing through a charge ...
6
votes
1answer
732 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 ...
5
votes
3answers
211 views
Is it true that $\vec{E}\neq 0$ inside a 1- or 2-dimensional conductor?
It is known that when a conductor is placed in an electric field, the charges redistribute themselves such that $E=0$ inside the conductor. I was also told that the same is NOT true for the 2D and 1D ...
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votes
2answers
201 views
Negative Mass and gravitation
Since Newtonian gravity is analogous to electrostatics shouldn't there be something called negative mass? Also, a moving charge generates electric field, but why doesn't a moving mass generate some ...
4
votes
4answers
2k views
Infinitely charged wire and Differential form of Gauss' Law
I have tried calculating the potential of a charged wire the direct way. If lambda is the charge density of the wire, then I get
$$\phi(r) = \frac{\lambda}{4 \pi \epsilon_0 r} \int_{-\infty}^\infty ...
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votes
6answers
327 views
Electrostatic Potential Energy Derivation
How is the boxed step , physically as well as mathematically justified and correct ?
Source:Wiki http://en.wikipedia.org/wiki/Electric_potential_energy
As work done = $- \Delta U $. for Conservative ...
3
votes
2answers
572 views
Electric field and electric potential of a point charge in 2D and 1D
in 3D, electric field of a piont charge is inversely proportional to the square of distance while the potential is inversely proportional to distance. We can derive it from Coulomb's law.
however, I ...
3
votes
2answers
138 views
Explanation for $E~$ not falling off at $1/r^2$ for infinite line and sheet charges?
For an infinite line charge, $E$ falls off with $1/r$; for an infinite sheet of charge it's independent of r! The infinitesimal contributions to $E$ fall off with $1/r^2$, so why doesn't the total $E$ ...
3
votes
1answer
802 views
How can a conductor be grounded yet there are induced charges on it?
A classic example for the method of images is the following, quoted from Griffiths's Introduction to Electrodynamics, page 121:
"Suppose a point charge $q$ is held a distance $d$ above an infinite ...
2
votes
3answers
4k views
In electrostatics, why the electric field inside a conductor is zero?
In electromagnetism books, such as Griffiths or the like, when they talk about the properties of conductors in case of electrostatics they say that the electric field inside a conductor is zero.
I ...
2
votes
4answers
708 views
How does instant charging of one plate affect the potential of the other plate of a floating capacitor?
If I have an uncharged floating capacitor and I instantaneously connect one plate to some potential, then that plate will acquire some charge. In practice, the other floating plate will ...
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vote
3answers
330 views
Is this really how a capacitor works? Why doesn't it behave like a resistor?
My book says a capacitor is two conducts being connected by an insulator. Now let's take a parallel plate capacitor to simplify the problem I have.
Suppose I got two parallel plate capacitor in ...
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vote
1answer
150 views
Can a super-extremal charged black hole be made out of electrons only?
In a previous Question it was argued that it would be impossible to add enough charge to a black hole to make it pass the extremal black hole limit since adding charge would increase the mass of the ...
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?
...
5
votes
1answer
510 views
What defines the maximum charge a capacitor can store?
The formula for a capacitor discharging is $Q=Q_0e^{-\frac{t}{RC}}$
Where $Q_0$ is the maximum charge. But what property defines the maximum charge a capacitor can store?
If it depends on ...
3
votes
2answers
97 views
Electric Field One-Form
I know for instance that we can interpret the electric field as the one-form that given a vector gives the change in potential in the direction of the vector, however I'm very unsure about how to ...
3
votes
1answer
423 views
Is the electric field at the edge of a uniformly charged disk infinite?
Consider a disk with a radius $R$ (I'll use $R=1$ at various points here) that has a constant surface charge density $\sigma$. Unlike the similar problem of the field in the vicinity of a infinitely ...
3
votes
2answers
472 views
When I connect in series two floating capacitors, one charged and the other not charged, does current flow?
Suppose I energize one capacitor by connecting it across a battery, allowing it to achieve some potential difference V0 across its plates, then disconnect it and allow both of its leads to float in ...
3
votes
3answers
5k views
Why a glass rod when rubbed with silk cloth aquire positive charge and not negative charge?
I have read many times in the topic of induction that a glass rod when rubbed against a silk cloth acquires a positive charge. Why does it acquire positive charge only, why not negative charge?
It ...
3
votes
1answer
406 views
Trying to understand Laplace's equation
I'm struggling here so please excuse if I'm writing nonsense.
I understand that the gravitational potential field, a scalar field, is given by $$\phi=\frac{-Gm}{r}$$
where $\phi$
is the ...
2
votes
2answers
2k views
Dirichlet and Neumann Boundary condition: physical example
Can anybody tell me some practical/physical example where we use Dirichlet and Neumann Boundary condition. Is it possible to use both conditions together at the same region?
If we have a cylindrical ...
2
votes
1answer
66 views
Electrostatic charge leakage
What are the ways electrostatic charged objects leak charge in humid conditions?
Can airborne particles pick up charge by contact, then be repelled hence removing charge? If so would it be a ...
2
votes
1answer
114 views
electrostatic potential, analytic properties
An electrostatic potential associated with some delocalized charge $\int \rho(\mathbf{r}) d{\mathbf{r}}$ is given by:
$$v_H(\mathbf{r}) = \int ...
2
votes
3answers
2k views
How does electricity flow in conductor when potential difference is applied?
Electrons move from higher potential to lower potential. When a conductor is connected to battery, electron move from negative terminal to positive terminal.
But the battery itself forms a Electric ...
2
votes
5answers
981 views
How can I prevent being zapped by static electricity every time I touch a doorknob or handle in the office?
I don't know what it is about this office, but it seems everything I touch (doorknob, bathroom faucet, edge of kitchen sink in the break room), I get zapped by static electricity. It's getting old. ...
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vote
1answer
165 views
What is the energy of interaction between a point charge and an infinite cylinder?
I don't remember enough from my electromagnetism course and I can't find any simple, full example on this subject.
I know, that I can consider the cylinder as a wire with the same charge density ...
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1answer
140 views
Potential for charge distribution, finiteness
Consider a potential for charge distribution:
$$v_H(\mathbf{r}) ~=~ \int \frac{\rho(\mathbf{r'})}{|\mathbf{r}-\mathbf{r'}|}d\mathbf{r'}$$
where $\rho(\mathbf{r'})$ is the charge density.
This ...
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vote
1answer
542 views
How is Gauss' Law (integral form) arrived at from Coulomb's Law, and how is the differential form arrived at from that?
On a similar note: when using Gauss' Law, do you even begin with Coulomb's law, or does one take it as given that flux is the surface integral of the Electric field in the direction of the normal to ...
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vote
2answers
157 views
In which cases is it better to use Gauss' law?
I could, for example calculate the electric field near a charged rod of infinite length using the classic definition of the electric field, and integrating the: $$
\overrightarrow{dE} = \frac{dq}{4 ...
0
votes
0answers
22 views
Electrostats and Photoelectric effect [closed]
Here I think ,
one beam will knock out just one electron. So I am not able to even understand what the question says , please someone give a hint as to what the question asks
