Tagged Questions
0
votes
2answers
41 views
Do black holes have charges?
Do black holes have charges? If so, how would they be measured? Also, does electricity behave the same way? Black holes affect photons, which are carriers of EM radiation, so do black holes have any ...
3
votes
1answer
57 views
What are the limits of applicability of Coulomb's Law?
Coulomb's law is formally parallel to Newton's Law of Universal Gravitation, which is known to give way to General Relativity for very large masses. Does Coulomb's Law have any similar limits of ...
0
votes
0answers
44 views
The electrical field of a conductive sphere containing a charge - gounded vs not grounded
Let's suppose we have a sphere but unlike theoretical ones it'll has have some thickness say $\Delta r$ and inner radius $R$. What I was wondering about is how will it behave if we place some charge ...
0
votes
0answers
55 views
Curie's principle in electromagnetic field theory
I am looking for some explanation and if possible also some references about the applications of Curie's principle in electromagnetic field Theory, precisely in the computation of magnetic (resp. ...
2
votes
1answer
71 views
Taylor expansion of an integral in spherical co-ordinates
I've some difficulty deriving this equation from jackson electrodynamics (The equation after 1.30)
$\nabla^2 \Phi_a\left({\textbf{x}}\right)=-\frac{1}{\epsilon_0}\int_{0}^{R} ...
2
votes
2answers
68 views
continuity of the electric potential due to a surface charge
The Electric potential due to a charge distribution on a surface is :
$\Phi \left ( x \right )=\int \frac{\sigma \left ( {x^{}}' \right )dx{}'}{\left \| x-x{}' \right \|}da$ I want to show that it's ...
0
votes
1answer
59 views
how to find the diameter of this fuse? [closed]
A fuse blows if the current exceeds 1.0 A. It is made of material that melts at a current density
of 620A/cm2. What is the diameter of the wire, assumed to have a circular profile, in the
fuse?
1
vote
1answer
70 views
Should the electric potential of a positively charged sphere be negative?
Because the indefinite integral of the electric field results in a negative value? (As the function is proportional to $r^{-2}$?
I've got to be missing something... Help please!! Thanks!
Also, I ...
2
votes
3answers
253 views
Maxwell Stress Tensor in the absence of a magnetic field
I'm having some trouble calculating the stress tensor in the case of a static electric field without a magnetic field. Following the derivation on Wikipedia,
Start with Lorentz force:
$$\mathbf{F} = ...
0
votes
0answers
45 views
numerical investigation of stability [closed]
I am trying to find the required specifications of an paul RF trap, in which a proton can be confined.(trap dimensions,voltage frequency and amplitude used, etc). I have to solve the equations of ...
0
votes
1answer
144 views
Finding the electric field
The charge per unit length on a long, straight filament is -91.9 µC/m.
(a) Find the electric field 10.0 cm from the filament, where distances are measured perpendicular to the length of 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 ...
0
votes
2answers
114 views
Calculate force of electric charges “suspended” by strings
In a question:
Two small plastic balls hang from threads of negligible mass. Each
ball has a mass of 0.110g and a charge of magnitude q. The balls are
attracted to each other, and the ...
2
votes
0answers
101 views
Metal sphere and charged ring
I think this is an interesting question, to which I don't really know the answer to. (Also, not a homework question.)
Say you have an uncharged metal sphere constrained to move in the z-axis. There ...
8
votes
3answers
321 views
Where's the energy in a boosted capacitor?
Suppose I look at a parallel plate capacitor in its rest frame and calculate the electrostatic energy, $E$.
Next, I look at the same capacitor in a primed frame boosted in the direction perpendicular ...
3
votes
1answer
367 views
Electric Field due to a charged sphere
Suppose we have a spherical surface with a surface charge density varying as $cos(\theta)$. Apparently one can find the electric field both outside and inside such a spherical surface by superposing ...
3
votes
1answer
162 views
Revealing Electric field lines through Grass seeds in mineral oil
In one of Walter Lewin's famous lectures, he takes a small container almost completely filled with mineral oil and grass seeds (I'll keep looking for the video to link to it). He connects two ...
1
vote
1answer
36 views
Effect of electric field of plasma sheaths
How will an electric fields affect the formation of plasma sheaths?
0
votes
1answer
88 views
Is it possible to charge a capacitor to any potential? Does it depend on the dielectric placed between them?
Is it possible to charge a capacitor to any potential? Does it depend on the dielectric placed between them? Or, are there other factors affecting it?
1
vote
4answers
305 views
No magnetic field from a static charge - Is there a simple physical argument to show why?
For a charge moving in an electric field $\vec E$, its equation of motion is given by the electric part of the Lorentz force $$\frac d {dt}\gamma m \vec v = e\vec E$$This comes from the conservation ...
1
vote
1answer
66 views
Question regarding the direction of electric field
Suppose that there are negative charges (e.g. electrons) only. There are more negative charges on left than on right. How would electric field be constructed? (So, What would be the direction?) And ...
0
votes
3answers
264 views
Is there is any difference between Electric Flux ($\Psi$) and Total number of Electric Field lines ($E\times$Area)?
Is there is any difference between Electric Flux ($\Psi$) and Total number of Electric Field lines ($E\times$Area)?
$\psi = \Sigma Q$, and $\phi$ = Electric field intensity $\times$ Area
where Area ...
0
votes
1answer
95 views
What is the energy of interaction between two infinite charged wires?
This is related to my previous question. I assume that the two wires have the same charge density with the same sign ($\lambda_1 = \lambda_2 = \lambda$)
If I want to take the same approach, the ...
0
votes
1answer
93 views
Will I recieve a shock if I charged a capacitor then i touch the 2 poles in same time?
What happens if I charged a capacitor then i touch the 2 poles in same time?
0
votes
1answer
46 views
Can we transfer the charge on a plate of a capacitor elsewhere?
I know it is hard , but can we transfer the charge on a capacitor plate elsewhere?
0
votes
0answers
58 views
Is there a minimum charge for the electroscope to work? [closed]
I built an electroscope, and I noticed that some charged objects didn't have enough charge to show any interaction.
So i was wondering if there is a threshold minimum charge for the electroscope to ...
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 ...
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 ...
1
vote
0answers
44 views
outward pressure in an confined electron gas
Suppose i have some electrons stored in a empty shell container with a negative ion layer in the inner surface so the electrons keep bouncing inside without being able to leave the inner cavity.
I ...
0
votes
0answers
69 views
Subtle problem on capacitors [closed]
Imagine we have 2 capacitors of capacitance $C_1$ and $C_2$ respectively. One is charged with charge $Q_1$, and the other is not charged.
Now we connect them in series as shown in figure:
What is ...
0
votes
2answers
192 views
Are electrons on a conductor, in electrostatic equilibrium, stationary?
Electrostatics basically means dealing with time independent electric fields (which was produced by stationary charges)
Now consider a neutral conductor. We know that putting a net negative charge on ...
1
vote
1answer
133 views
How will water affect the electric field?
How will 2 charged bodies act when the separation medium between them is water ?
0
votes
0answers
136 views
A crazy problem
Easy
Consider the following figure
Each red point is a particle of a known mass that carries charge Q/2 and connected to a the box by a thread of known length. This problem can be solved easily to ...
2
votes
2answers
93 views
Will the positive ions in an aqueous solution be attracted to a charged body ?
If I had a negatively charged body , say an electret , and i put it in a container of NaCl solution . Will the positive ions of sodium be attracted to it ? and why? If no, why do the positive ions ...
1
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 ...
-1
votes
1answer
122 views
If I had 2 charged bodies attracted to each other when i move one , will the other move too? [closed]
Does the force of attraction between them have to be higher than a certain number to be able to move with each other?
How will the weight of the objects influence the process?
Will they be moving with ...
3
votes
3answers
383 views
Will two magnets lose magnetism if they stick to each other for infinitely long time?
Here we have two magnets and they are sticking to each other. What I've learned that could possibly explain it is one magnet holds positive charge and the other one holds negative. But when the ...
1
vote
1answer
76 views
Fields of Steady Currents Using Electrostatics
Suppose you have a uniform ring charge rotating at constant angular velocity so that you also have a uniform ring of steady current, and thus you can use the Biot-Savart Law to compute the magnetic ...
1
vote
1answer
162 views
What is discontinuity in Vector Fields
I am reading David J. Griffiths and have a problem understanding the concept of discontinuity for E-field.
The E-field has apparently to components. (How does he decompose the vector field into the ...
6
votes
1answer
74 views
What causes a spark to move along rods that are not parallel?
I took my son to a science museum where they had a gadget that many of us probably saw in movies involving a mad scientist. The gadget had two metal rods about two inches apart at the bottom. The rods ...
4
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 ...
1
vote
1answer
43 views
Is the stress finite at the centre of a spherical continuous charge distribution?
At the centre of a spherical continuous charge distribution with no external electric fields, the electric field is zero from symmetry arguments. But does the stress at the centre remain finite?
0
votes
1answer
173 views
Faraday's law and superconductivity
According to Faraday's law of induction, volts = -Number of coils in a solenoid * change in strength of magnet / change in time. This doesn't take into account distance or speed, only time. If amps = ...
13
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 ...
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 ...
0
votes
1answer
318 views
linear charge density, surface charge density and volume charge density
What is the difference among linear charge density, surface charge density and volume charge
density.
1
vote
3answers
2k views
How is calculated the potential between two capacitors in series?
Suppose to have two capacitors in series:
The voltage in the middle point will be:
$$
V_X = V_1 \frac{C_1}{C_1+C_2}
$$
How can this be explained? It's been asked in electronics, and explained in ...
2
votes
2answers
239 views
A Question From Jackson Electrodynamics
I have a question regarding Jackson's Classical Electrodynamics. Consider the equation
$$\varphi \left ( x\right )=\tfrac{1}{4\pi\epsilon _{0}} \int_V \frac{\varrho ( x )}{R}d^{3}x+\tfrac{1}{4\pi} ...
0
votes
2answers
392 views
Electricity & Magnetism - Is an electric field infinite?
The inverse square law for an electric field is:
$$
E = \frac{Q}{4\pi\varepsilon_{0}r^2}
$$
Here: $$\frac{Q}{\varepsilon_{0}}$$
is the source strength of the charge. It is the point charge divided ...
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 ...
