Electrostatics is concerned with the electrical fields and scalar potentials of stationary electrical charges and charge distributions. Use this for questions about electromagnetic situations in which currents and magnetic fields are absent, otherwise use [tag:electromagnetism] and/or ...

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Relativity Paradox involving two charged particles

Suppose there are two charged particles separated by some distance $d$ both with an equal positive charge of $q$. The particles also have equal masses of $m$. $m$ and $q$ are chosen so the ...
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0answers
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Can a charge moving in an open trajectory qualify as current?

It is sometimes said that a point charge is equivalent to an electric current. If it were a steady current, I should be able to find it from Ampere’s law or Biot-Savart’s law. Even if the current is ...
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2answers
89 views

Using Electric Potential to Float an Object

I've been trying to answer the following question but I'm stuck at one step. The question essentially states that a magician is trying to perform a "floating objects" act, for which she has a thin ...
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0answers
15 views

Energy density in electrostatics

Imagine two hollow spheres with radius $R$ with charges $q$ and $-q$. $L>2R$ is distance between them. Potential energy of the each sphere is \begin{equation*} W_1=\frac 1{8\pi \epsilon_0}\frac ...
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1answer
58 views

How should I interpret the math in showing that the potential difference and the emf in an ideal battery are the same?

I was reading Griffiths' Introduction to Electrodyamics where he says that in order to have the same current through out a circuit there are two force per unit charges acting on the circuit, $f=f_s+E$ ...
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1answer
39 views

Divergence of a vector field, going through the math [closed]

The example I'm working on has this given identity: $\bigtriangledown \cdot \mathbf{\bar{r}}=3$. The question is: find the divergence of a vector field $\bar{\mathbf{E}}=\frac{\mathbf{r}}{r^{3}}$. ...
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0answers
39 views

Why does charge build up at the boundary surface of two media?

On a homework problem, we are asked to to use the first two Maxwell equations, $$\nabla\cdot \mathbf{B} = 0$$ $$\nabla \cdot \mathbf{D} = \rho$$ to show that along the boundary surface of two ...
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1answer
60 views

Energy density what volume?

In a derivation of energy density for an electric field (see here) We get rid of the $$\epsilon_0\oint(V \vec E \cdot d\vec a)$$ term by choosing a surface sufficiently far away so that this term ...
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2answers
38 views

Is it inevitable to compute the quadruople tensor in components? Why? [closed]

I was trying to determine the quadrupole tensor for a given charge distribution in one go from this equation: $$\overleftrightarrow{D}=\int d^3r \varrho(\vec{r})\left(3\vec{r} \circ ...
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0answers
38 views

Motion of charge particle under electric and gravitational fields

I am working on one experiment in which the charged particle falls under the effect of gravity and passes through the electric field in between. I want to know what is the charge of the particle? ...
2
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1answer
46 views

How should one interpret $\vec{f}=0$ in an ideal battery?

In a circuit there are two forces that act on the charges to keep the current uniform through out,$\vec{f}=\vec{E}+\vec{f_s}$, where $\vec{E}$ is the electrostatic field and $\vec{f_s}$ is the ...
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3answers
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Why is electric field zero inside a hollow metal sphere ?

The figure below shows a hollow metal sphere with a positive point charge $Q$ sitting outside it. What is the electric field at the center of sphere ? The answer is zero (look at here at the beginning ...
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1answer
67 views

What is the charge density in the proof of Earnshaw's theorem?

I am trying to understand the proof for Earnshaw's theorem. Though the theorem states that a collection of point charges cannot be maintained in a stable stationary equilibrium configuration ...
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1answer
29 views

Electrostatic induction in spheres

if we have a charged sphere with charge Q and radius r, the voltage on it's surface is calculated by gauss's law, what if we approach to it another sphere with radius R , R>r, earth it, remove earth, ...
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1answer
39 views

How to determine electric field strength without Gauss's Law

For objects which do not subscribe to Gauss's Law, I don't understand how one would determine the electric field strength. I know that the equation $E=k\int \frac{\mathrm{d}q}{r^2}$ can be used to ...
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2answers
64 views

Electrostatics Basic Question

Why ,if we increase the charge on a conductor its potential also increases? That is, Q directly proportional to V. Why ,if an insulated conductor is given some charge it acquires a certain ...
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4answers
1k views

When to use method of images in Electrostatics?

I am a bit confused about when to use the method of images in E&M? For example, in Griffith's Electrodynamics Example 3.2, the problem reads: A point charge $q$ is situated a distance $a$ from ...
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1answer
36 views

Potential at infinity due to a point positive charge

I know it is quite basic for this site, but I am confused...is it just a convention to take it as zero, or there is some logic? I really dont think that V = kq/r r->infinity, v -> 0 is right, because ...
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2answers
44 views

Current flow through car struck by lightning

It is safe to sit under a car rather than to stand under a tree during lightning. The electric field inside the car is zero when the lightning falls on it so, after falling on car, where do the ...
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4answers
1k views

Is there a limitation on Gauss' law? [duplicate]

Recently I had a question to find the electric field at a distance $R$ from the origin, where the space is filled with charge of density $\rho$. I did this by assuming a Gaussian surface of radius ...
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2answers
355 views

Two spherical cavities hollowed out from the interior of a conducting sphere. At the center of each cavity a point charge is placed

From Griffiths: Two spherical cavities, of radii a and b, are hollowed out from the interior of a (neutral) conducting sphere of radius R. At the center of each cavity a point charge is placed – ...
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1answer
37 views

Why defects reduce electric field?

I am running a simulation with drift-diffusion equation, Poisson equation and Fourier heat equation with a metal/insulator/metal structure. In my simulation, initially electric field is at the top of ...
2
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1answer
65 views

Why does a conductor shield its inside from its outside, but not vice versa?

A point charge outside a hollow conducting sphere will produce no field in the hollow interior: the metal shields the interior. But a point charge inside the hollow interior will produce a field ...
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0answers
152 views

Fluctuation interaction between two uncharged spheres

TL;DR: The problem is to determine force, acting between two uncharged conducting spheres, induced by correlated fluctuations of charge densities in these spheres. I've got stucked along the way and ...
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1answer
55 views

Calculating electrostatic potential [closed]

A continuous charge distribution is spherically symmetric and has a volume charge density $$\rho(r) = \rho_oe^{−\alpha r}$$ I need to find the potential as a function of '$r$' i.e. $V(r)$. It seems ...
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1answer
60 views

Why does $E = 0$ inside conducting sphere? [duplicate]

Given this picture, I've understood that giver r>R, E = q/(4πεR^2) but I just can't get why inside the sphere, E equals 0? And the notes don't explain something. I've tried to look it up on the web ...
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5answers
396 views

Why is the electric field inside a conductor zero in equilibrium?

My textbook says the field inside a conductor must be zero in order for the system to be equilibrium and therefore there must be no excess charge inside. Their proof: 1) Place a gaussian surface ...
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1answer
27 views

What does “touching” mean in the context of charge by conduction in electrostatics?

So in our physics class today, we had a demonstration involving a charged rod and a neutral ball attached to a string (a physical pendulum). At first, when the rod was placed near the ball, the ball ...
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1answer
76 views

Where did earth's electric charge come from?

I get between .7 and 1.3 volts out of a 20 foot tree depending on the weather. I understand this is "telluric" electricity with pretty low amperage. Where is this charge coming from? The sun, right? ...
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1answer
29 views

How to calculate the electric energy caused by eletrostatic field stored in a region, given $V$?

I have problem in apply the rules to find the energy stored in free space here is the problem Find the energy stored in free space for the region $$0.002<r<0.003m,\quad ...
2
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2answers
88 views

Continuity of electric field

I was solving a problem in which i came across a radially symmetric discontinuous electric field, which puzzled me for a moment but then I figured that there must be surface charge density at the ...
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2answers
177 views

Question on conduction

I already know that a charged polyethylene plastic (done by rubbing it with paper) can be used to attract a cardboard. Now, can I 'charge' the cardboard by touching it with the plastic? Supposedly, ...
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3answers
50 views

Vector question, differentials, Electromagnetism

I was reading this demonstration of electric potential in my book: Let $q$ be a point charge at point $P$ The Electric field created at point $M$ by $q$ is : $$\vec{E}(M) = ...
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2answers
505 views

Why is Xenon favored as the propellant in electric thrusters?

Most of the articles I've read on electric thrusters mention that Xenon is generally, with some exceptions, used as the propellant (or would it be termed reaction mass?). They never mention why ...
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1answer
45 views

How is the electric potential of a localized charge distribution scaled when scaling the geometry of the problem?

I am trying to find the potential at a point on the surface of a charged polygon (rectangular). I have find a solution to the problem, but it relies on the following statement: If the potential at ...
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1answer
76 views

Gauss's law not making sense

If we have a point charge and outside of it we have a non-conducting Gaussian sphere, then Gauss's law says that the net flux should be zero. I agree that the total field lines coming in are equal to ...
5
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2answers
95 views

Poisson equation in 2D and 3D: geometrical reason for the difference

The Poisson equation in 3D shows a fundamental solution in 3D which decays with $\sim 1/r$, whilst in 2D it shows a much different decay $\sim -\ln r$. While in 3D not only the solution, but also its ...
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1answer
38 views

Repulsive force between same charged plates?

I understand that the attraction force between parallel plates of capacitor is the derivative of stored energy with respect to distance of the plates. But how could we find the repulsive force between ...
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2answers
54 views

Potentials with no sources

I have been reading Electrostatics from Panofsky's book. One of the exercises in his first chapter is Functions of the type $\phi = x$, or indeed $\phi = x^2 + 2 y^2 - 3z^2$ , satisfy Laplace's ...
2
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2answers
368 views

Electrostatics - Inserting a brass plate between two charges

The question is: if I were to insert a brass plate between two charges, what will happen to the force between the charges? Would it increase, decrease or stay the same? Does the brass plate increase ...
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2answers
3k views

Derivation of Electric Force between Parallel Plates

So the electric field between two parallel plates is given by $E = V/d.$ How do you derive this?
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0answers
32 views

Laplace equation between circles [closed]

I need to solve the simple Laplace equation $$\nabla^2f(r,\theta)=0$$ with boundary conditions: $$f(a,0)=g(\theta)$$ $$\lim_{A\rightarrow\infty}f(A,\theta)=1$$ what would be a straightforward way to ...
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1answer
110 views

What exactly is electric dipole moment & how it works?? and Why Electric Dipole Moment vector's direction is from negative charge to positive charge?

What exactly is electric dipole moment & how it works/used? (don't go into deep mathematical calculations) Please answer the question about the direction with good details. Don't just say that ...
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1answer
24 views

Potential of 2 parallel plates

How can I write down a potential of 2 parallel plates (area $A$) with charge $+Q$, $-Q$ such that V(inf)=0 and V(in) is what I would expect, $Ed = Qd/A\epsilon_0$. Do I have to write a discontinuous ...
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1answer
44 views

Does wearing jewelry increase chance of static electricity

I am the only one in my office repeatedly subject to static stock. Does wearing silver jewelry increase my chance of getting a shock?
4
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2answers
91 views

How to get the electric field strength of a plate as approximation of a sphere

Suppose you know that the electric field in distance $r > R$ from the center of a charged sphere with charge $Q$ and radius $R$ is given by: $$ E = \frac{1}{4\pi\epsilon_0} \frac{Q}{r^2} $$ ...
0
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1answer
30 views

2D room with electron-point-charges (stability)

Is there a fast way to find out stable #"point charges" & constellation for this kind of Simulation. The 2D repeating itselve in each direction till infinity, so i've a kind of symetrie here. ...
3
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4answers
2k 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 ...
2
votes
3answers
497 views

How much energy is needed to create an electron?

I know how to calculate the electrostatic energy of a sphere (it has a well defined radius). But how can I calculate the electrostatic energy of an electron as it is a point particle? By electrostatic ...
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0answers
27 views

Interaction between charged particles through static electric fields [duplicate]

I generally find it pretty primitive to accept that static charges communicate with the help of static electric fields without any exchange going on between them such as electromagnetic waves. I ...