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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Flux through side of a cube

I am looking at Griffiths introduction to Electrodynamics 3rd ED. Problem 2.10 asks for the flux of $E$ through the right face of the cube, when a charge $q$ is in the back left corner of the cube. ...
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345 views

Why is the radial direction the preferred one in spherical symmetry?

I am learning about electricity and magnetism by watching MIT video lectures. In the lecture about Gauss's law, while trying to calculate the flux through a sphere with charge in it, the lecturer ...
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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 ...
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265 views

The appearance of volume $V$ in the Fourier series representation of a periodic cubic system

In the textbook Understanding Molecular Simulation by Frenkel and Smit (Second Edition), the authors represent a function $f(\textbf{r})$ (which depends on the coordinates of a periodic system) as a ...
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30 views

What does a hexadecapole look like

Two dipoles can form a quadrupole, two quadropoles an octopole. The textbook by Griffith then says ' and so on'. So how would a hexadecapole really look like? My impression was that the construction ...
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Semiconductors and energy bands

The valence and conduction band of a semi-conductor are often drawn as here click. This plot has essentially two features and I would like to understand them. The peak and the valley of the two ...
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Is a capacitor in an open circuit charged?

Say I have a circuit consisting of a battery, a wire, an open switch, and a capacitor. The circuit is open since the switch is open. My book says that the capacitor will only be charged when the ...
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Motion of a dipole in an electric field

Assume that we have some non-constant electric field $E(x,t)$ and a point-dipole at a position $q$ with a constant dipole moment $\vec{p}$. How would you describe the time evolution, i.e. the motion ...
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Why 3 dipole terms in a multipole expansion?

As can be seen on this page http://en.wikipedia.org/wiki/Multipole_expansion when we take a multipole expansion without assuming azimuthal symmetry we end up with $2l+1$ coefficients for the $l^{th}$ ...
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1k views

Prove a dielectric with infinite dielectric constant behaves as a conductor for static fields

I read the following problem: Prove that a dielectric medium for which $\varepsilon \to \infty$ behaves as a perfect conductor in the presence of static electric fields. So, the easy part is that the ...
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1answer
511 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 ...
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Electric Potential of a Charged Sphere

Problem Consider a sphere with radius $R$, and with a charge distribution $\rho(r)=\rho_0r$. Using Poisson's equation, calculate the electric potential inside and outside the sphere. Solution I don't ...
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189 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 ...
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842 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 ...
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3answers
137 views

Electric field in a conductor

Is it always true that the electric field in a conductor is zero? What would happen if I put a very big charge inside an ungrounded hollow conducting sphere like this image? The charges inside the ...
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1answer
43 views

Potential energies of charges and spring

I wonder if someone could sanity check this very simple calculation. Consider a pair of charges $+q$ at rest separated by a spring of length $d$ and stiffness $k$. The spring provides the force that ...
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3answers
282 views

How would you define electrostatics and magnetostatics starting from Maxwell's equations?

I'm reading Griffith's text, and he starts by defining Electrostatics as requiring the source charges don't move. I've seen a few slightly different definitions of electrostatics and magnetostatics. ...
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255 views

How can I calculate the force that is applied on a tube by another tube?

Let's say there is two tubes(cylinders with no tops or bottoms) with charges $q_1$ and $q_2$, radii $b_1$ and $b_2$, lengths $l_1$ and $l_2$. These tubes are located along the axis of each other's ...
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295 views

How to solve the Laplace Equation in the hollow square region?

Suppose the values of $a$, $b$, $V_1$ and $V_2$ is given. I want to find the solution of the Laplace equation, $$\frac{\partial^2 \phi}{\partial x^2}+\frac{\partial^2 \phi}{\partial y^2}=0$$ in the ...
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Charge distribution on conductors?

You have seen that the excess charge on an isolated conductor moves entirely to the conductor’s surface. However, unless the conductor is spherical, the charge does not distribute itself uniformly. ...
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289 views

Are the axial electric field lines of a dipole the only ones that extend to infinity?

Consider an electric dipole and its electric field lines. There will be many field lines that do not extend to or originate from infinity, but rather begin at the positive charge and loop back ...
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1answer
891 views

Why does Hauksbee's electrostatic machine produce light?

I'm reading on the history of the discovery of electricity and the electron, and I've went from reading about Rutherford's gold leaf experiment all the way back to Francis Hauksbee's spinning glass ...
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1answer
162 views

Local nature of a surface charge density

Boundary S of a cavity in a very large (perfect) conductor is a connected compact (smooth) surface. A positive point charge +q is placed inside this cavity. From Gauss' law we know that the total ...
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1answer
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Electric Field Between Two Parallel Infinite Plates of Positive Charge and a Gaussian Cylinder

Is the electric field between two positively charged parallel infinite plates one with a charge density twice the other effect the electric field on the outside of the plates? I am thinking no, ...
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1answer
379 views

Information content of the electrostatic Maxwell equations vs Coulomb's Law vs Poisson's Equation

In electrostatics, we have Maxwell's equations: $\nabla \cdot E = \rho$ $\nabla \times E = 0$ These four equations (the second line standing for three equations) can also be written in terms of the ...
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Moving the plates of a charged capacitor to calculate energy density - where's the flaw in my argument?

For a charged air-dielectric capacitor, let the plates be parallel to the $xy$ plane, with the top carrying a positive charge $+Q$, the bottom a charge $-Q$. The force on an infinitesimal charge ...
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313 views

Coulomb potential

It is known that the Coulomb potential can be obtained by Fourier transform of the propagator from E&M. Is this because one of Maxwell's equations have the form $\nabla \cdot \mathbf{E}=\rho$?
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Electric field a distance $z$ above the center of a circular loop. The Hard way [closed]

Problem 2.5: Find the electric field a distance $z$ above the center of a circular loop of radius $r$ which carries a uniform line charge $\lambda$. This problem is in refereced here (with ...
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1answer
262 views

Make water droplets charged?

Normally water molecules are electrically neutral. But I have seen somewhere ideas about electric energy generators mentioning that water droplets might be used in some applications as they are ...
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2answers
178 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 ...
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2answers
481 views

Electric field lines

If electric field lines cannot terminate in empty space, in the simple example of two equal charges what happens to the line starting from one of the charges toward the other in the middle?
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1answer
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What is the role of battery across a Parallel plate capacitor having equal charge?

Each plate of a parallel plate capacitor has a charge $q$ on it. The capacitor is now connected to a battery. Now, which of the following options are correct? a) The facing surfaces of the ...
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1answer
91 views

Unrolling electrolytic capacitors

If I take an electrolytic capacitor, remove the aluminium casing, unroll the foil/ electrolyte-soaked paper, if charged, will it still work? And what if I took of the cathode foil and wrapped the ...
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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 ...
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1answer
112 views

Scaling of Static Electric Field

The electric field of a point charge goes like $\displaystyle\frac{1}{r^2}$ The electric field of an infinite line goes like $\displaystyle\frac{1}{s}$ The electric field of an infinite plane is ...
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1answer
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Given charge distribution find electric field

Given a charge distribution $\rho(\vec{r})$ where $\vec{r}$ is the position vector and that $\rho$ is a function of only $|x|$, Why is it that the corresponding electric field $E$ is necessarily of ...
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2answers
252 views

Finding the direction of the magnetic force acting on a conducting wire

I have a problem in finding the direction of the force when a conducting wire is placed in a magnetic field. If I use Fleming's Right Hand rule I get a circular magnetic field, so what will be ...
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157 views

Electric flux through nonspherical surphace

Here's an explanation from a book that I read: Now, I have a few problems with this explanation as well as with the "cone method". I guess that all my problems boil down to the concept of ...
3
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2answers
146 views

When to use which representation for an electric field

In class we covered three types of possibilities to evaluate the electric field for static problems. Unfortunately, most physics textbooks cover these ways without addressing the question of ...
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1answer
132 views

Solving non-linear ODE for divalent solution at a 1-D surface boudary

I am trying to solve the following equation for a positively charged plane with charge density $\sigma$ at $z = 0$. $$ \phi''(z)=-\frac{e}{\epsilon \epsilon_0} \big(z_+n_{+} e^{-\beta z_+ ...
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1answer
75 views

Does it really make sense to talk about field lines?

Field lines should only provide a visual representation of a field. There is a rule for their construction: take an object subject to a field, move it by d$\mathbf{r}$ and draw the direction of the ...
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1answer
78 views

Correct formula to express the potential generated by a single layer charge distribution

Assume that the closed surface $S$ encircles a volume $V$, and that a surface charge with density $\sigma$ ("single layer") is distributed over $S$. My question regards the electrostatic potential ...
3
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1answer
172 views

Polarization vector and bound charge

Why is it that the bound charge is $Q_b = - \oint_S{\mathbf{P} \cdot d\mathbf{S}}$? In particular, why is there a negative sign? Hayt's book on electromagnetism describes this as the "net increase in ...
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1answer
453 views

What determines how much electrical charge an object can hold?

What determines how much electrical charge an object can hold? Does increase voltage force more electrical charge to be store in an object (Van de Graaff generator), since electric field increase as ...
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Electric field intensity in a dielectric inside a capacitor

In the question A parallel plate capacitor is charged from a cell and then isolated from it. A dielectric slab of dielectric constant $K$ is now introduced in the left half region between the ...
3
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1answer
188 views

A dielectric sphere in an initially uniform electric field and representation theory of SO(3)

I learned recently that the highest order spherical harmonic required to represent the spatial distribution of decay products of a particle can be used to determine its spin, by using arguments ...
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2answers
71 views

What is the potential field of an ion near the Bohr radius?

I figure that at large enough distances, the potential field of an ion is just the Coulomb potential for its net charge. But what happens at scales comparable to the ion's Bohr radius? Could there be, ...
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1answer
129 views

Realistic vs Idealistic capacitance

I am doing an investigation into the differences of calculating capacitance using the well know formula for an idealistic parallel plate capacitor, based on the assumption of a uniformly distributed ...
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1answer
444 views

Frank-Hertz experiment setup

The usual schematic representing the setup of Frank-Hertz experiment is the following: However, sometimes, you can see a bit different schematic: My question is: what function does $V_{G_1K}$ ...
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1answer
363 views

what is the electric field intensity inside a charged metallic shell if there is a point charge inside the shell?

What I know so far: - Charges (electrons) inside a conductor will repel (Coulomb's law). - The charges will experience repulsion which results in maximum separation distances between the charges. - ...