Questions tagged [dipole]

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31
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3answers
7k views

Why there is no dipole gravitational wave?

I have read that "thanks to conservation of momentum" there is no dipole gravitational radiation. I am confused about this, since I cannot see the difference with e.m. radiation. Is this due to the ...
21
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3answers
3k views

Why is dipole the simplest source in electrodynamics?

I see this sort of statement in many materials, for example this: The smallest radiating unit is a dipole, an electromagnetic point source. and this: The simplest infinitesimal radiating ...
19
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1answer
3k views

In the Lennard-Jones potential, why does the attractive part (dispersion) have an $r^{-6}$ dependence?

The Lennard-Jones potential has the form: $$U(r) = 4\epsilon\left[ \left(\frac{\sigma}{r}\right)^{12} - \left(\frac{\sigma}{r}\right)^{6} \right]$$ The (attractive) $r^{-6}$ term describes the ...
17
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1answer
1k views

Do neutrons exhibit momentary small charges due to the movement of its quarks?

So in a similar way to electrons moving in atoms, causing induced dipole-dipole interactions, can neutrons momentarily attract or repel?
16
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1answer
762 views

Electric dipole moment of electron: about what point is the moment taken?

There is a lot of experimental research activity into whether the electron has an electric dipole moment. The electron, however, has a net charge, and so its dipole moment $$ {\bf \mu}= \int ({\...
14
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2answers
1k views

Does an accelerating electric dipole radiate?

For such a simple question I'm finding it remarkably hard to get a definitive answer. Googling has not helped me. Consider an ideal electric dipole that is constant i.e. neither its magnitude nor ...
14
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3answers
897 views

Electromagnetic field energy “paradox”

The starting point is the energy density in electromagnetic fields: $$ u = \frac{1}{2} \left( \epsilon_0 |\mathbf{E}|^2 + \frac{1}{\mu_0} |\mathbf{B}|^2 \right) \tag{1}$$ The "paradox" is if we use ...
13
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3answers
424 views

(Special relativity) A tilted dipole composed of $+q$ and $-q$ point charges moves at a horizontal velocity. Will it rotate?

Two point charges $+q$ and $-q$ are connected with a rigid insulating stick: This dipole moves at a velocity $v$ to the right. Therefore, the charges should exert a magnetic Lorentz force $F_B$ on ...
13
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2answers
2k views

Magnets arranged in a sphere

If I was to take a bunch of magnets and arrange them in a sphere (And keep them there with glue or plastic or something) so that the north pole faces the outside of the sphere and the south pole faces ...
12
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5answers
12k views

What is the physical sense of the transition dipole moment?

So if the states are the same we achieve the expectation value of the dipole moment for a given state. I mean $ \langle \mathbf{\mu} \rangle = \langle \psi \vert \hat{\mathbf{\mu}} \vert \psi \rangle$...
9
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2answers
3k views

Why is there a factor of 1/2 in the interaction energy of an induced dipole with the field that induces it?

In this paper, there's the following sentence: ...and the factor 1/2 takes into account that the dipole moment is an induced, not a permanent one. Without any further explanation. I looked through ...
9
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1answer
249 views

How do temporary dipole-dipole interactions work in quantum mechanics?

The standard presentation of temporary dipole-dipole interactions (in high school at least) is classical: the electrons in an atom/molecule 'orbit' around its nucleus/nuclei. As a direct result of ...
9
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2answers
3k views

What is the “direction” of the transition dipole moment? (Understanding Eq. 9.29, Charge and Energy Transfer 3rd Ed, May & Kuhn)

For a real vector $\mathbf{r}$, the direction is given by: $\hat{\mathbf{n}}=\mathbf{r}/\left|\mathbf{r}\right|$. The transition dipole moment is a complex vector. How do you define its direction? ...
8
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3answers
2k views

Electric field due to a hydrogen atom

Electric field due to two opposite charges According to the answers in the previous questions, if you separate the two charges from each other, they will produce a non-zero electric field ...
8
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5answers
3k views

Direction of rotation of proton in magnetic field--opposite to a dipole

Chatroom created by @pcr for discussing this: http://chat.stackexchange.com/rooms/2824/direction-of-rotation-of-proton-in-magnetic-field Here's a small paradoxical question I was asked a long time ...
7
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4answers
1k views

Why does an atom in an oscillating electric field behave like an electric dipole?

How can I understand that an atom subjected to an oscillating electric field (e.g., $\vec{E}=\hat{i} E_0\cos\omega t$) behaves like an oscillating electric dipole? What is the underlying picture that ...
7
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1answer
9k views

Why is the geomagnetic north pole at a different location from the magnetic north pole?

According to this site Magnetic North, Geomagnetic and Magnetic Poles The Geomagnetic poles (dipole poles) are the intersections of the Earth's surface and the axis of a bar magnet hypothetically ...
7
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5answers
5k views

Electric field falls off faster than $\frac{1}{r^2}$ for large distances

An excerpt from a book; The electric field due to a charge configuration with total charge zero, is not zero; but for distances large compared to the size of the configuration, its field falls off ...
6
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4answers
62k views

Convention for the potential energy of a dipole in a uniform electric field?

When finding the potential energy of a dipole in a uniform electric field, I was told by my lecturer that the convention is that the potential energy is 0 when the dipole moment and electric field ...
6
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4answers
221 views

Difference between magnetic dipole interactions and spin interactions

$\def\b#1{{\mathbf #1}}$ Referring to the following formulae from wiki, it seems like dipole-dipole interactions can be qualitatively described using the formula: $$H = -\frac{\mu_0}{4\pi|\b{r}|^3}\...
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6answers
6k views

What is the origin of the Dirac delta term in the dipole electric field?

I am a bit lost how one has deduced the formula for electric field with electric dipole because of some inconsistency between different sources. The Wikipedia article contains a delta function in the ...
5
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2answers
2k views

Meaning of the phrase “dipole moment of the combination”

Here is a question I came across in a book: Three point charges $-q$,$-q$ and $2q$ are placed on the vertices of an equilateral triangle of side length $d$ units.What is the dipole moment of the ...
5
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2answers
2k views

Will there be any force of attraction or repulsion between an electrified body and a non-electrified body?

Up to my knowledge an electrified (charged) body can attract a non-electrified (neutral) body. I thought this because, when we bring a charged (suppose negatively charged) body near a neutral one. ...
5
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1answer
331 views

Is there a finite-sized charge distribution whose electric field is *exactly* that of a point dipole?

This kinda gets lost in the telling when electric dipoles are introduced in textbooks, and it ends up causing a good deal of confusion (as in e.g. this recent example). Point electric dipoles, and the ...
5
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2answers
391 views

How do you fit a dipole in an electron?

Experiments used to observe particle spin properties (such as Stern-Gerlach) rely on a varied magnetic field and a dipole-like reaction in the particle, deflecting it in one direction or another. In ...
5
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2answers
88 views

Definition of electric polarisation and the potential due to a polarised body

I've two questions, the second one depends on the first. $\mathbf{1}$ How exactly is polarisation defined? Griffiths says $\mathbf{P} \equiv$ dipole moment per unit volume How exactly do we go about ...
5
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1answer
19k views

Force between two point dipoles

What would the force between two parallel point dipoles be? I was thinking of doing it the way force between two point charges is found out, by finding the field and then the force but I am not able ...
5
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1answer
146 views

Direct detection of the electric field of an electromagnetic wave from an oscillating dipole

The principle behind the detection of gravitational waves is that stellar-mass sources can act as coherent oscillators, producing a gravitational wave with a well defined, and relatively slowly ...
5
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1answer
4k views

Difference between a “source dipole” and a “force dipole”

I know quite well what a dipole is and in general what multipole moments are (in the context of, for instance, electrodynamics). What I find myself confused by is something called a "force dipole" in ...
5
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2answers
364 views

Near energy In the null of a Hertzian dipole

Since $\mathbf E = -∇Φ - ∂\mathbf A/∂t$ one expects an oscillating $\mathbf E$ field even in the null of a Hertzian Dipole unless the two right hand side terms cancel -- which they do in the far field ...
4
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3answers
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What do we mean with magnetic monopole and dipole?

What do we mean with magnetic monopole and dipole? I can not find a way to relate magnetic monopoles and dipoles with electric ones. I do not understand their outcomes. Also,what is their role in ...
4
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3answers
111 views

Energy of magnetized vs not magnetized ferromagnet?

A ferromagnet conststs of many, many microscopic magnetic dipoles that can be pointed in different orientations. If the ferromagnet is not magnetized by applying an strong external magnetic field, the ...
4
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1answer
5k views

Derivation of Interaction energy of Dipole - Induced Dipole Interaction

I see that the formula giving the potential (interaction) energy of a dipole and an induced dipole is $$V=-\frac{C}{r^6}$$ where $$C=\frac{\mu_1^2 \alpha'_2}{4 \pi \epsilon_0}$$ and that the formula ...
4
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1answer
201 views

What is a neutron's polarizability?

It is undergraduate textbook level knowledge that atoms are polarizable – that is, they become electric dipoles in electric fields due to the deformation of the electron wave function(s). ...
4
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1answer
101 views

A Classical model for diamagnetism

I'm trying to derive a classical model to describe diamagnetism, and I'd like to understand if it is possible to understand the basic properties of diamagnetic materials with it i.e.: ''A diamagnetic ...
4
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2answers
179 views

Neat expression for the dipole potential in Fourier space?

In textbook electromagnetism we are used to seeing neat, coordinate-free, expressions for the scalar potential from an electric dipole (using Gaussian units) $$\phi(\mathbf{r}) = \frac{\mathbf{p} \...
4
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1answer
11k views

Force from point charge on perfect dipole

Have a point charge and a perfect dipole $\vec{p}$ a distance $r$ away. Angle between $\vec{p}$ and $\hat{r}$ is $\theta$. Want to find force on dipole. I'm having more than a little difficulty ...
4
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1answer
768 views

How do electric dipoles exist?

Ok, this might seem like a trivial question, but how exactly does an electric dipole exist, since positive and negative charges attract each other on the basis of Coulomb's law? Adding to this, I ...
4
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3answers
395 views

How does an electric dipole lose its energy while aligning with a uniform electric field?

How does an electric dipole lose its energy while aligning with a uniform electric field--through heat or light?
4
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1answer
180 views

What happens to the incident electromagnetic wave in Rayleigh scattering?

In Rayleigh scattering an incident EM wave causes an induced dipole oscillation of an atom/molecule, which in turn causes radiation at the same frequency of the incident wave. But what happens to the ...
4
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2answers
261 views

Dipole field issue in particle-mesh Ewald method with periodic boundary conditions

I am working on a thesis that makes a great use of molecular-dynamics simulations, and I am trying to understand how the particle-mesh Ewald method works. The problem is, I have difficulties ...
3
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3answers
3k views

Does a neutral particle feel the electric pull?

Do neutral atoms, neutrons, neutrinos, etc. feel zero electrostatic force, or if they feel two equal and opposite forces that cancel each other?
3
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2answers
163 views

Is the vector potential component $A_\phi$ for a dipole necessarily 0 due to symmetry about the $z$-axis?

Consider an electric dipole (+$q(t)$, -$q(t)$) [where say $q(t)=q_0\cos\omega t$] is placed along the $\hat z$ axis. In the spherical polar coordinates, its vector potential $$\vec{A} = A_r \hat r + ...
3
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1answer
5k views

Why does the field of an electric dipole drop off as $1/r^3$?

Electric field of a point charge drops off inverse squarely but that of a dipole varies as $1/r^3$. Why the field drops off more quickly for a dipole?
3
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1answer
211 views

How to derive the $\frac{4\pi}{3}\vec{p}\delta^3(\vec{r})$ element for the dipole field, from its potential?

This might be a bit more general question about how to figure out what is the appropriate (delta) expression in singular points, but e.g. for the dipole, we can derive its potential by a taylor ...
3
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2answers
2k views

Why does the dipole moment depend on the distance?

Why does the dipole moment of an electric dipole of different charges depend on the distance from the origin? Physically, I don't understand why something that measures how much of a dipole something ...
3
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1answer
2k views

What is the charge density of a pure electric dipole?

I've only found two definitions of the charge density of a pure electric dipole: \begin{equation} \rho(\vec{r})=\vec{p}\cdot\nabla\delta(\vec{r}-\vec{r}_0) \end{equation} and \begin{equation} \rho(\...
3
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1answer
349 views

Intuitive explanation of difference in $r$-dependence between dipole and monopole

For an electric monopole, its potential scales with $\frac{1}{r}$, where $r$ is the distance from the point of interest to the charge. However, for a dipole, its potential scales with $\frac{1}{r^2}$. ...
3
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1answer
70 views

Electric field of a dipole from superposition and from multipole expansion

This question came in my physics test: Charge density in a one dimensional space is given by $\rho=Q[\delta(x-x_0)-\delta(x+x_0)]$. The electric field due to this charge distribution at point (2$x_0$,...
3
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1answer
77 views

Do neutral particles interact with EM radiation?

This question has been bugging me for a while. I asked my professor, I read a couple of posts here and from other sources and some people say that neutral particles interact with light and some people ...

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