# Griffiths page 150

1. Feb 24, 2008

### ehrenfest

[SOLVED] Griffiths page 150

1. The problem statement, all variables and given/known data

On this page, Griffith's start talking about "pure" and "physical" dipoles. Can someone define what these terms mean?

2. Relevant equations

3. The attempt at a solution

2. Feb 24, 2008

### siddharth

Isn't it explained pretty clearly in the same page?

Where exactly are you having difficulty?

Last edited: Feb 24, 2008
3. Feb 24, 2008

### D H

Staff Emeritus
A physical dipole comprises a pair of equal but opposite charges $q$ separated by a vector $2a\hat r$. The dipole moment is $2aq\hat r$. By decreasing the separation distance but increasing the charge you can keep the dipole moment constant. A pure dipole has a zero separation distance but a non-zero dipole moment. Such a thing is not physically realizable.

4. Feb 25, 2008

### pam

What G means in Ex. 3.8 is that the point charges example is one physical configuration that has a dominant dipole moment. He seems to define a "pure" dipole as a configuration that has ONLY a dipole moment, and no higher moments. As he says, that point charges model is only a "pure" dipole in the limit -->0. The sphere with with cos charge distribution is a pure dipole because its potential for r>R is pure dipole.
None of this is too clear in G because he does not discuss higher dipole moments in good detail. Some things are clearer in more advanced texts.

5. Mar 18, 2008

### astrosona

6. Mar 18, 2008

### neelakash

DH and pam explained nce...

You may try to get a copy of Corson and Lorrain for more rigorous treatment of higher terms.