# Griffiths page 150

[SOLVED] Griffiths page 150

## Homework Statement

Please stop reading unless you have Griffith's E and M book.

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

## Answers and Replies

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siddharth
Homework Helper
Gold Member
Isn't it explained pretty clearly in the same page?

A physical dipole (one with a finite separation between charges)
Where exactly are you having difficulty?

Last edited:
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.

pam

## Homework Statement

Please stop reading unless you have Griffith's E and M book.

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

DH and pam explained nce...

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