Griffiths Page 150: Define "Pure" & "Physical" Dipoles

[ehrenfest]

[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?

Homework Equations

The Attempt at a Solution

 

[siddharth]

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?

 

[D H]

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.

 

[astrosona]

https://www.physicsforums.com/showthread.php?p=1652920#post1652920

 

[neelakash]

DH and pam explained nce...

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