# Pure/Physical Dipole?

1. Mar 17, 2008

### astrosona

[SOLVED] Pure/Physical Dipole?

Hi,

In the Griffith's book page 154 is two pictures showing pure and physical dipoles. It also writes about these in previous pages but i do not get the point! i mean what is the big deal in the difference between them? Why should Griffith's pay this much attention on it? is there a physical difference between them? actually i see no difference at all! pure is a kind of physical dipole! i do not why even Griffith's names them differently? why does Griffith's keep mentioning this and makes a big deal out of it?

Ok, let's say there is Pure dipole in some where and we are going to calculate the force exerting to a charge q in some where else, so what would have make the difference if it was a physical dipole instead of the pure dipole?

I am sure i am not getting some thing in it somewhere......

thanks

Last edited: Mar 17, 2008
2. Mar 18, 2008

### ehrenfest

3. Mar 18, 2008

### astrosona

OK.cooooooooool, i found it myself!!!

In page 60 Nayfeh it option 2 it says the Dipole potential from multiple expansions is only correct when we have $$\delta$$/r $$\rightarrow$$0 and it is not correct when we have $$\delta$$/r ~1

cooooool, so actually the pure dipole works for good estimation of the multiple expansions method and we have to be careful for the physical dipoles.... i can see we have to treat the physical dipoles as two monopole when we are enough close to them..

any commands?

Last edited: Mar 18, 2008
4. Mar 18, 2008

### astrosona

i guess this is a solved one! yes?

5. Mar 18, 2008

### D H

Staff Emeritus
Yes. The only terms in the spherical harmonic expansion (aka multipole expansion) of the electrical potential for a pure dipole are the $C_1^m$ terms - the dipole terms. A pure dipole is called "pure" is because it only has a dipole moment. A physical dipole will have quadrupole moments, etc. The spherical harmonic expansion of the potential has an infinite number of non-zero terms.

6. Mar 19, 2008

### astrosona

oh, i see. thank you...