Solved: Pure vs Physical Dipole | Griffith's Book

[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...

please release me.!

thanks

 

[ehrenfest]

I asked a similar question several weeks ago.

https://www.physicsforums.com/showthread.php?t=217621

 

[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 \rightarrow0 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?

 

[astrosona]

i guess this is a solved one! yes?

 

[D H]

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.

 

[astrosona]

oh, i see. thank you...