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Question about dipoles...

by marc32123
Tags: dipoles
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marc32123
#1
Mar27-14, 10:36 PM
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I read somewhere that a dipole is a seperation of two equal but opposite charges. Is this true or can the two charges of a dipole have a different amount of charge?
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DrDu
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Mar28-14, 02:22 AM
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As I said in your other post, you can describe the field of a charge distribution outside a sphere containing it in terms of a sum of multipole moments. If you have two unequal charges, the most important ones are the monopole (the total charge) and the dipole, although also higher moments, like quadrupole and hexapole moments will be present. However their importance rapidly decreases as the field of a monopole falls off like ##1/r^2##, that of a dipole as ##1/r^3## and that of a quadrupole as ##1/r^4## and so on, so that the higher multipole moments are only of importance for very short distances.
Simon Bridge
#3
Mar28-14, 04:09 AM
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Quote Quote by marc32123 View Post
I read somewhere that a dipole is a seperation of two equal but opposite charges.

Is this true
yes.

... or can the two charges of a dipole have a different amount of charge?
Short answer: No - by definition a "dipole" has equal and opposite charges.

You seem to be having trouble grokking what is being explained in other threads.

The electric field of a charge distribution can be represented in lots of different ways.
It can be just itself of course.

Because of the superposition principle, it can be a sum of fields due to different charge distributions too:

It can the the sum of the fields due to each charge by itself - this is probably the first one you learned.

It can also be the sum of the fields due to different ways that charges can be separated.
This is called the "multipole expansion". Which you probably have not covered yet.

Or pretty much anything you like - so long as they all add up to the field you need.
Some ways of adding up fields are easier than others - the first two are the easiest.

See lecture:
http://www.pas.rochester.edu/~dmw/ph...s/Lect_20b.pdf
... it's a bit like how you can construct complicated wave patterns by summing sines and cosines.

Two charges -q and Q separated by a distance d has two poles but it is not a "dipole".
It will have a multipole expansion that includes a dipole component.
You can have a go calculating the expansion if you like.


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