Someone help explain electric dipole situation

  • #1

Ok so she says that electric dipoles are of opposite charge but equal magnitude at 3:40. But then at 5:33 she shows 2Q with -Q, at that point the magnitude of the 2Q particle wouldn't be equal to the -Q so they wouldn't be electrical dipoles right?
 

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  • #2
DrClaude
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Ok so she says that electric dipoles are of opposite charge but equal magnitude at 3:40. But then at 5:33 she shows 2Q with -Q, at that point the magnitude of the 2Q particle wouldn't be equal to the -Q so they wouldn't be electrical dipoles right?
At 3:40, she gives an example of a dipole, not a definition. A dipole is described by two opposite charges, but not necessarily of the same magnitude.
 
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  • #3
bobob
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Ok so she says that electric dipoles are of opposite charge but equal magnitude at 3:40. But then at 5:33 she shows 2Q with -Q, at that point the magnitude of the 2Q particle wouldn't be equal to the -Q so they wouldn't be electrical dipoles right?
Well, That is not really the way to look at it. First of all, there is a theorem that says the lowest nonvanishing multipole moment is independent of origin. The lowest multipole moment is just the total charge and if the charge configurtation contains equal and opposite charges, that vanishes. The next multipole is the dipole and with two equal and opposite charges, the "monopole" moment vanishes and therefore the dipole moment does not depend on how you choose your coodinates. Thst is what makes it a dipole and not just a charge distribution with a dipole moment. If they are not equal as in your example with charges 2Q and -Q, the lowest moment does not vanish because the total charge is Q, so there is a term proportional to Q and any dipole moment of the configuration depends on the position of the charges. You can have a charge distribution that has a dipole moment without the distribution itself being a dipole.
 
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