Ok, this might seem like a trivial question, but how exactly does an electric dipole exist, since positive and negative charges attract each other on the basis of Coulomb's law? Adding to this, I would like to know what would happen in a hypothetical situation where you had free positive and negative charges, and you let them attract each other. Would they stick to each other? Or something else?
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All you need to get a dipole is having equal positive and negative charges not able to come together, somehow. So they attract each other enough to be nearby, but there is some other repulsive force that keeps them from going all of the way.
For an easy example of this happening, imagine a salt molecule. The rules of quantum mechanics keep the electrons in the sodium ion and chlorine ion from overlapping too much, but each atom retains its ionic nature. So, you end up with a dipole, where you have net zero charge, but seperated net positive and net negative charge regions${}^{1}$.
${}^{1}$ Yes, I know that this isn't 100% right, but an ionic molecule is a clearer example of this concept than working through the details of covalent bonds with net electronegativity, or atoms with net zero charge but net nonzero angular momentum and higher multipole moments.
answered Feb 17, 2018 at 15:58
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$\begingroup$ Maybe an ionic crystal is not the best example for a dipole. Much better would be the water molecule which has a strong permanent dipole. $\endgroup$ Commented Feb 17, 2018 at 16:14
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$\begingroup$ Also: the electric dipole moment of the neutron, small (not yet measured), and CP violating--but it is predicted to be nonzero. There are also of course induced dipole moments, c.f. the dielectric constant. $\endgroup$– JEBCommented Feb 17, 2018 at 19:30
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$\begingroup$ @freecharly: my issue there, from a learning perspective, is explaining why a water molecule has a dipole moment, which requires explaining orbitals and electronegativity, rather than there just being ions.. There is no abstraction in a single molecule of an ionic compound -- simply a positive charge and a negative charge. $\endgroup$ Commented Feb 19, 2018 at 14:06
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$\begingroup$ @JerrySchirmer - You are right. A NaCl molecule dipole in a gas, not crystal, can be understood easier. $\endgroup$ Commented Feb 19, 2018 at 16:09
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1$\begingroup$ @HarryHolmes the question was "how do you have any dipoles at all"? And an isolated atom is typically not a dipole at all -- the non-molecular hydrogen atom has no dipole moment, for example. $\endgroup$ Commented Feb 24, 2021 at 15:24