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Quoting Feynman from Chapter 2 of his book Six Easy Pieces (emphasis mine):

Suppose that we have two unlikes that attract each other, a plus and a minus, and that they stick very close together. Suppose that we have another charge some distance away. Would if feel any attraction? It would feel practically none.

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On the other hand, if we get very close with the extra charge, attraction arises, because the repulsion of likes and attraction of unlikes will tend to bring unlikes closer together and push likes farther apart. Then the repulsion will be less than the attraction.

I get the first part. The two unlikes are so closely bonded that the distant charge can't affect them in any way. However, the second part is confusing. Why would the repulsion of the two bonded unlikes with respect to the third charge "tend" to be less than their attraction?

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  • $\begingroup$ Maybe this will help? en.wikipedia.org/wiki/Electric_dipole_moment $\endgroup$
    – Solomon Slow
    Commented Nov 19, 2020 at 19:05
  • $\begingroup$ Consider the positions of the two bonded unlikes to be slightly diferent. The bond axis can point in any direction. $\endgroup$
    – Whit3rd
    Commented Nov 19, 2020 at 19:28
  • $\begingroup$ He means the repulsion between the third charge and the initial pair. It has nothing to do with the repulsion between the components of the pair. $\endgroup$
    – nasu
    Commented Nov 19, 2020 at 19:41
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    $\begingroup$ Think of the pair rotating from the force of the nearing third charge so the like charges will become more opposite from each other with the unlike charge in between. $\endgroup$
    – Adrian Howard
    Commented Nov 19, 2020 at 19:45

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As the third charge is moved closer, if the orientation of the bonded charges can change it would do so in a way that the like charge will tend to move farther, and the unlike charge will tend to move closer, to the third charge. Now as the third charge is close to the pair, the difference in the distances becomes significant and the repulsion of the like charge, being further away, will be less than the attraction of the unlike charge, which is closer.

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  • $\begingroup$ Is it possible to arrange the orientation of the pair in such a way that the third charge cannot unbond them? $\endgroup$
    – Paul Razvan Berg
    Commented Nov 19, 2020 at 22:33
  • $\begingroup$ @PaulRazvanBerg Anything can un-bond if supplied enough energy. $\endgroup$
    – electronpusher
    Commented Nov 20, 2020 at 7:34

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