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I found this in my physics book :

The force between two conducting spheres having opposite charge is more than the force between point charges of the same magnitudes and separated by the same distance.

I was wondering why this is so. Can anyone help?

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  • $\begingroup$ If the spheres are separated surface to surface by the same distance as the point charges, then I would say that the statement is false. $\endgroup$
    – R.W. Bird
    May 12 '20 at 15:07
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If the spheres were uniformly charged then there would be no difference as the field from each sphere would be uniform - just like a point charge.

However, since they are conducting spheres, the electrons within the conductors are free to move around and distribute the total charge of the conducting sphere in whatever way suits them. In this case, as you bring, say, the negatively charged conducting sphere closer to the positively charged one, the electrons in the positively charged sphere will be repelled to the far side of the sphere. Similarly, in the negatively charged sphere, the electrons will be attracted to the positively charged sphere so will gather more on that side.

Overall this means that the positively charged sphere appears more positively charged than it actually is to the negatively charged sphere as the distribution of electrons have been pushed away to make the closer side more positive. Similarly, the negatively charged sphere appears more negative as the electrons have been pulled closer to the positively charged sphere.

Therefore the spheres spheres experience will have a stronger electrical attraction than point charges.

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