Could anyone explain the difference between those two words, and how to use one instead of another?
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1 Answer
That's a really interesting question. I don't have an authoritative basis for it (I would have put this as a comment but ran out of space), but my suspicion is that it's because "same" would imply that the charges are identical in both polarity AND magnitude. Definition 1 of "Same" in the OED is:
Identical; not different
While it's true that in every day usage we play a little loose with that term (for example two people who are 30 years old are said to have the "same" age even if one is 11 months older than the other), physics tends to demand greater precision.
It is not of course necessary for the charges to be identical in magnitude for a repulsive force to apply between two charged materials. They need to be of "like" charge:
Having the same characteristics or qualities as; similar to
but not necessarily the "same" charge.
"Opposite" on the other hand merely implies that one surface has a negative charge and the other a positive charge, without any suggestion of magnitude.
It may well be that someone else can provide a more definitive reason, but this one strikes me as likely.
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1I think you explained it very well. The Wikipedia article has "The force is along the straight line joining them. If the two charges have the same sign, the electrostatic force between them is repulsive; if they have different signs, the force between them is attractive".– user140086Dec 17, 2015 at 8:33