Uncontracted Negatives

(a) Isn't Zelda leaving for Paris?

(b) Is Zelda not leaving for Paris?

A linguist explains that (a) is the question for "Zelda is not leaving

for Paris."

She then adds that (b) is "word negation, rather than

sentence negation, and the scope of negation is different." She also

says that "not" emphasizes one word but does not tell the reader

which word. (I guess she assumes that her readers are smart enough

to guess. Sadly, I am not.)

Would someone please explain to me in simple English that I

can understand what she was saying? In other words, what is

the difference in meaning between (a) and (b)?

Thank you VERY much.