when "will not be" used and when "won't be" is used

Will not be and won't be both are same because "won't be" is the abbreviation form of will not be.So,both can be used interchangeably. It's generally used in future continuous tense. For ex:- She won't be reading. They won't be watching this show tomorrow. It's also used to frame this type of sentence related to future tense. Ex:- He won't be an engineer. We won't be blamed. (passive form)