أَسَافَرَ عَلِيٌّ؟
Did ʿAlī travel?
The unknown is not which person travelled; rather, whether ʿAlī's travelling is true. The response may be “yes, he did” or “no, he did not.”
أَنَجَحَ الطَّالِبُ؟
Did the student succeed?
If the speaker does not know whether this happened, this also seeks assent. No contrasting alternative with am is required.
Grammatical point: interrogative hamzah is a particle, not an operator that changes the case or mood of the following noun or verb. Its natural position is at the head of the question.