Leaving A Negative Impression

(I am completing my last chemistry lab of the year. The lab was designed based on a textbook the school no longer uses, so our instructor is explaining how to finish the lab report. This teaching assistant is good at demonstrations, but rarely listens to your questions before answering them.)

Teacher’s Assistant: “So, when copper is the cathode, you make the number negative before you solve the equation.” *points to a number on a piece of paper I can’t see*

Me: “Which number?” *there are two possibilities*

Teacher’s Assistant: “When the copper is the cathode.”

Me: “Yes, but which number do I make negative?”

Teacher’s Assistant: “When the copper is the cathode.”

Me: “I asked which number, not when to do it.”

Teacher’s Assistant: *walks over to me, points and where I wrote “Cu” on my lab report* “You see how there the copper is a cathode?”

Me: “Yes! But, do I make this number—” *points* “—or this number—” *points* “negative!”

Teacher’s Assistant: “The voltage of copper.” *leaves annoyed*

(If it was that easy why not tell me the first time I asked?)