Not the calculation—the explanation . They will show you a potentiometer or a Wheatstone Bridge circuit. They won't ask for the answer. They'll ask: "Explain why the galvanometer reading is zero." If you write "because no current flows," you get 0 marks. You must say: "The potential difference between points X and Y is equal, hence potential difference across galvanometer is zero." Precision is everything.
Often rushed at the end of Term 3, this topic appears heavily in Paper 2. jc1 h2 physics promo papers