sentence2_structure: We<>use<>voltage<>to keep track of electric potential energy$$$We<>use voltage to keep<>track of electric potential energy
sentence1: But, if the bias voltage is raised to the gap voltage the flow of quasi-particles across the junction becomes possible because the energy gained across the drop of electrical potential compensates for the energy spent in breaking a Cooper pair.
sentence2: We use voltage to keep track of electric potential energy.
question: What do we use to keep track of electric potential energy?