sentence2_structure: We<>use<>voltage<>to keep track of electric potential energy$$$We<>use voltage to keep<>track of electric potential energy
sentence1: Finally, to keep electric field constant, voltage is reduced by 30%, reducing energy by 65% and power (at 1.4x frequency) by 50%.
sentence2: We use voltage to keep track of electric potential energy.
question: What do we use to keep track of electric potential energy?