a student in an electronics lab is studying the electrical properties of pieces of graphite. the student applies a potential difference of 24 v across the length of a cylindrical piece of graphite with a radius of 0.5 mm. the student has another piece of graphite of the same length but with a radius of 0.7 mm. the student wants the same current in both pieces of graphite. what potential difference should the student apply across the 0.7 mm piece?