Problem: A parallel-plate capacitor is designed so that the plates can be pulled apart. The capacitor is initially charged to a potential difference of 20 Volts when the plates are 1.0 mm apart. The plates are insulated so that the charge cannot leak off.
Part A: What is the potential difference between the plates when they are pulled to a new separation 3.5 mm apart?
I know the correct solution but explain how to get this answer!