Pertaining to DC circuits: a. What Phisically happens in Voltage Drop? b. What exactly do electrons resist against in a conductor? c. With batteries connected in series, why does Voltage phisically add up? d. With batteries connected in parallel, why does current physically add up? I understand the theoredic principles of these question, its just that I have a hard time grasping what actually goes on PHYSICALLY. Please be as Scientific and Deep as possible.