A: a) Voltage in essence is a measure of the amount of work done per unit charge as it moves through a circuit. Now, voltage can be differentiated into two types depending on how the unit charge moves. If the unit charge is moving between two points of a component/conductor/circuit, then that is a potential difference. If the unit charge is moving round a complete circuit, then that is electromotive force, or emf.
b) Current is defined as the rate of flow of charge, or amount of charge flowing through a circuit per unit time.
I = Q/T