The movement of electrons in a conductor requires some form of energy or work, usually provided by an external force, like a battery. This force is called the electromotive force or voltage. The voltage between two points, referred to as points "a" and "b," in an electric circuit is the energy (or work) needed to move a unit charge from point "a" to point "b," and this relationship is expressed mathematically as

In this equation, "w" represents the energy measured in joules (J), and "q" represents the charge measured in coulombs (C). The voltage, denoted as "v_ab" is measured in volts (V).

Voltage, often referred to as the potential difference, signifies the energy required to move a unit charge through an element within the circuit. It is important to note that the value of a voltage can be either positive or negative, with its direction determined by its polarities, indicated as (+) and (-). In electrical terminology, it is customary to state that a voltage exists across an element. The notation "v_ab" represents the voltage between points "a" and "b" and can be interpreted in two distinct ways:

• Point "a" is at a potential of "v_ab" volts higher than point "b."
• The potential at point "a" with respect to point "b" is "v_ab" volts.

A constant voltage is categorized as a direct current (DC) voltage, typically represented as "V." DC voltages are commonly generated by sources such as batteries. On the other hand, a voltage that varies sinusoidally with time is called an alternating current (AC) voltage, represented as "v." AC voltages are typically produced by electric generators.