From our previous lesson in Atoms and Electric Charge, we have learned that some atoms can lose or gain electrons.
Throughout this process, the atom is no longer electrically neutral; in other words, the number of electrons and protons in an atom is not equal. When an atom loses (gives away) one or more electrons, it transforms into a positively charged atom. Conversely, an atom becomes negatively charged as it gains (receives) electrons.
When a point has a concentration of positively charged atoms and another has a concentration of negatively charged atoms, a potential difference arises between the two points. The potential difference is defined as the strength of the electrostatic force between two charged objects. Due to the electrostatic force, charged objects gravitate toward or away from one another depending on their polarity. Thus, once a free electron is placed between two charged objects of different polarities, the electron will be pulled towards the positively-charged object. This is because an electron (which is negatively charged) will be pushed (repelled) by a negatively charged object and drawn (attracted) by a positively charged one.
The ability of any two oppositely charged objects to perform work is defined by their potential difference. And the electromotive force (EMF) is the summation of the potential differences existing in any given electrostatic field.
Volts [V] is the unit of EMF, named after Alessandro Volta. With this in mind, the electromotive force is sometimes referred to as the voltage. EMF and voltage will be utilized interchangeably throughout our lectures.
We can derive from the discussion above that voltage can be defined as the rate of work performed per unit charge. Voltage can be expressed mathematically as:
$$\begin{align} V = \frac{W}{Q} \end{align}$$
\(\begin{align*} \text{Where:}\quad V &= \text{electromotive force or voltage, [V]} & \\ W &= \text{work, [J]} \\ Q &= \text{charge, [C]} \end{align*}\)
Voltage is always measured between two points, and these points are commonly identified as either positive ($+$) or negative ($-$). The sign denotes the polarity of the charged object at a given point.
Since voltage is measured between points, for instance, between points 1 and 2, the reference of measure is of importance. Because the voltage between points 1 and 2 is not equal in polarity with the voltage between points 2 and 1. Though they may be equal in magnitude, they will be opposite in polarity. This concept is shown in ($\fref{1}$).
Suppose a voltage has a positive (+) polarity. In that case, it will be identified as a voltage rise ($\fref{1a}$). If it has negative ($-$) polarity, it will be identified as a voltage drop ($\fref{1b}$). Hence, $V_{12}$ is a voltage rise, and $V_{21}$ is a voltage drop.
Figure 1: The polarity will depend of the direction.
As with current, a voltage can be constant or variable. Thus, a constant voltage is referred to as Direct Current voltage (DC), whereas a sinusoidal voltage is referred to as Alternating Current voltage (AC).