Encyclopedia of Electrical Engineering
Encyclopedia of Electrical Engineering

Potential Difference

What is potential difference?

A charge repels other like charges (with same sign). That means, an electron repels other electrons. If you at one point have, say, 10 electrons then they will try to move as far away from each other as they can. This point with many electrons (that is, this point that electrons are strongly repelled from) is said to have high potential. A point of less repulsion is said to have lower potential.
In this way, electrons will always try to move towards the lower potential. In other words: if there is a potential difference between two points then electrons will try to move because they experience an electric force towards the lower potential.
Voltage and the term "potential difference" are often used interchangeably. The potential difference might be better defined as the potential energy difference between two points in a circuit. The amount of difference (expressed in volts) determines how much potential energy exists to move electrons from one specific point to another. The quantity identifies how much work, potentially, can be done through the circuit.
A household AA alkaline battery, for example, offers 1.5 V. Typical household electrical outlets offer 120 V. The greater the voltage in a circuit, the greater its ability to "push" more electrons and do work.
Voltage/potential difference can be compared to water stored in a tank. The larger the tank, and the greater its height (and thus its potential velocity), the greater the water's capacity to create an impact when a valve is opened and the water (like electrons) can flow.

How a potential difference can be established?

To create and sustain a potential difference you need something to move charges "the wrong way". That is, towards the point of higher potential. You just need a force larger than the repelling force.
Every source of voltage is established by simply creating a separation of positive and negative charges.
for example, a region of positive charge has been established by a packaged number of positive ions, and a region of negative charge by a similar number of electrons, separated by a distance r.
Fig. No. 2 Defining the voltage between two points.
Since the voltage established by the separation of a single electron, a package of electrons called a coulomb (C) of charge was defined as follows:
One coulomb of charge is the total charge associated with $6.242 \times 10^{18}$ electrons.
Conversely, the negative charge associated with a single electron is $$ Q_e = {1 \over 6.242 \times 10^{18}}C = 0.1602 \times 10^{-18} C$$
$$ Q_e = 1.602 \times 10^{-19} C$$
If we take a coulomb of negative charge near the surface of the positive charge and move it toward the negative charge, we must expend energy to overcome the repulsive forces of the larger negative charge and the attractive forces of the positive charge as shown in Fig. 2.(b), if a total of 1 joule (J) of energy is used to move the negative charge of 1 coulomb (C), there is a difference of 1 volt (V) between the two points. The defining equation is $$V = {W \over Q} $$ Take particular note that the charge is measured in coulombs, the energy in joules, and the voltage in volts. The unit of measurement, volt, was chosen to honor the efforts of Alessandro Volta, who first demonstrated that a voltage could be established through chemical action.

What is The electron volt?

It is the level of energy required to move an electron through a potential difference of 1 volt. Applying the basic energy equation, $$W = QV = (1.602 \times 10^{-19} C)(1 volt)$$ $$ = 1.602 \times 10^{-19} J $$ $$ 1 eV= 1.602 \times 10^{-19} J $$ Voltage is the pressure from an electrical circuit's power source that pushes charged electrons (current) through a conducting loop, enabling them to do work such as illuminating a light.
In brief, voltage = pressure, and it is measured in volts (V). The term recognizes Italian physicist Alessandro Volta (1745-1827), inventor of the voltaic pile the forerunner of today's household battery. In electricity's early days, voltage was known as electromotive force (emf). This is why in equations such as Ohm's Law, voltage is represented by the symbol E.
Voltage is either alternating current (ac) voltage or direct current (dc) voltage. Example of voltage in a simple direct current (dc) circuit:
Alessandro Volta Italian physicist Alessandro Volta (1745-1827)
voltage as a pressure
Fig. No.3: Image shows voltage as a pressure to force electron in a conductor.