Voltage is defined as potential difference between two points of a conducting wire carrying a constant current of one ampere when the power dissipated between these points is one watt. Voltage is also referred to as electromotive force (EMF). It is the force that pushes electrons through a wire, and is often referred to as electrical pressure. Voltage in an electrical circuit is like a pressure in water system. Voltage pushes current through the circuit, but cannot flow through the wire; it is the current that flows.
Voltage is often thought as a potential to do something and it is present at a common wall outlet, but there is no current flow until some device is connected and a complete circuit exists.
The SI unit of both potential and potential difference is the volt, named in honour of the Italian physicist, Count Alessandro Volta (1745–1827). The volt (symbol: V) is defined as ‘the potential difference between two points such that the energy used in conveying a charge of one coulomb from one point to the other is one joule’. A volt is the amount of potential necessary to cause one coulomb to produce one joule of work.

This definition may be expressed in the form of an equation: E=W/Q.

E - potential difference (volts, V)
W - work (joules, J)
Q - electric charge (coulombs, C)

Example:
The work done by a generator in separating a charge of 20 C is 50 kJ. What is the resulting potential difference across its terminals?

Solution:
E = W/Q = (50x1000 J)/(20 C)
E = 2500 V

All materials, including conductors, are usually electrically neutral because, under normal circumstances, their atoms contain equal numbers of protons and electrons whose equal, but opposite, charges act to neutralize each other.
In order to acquire a charge, an object must either gain or lose electrons – thereby acquiring an excess or a deficiency of negative charge. For example, if an object has more electrons than protons, it is negatively charged; if it has more protons than electrons, then it is positively charged.
However, it’s not necessary for two objects to be literally negatively and positively charged for a potential difference to exist between them. For example, if two objects are both negatively charged, but one is less negatively charged than the other, then a potential difference will appear between them also.

For example, if object A is less negative than object B, then we can say that object A is ‘positive with respect to object B’; or object B is ‘negative with respect to object A’.
In practice, this is by far the most common situation we encounter in any circuit, and is practically always the case for electrodes in cells and batteries: with the battery’s so-called ‘positive’ electrode actually being negatively charged, but less negatively charged than (or ‘positive with respect to’) the ‘negative’ electrode.

The process by which this can be made to happen is called ‘charge separation’. There are many ways in which charge separation can be achieved, and we'll look a few of those here.

Frictional contact (triboelectricity)

Probably the very earliest-known method of charge separation was through frictional contact. The ancient Greek philosopher, mathematician and astronomer, Thales of Miletus (circa 624–546 BC), recorded that whenever amber was rubbed with wool, the amber would acquire an electric charge (although, of course, he wouldn’t have used that expression). It is believed that the ancient Greeks would amuse themselves by charging amber in this way in order to pick up pieces of paper.
The word ‘electricity’ is derived from the Greek ‘electra’, meaning ‘amber’. Another Greek word, ‘tribo’, meaning to ‘rub’, has given us the modern term, ‘triboelectricity’, which describes the charge separation that occurs whenever one type of material is rubbed by another.
Actually, it’s not the rubbing that’s important; it’s bringing the surfaces of two different materials into contact with each other. Rubbing the materials together merely brings them into more intimate and repeated contact, thus increasing the amount of charge separation.
Although the reason for this charge separation is not fully understood, it is clear that when different materials come into contact with each other, the surface of one material appears to ‘steal’ some electrons from the surface of the other. The material that steals electrons, therefore, acquires a negative charge while the material that loses those electrons acquires an equal positive charge.

Research into this phenomenon by a Swedish physicist, Johan Carl Wilcke (1732–1796), led him to publish what is known as the ‘triboelectric series’: a list of materials in order of the magnitude and relative polarity of the charge they acquire when touched or rubbed by another material.
Materials towards the bottom of the series, when rubbed by materials towards the top of the series acquire a more-negative charge. And, the further apart materials are within the series, the greater will be the potential difference between their charges when they are brought together by rubbing.
If, for example, we rub our hair with, say, a cotton handkerchief, then our hair will acquire a more-positive charge while the cotton handkerchief will acquire a more-negative charge.
We experience a similar effect when we walk across a synthetic carpet when the air is particularly dry. The potential buildup on our body is sufficient to produce a painful shock when we then touch an earthed metallic body such as a radiator!
Although the voltages created through frictional contact can be very large (several thousands of volts), the amount of energy involved is tiny, so triboelectricity is not able to sustain currents for more than a few microseconds – certainly not long enough to cause us any harm, but possibly enough to damage sensitive electronic components (which is why we should always ‘earth’ or ‘ground’ ourselves before handling electronic circuit boards).
However, current research into triboelectricity is leading to some very interesting developments. For example, ‘smart clothing’ is a term now being used to describe flexible fabrics which generate voltages as they bend and flex when worn. With clothing that can generate voltages, it might be possible, for example, to recharge mobile telephone batteries on the move. Similarly, triboelectricity materials might, one day, be built into computer-tablet touchscreens, enabling the tablet’s battery to be charged whenever the tablet is in use. These are just two examples of the current research into what are known as ‘triboelectric nanogenerators’.

Thermoelectricity

The term, ‘thermoelectricity’, describes the direct conversion of a temperature difference into a potential difference.
When two materials are in intimate contact with each other, there is a tendency for free electrons to diffuse in both directions across the junction from one material into the other. If the materials are different, then more free electrons cross the junction in one direction than in the opposite direction. The diffusion of electrons is short-lived but the resulting imbalance causes a small potential difference to appear across the junction.

If the temperature of the junction increases, the resulting higher energy level results in a somewhat greater imbalance, and the resulting potential difference will increase. This ‘thermoelectric effect’ was first observed by the German Physicist, Thomas Seebeck (1770–1831) in the early 1820s, and is known as the ‘Seebeck Effect’.
A simple device that utilizes the Seebeck Effect is the ‘thermocouple’. A basic thermocouple consists of two wires, manufactured from different materials, connected together to form junctions at opposite ends. For the reasons described in the previous paragraphs, a contact potential difference appears across each of the junctions. If the junctions are at the same temperature, then these two potential differences will be equal, but will act in opposition to each other, and there is no overall potential difference within the circuit. If, on the other hand, the temperature of one junction is higher or lower than the other, then there will be difference between the two junction potential differences which will cause a current around the circuit.
The potential difference appearing across the open circuit is quite small – typically in the millivolt range – so, for this reason, thermocouples are frequently connected in series to form what is termed a ‘thermopile’, where all the ‘hot’ junctions are subject to the higher temperatures, and all the ‘cold’ junctions are subject to the lower temperatures.

Interestingly, a French watchmaker and part-time physicist, Jean Charles Peltier (1785–1845), who was studying the same phenomenon, discovered that it was reversible. In other words, if a potential difference is applied to the circuit with two wires connected together to form junctions at opposite ends then a temperature difference will appear between the two junctions. This is known as the ‘Peltier Effect’. For this reason, the thermoelectric effect is also generally known as the ‘Seebeck-Peltier Effect’.

Cells and batteries

There are many different types of cells but, in its simplest form, a cell consists simply of two dissimilar conductors (e.g. zinc and copper), called ‘plates’ or ‘electrodes’, immersed in a conducting liquid (e.g. dilute sulfuric acid) called an ‘electrolyte’.

When the zinc electrode is inserted into the electrolyte, it reacts chemically with the electrolyte, and starts to dissolve. As the zinc dissolves, positively charged zinc ions are released into the electrolyte, leaving electrons behind to accumulate on the zinc electrode – which, therefore, acquires a negative charge. This action continues until the zinc electrode acquires sufficient negative charge to prevent any further positive ions from escaping.
A similar chemical reaction occurs at the copper electrode, with positive copper ions dissolving into the electrolyte, leaving electrons behind to accumulate on the copper electrode. The reaction of copper, however, is far less vigorous than it is for zinc, and the amount of negative charge acquired by the copper electrode is significantly less than the amount of negative charge acquired by the zinc electrode. We say that the copper electrode, therefore, is ‘positive with respect to the zinc electrode’, and is named the ‘positive electrode’ (or ‘positive plate’) while the zinc is named the ‘negative electrode’ (or ‘negative plate’).
The difference between these two amounts of negative charge results in a potential difference of about 1.1 V appearing between the two electrodes. This is a gross over-simplification of the chemical process that is actually taking place within the cell, but is more than adequate, at this stage, to explain how a chemical cell separates charges and provides a potential difference.

The open-circuit potential difference created by the charge separation process is called the electromotive force (e.m.f., symbol: E) of the source.
There are many other methods of separating charges, including the use of light (photovoltaic cells), pressure (piezoelectricity), heat (thermocouples) and, of course, magnetism (generators). The most important of these various methods is magnetism, and we will learn later how a generator uses magnetic fields to separate charges.

Resistance and resistors

Resistance is a measure of the opposition to current flow in an electrical circuit.
We will focus on the resistance of metal conductors and insulators, while the resistances of semiconductors, electrolytes (conducting liquids) and ionised gases behave somewhat differently and will not be examined here.
The origin of the term ‘resistance’, in the sense of opposing current, is credited to the German scientist, Georg Simon Ohm (1789–1854).
However, the concept of electrical resistance was the subject of experiments by the English physicist, Henry Cavendish (1731–1810). Long before the days of electrical measuring instruments, Cavendish studied the effects of the ‘opposition to current’ by different conductors, by subjecting himself to a series of electric shocks – the more intense the shock, the lower the material’s opposition to current. Judging from his research notes, Cavendish must have subjected himself to thousands of such shocks! His results apparently compare remarkably well with our today's knowledge about the electrical resistance of various materials.
Resistance (symbol: R) is, to some extent, dependent upon the quantity of free electrons available as charge carriers within a given volume of material, and the opposition to the drift of those free electrons due to the obstacles presented by fixed atomic structure and forces within that material. For example, conductors have very large numbers of free electrons available as charge carriers and, therefore, have low values of resistance. On the other hand, insulators have relatively few free electrons in comparison with conductors, and, therefore, have very high values of resistance.
But resistance is also the result of collisions between free electrons drifting through the conductor under the influence of the external electric field, and the stationary atoms. Such collisions represent a considerable reduction in the velocity of these electrons, with the resulting loss of their kinetic energy contributing to the rise of the conductor’s temperature. So it can be said that the consequence of resistance is heat.
Resistance, therefore, can be considered to be a useful property as it is responsible for the operation of incandescent lamps, heaters, etc.
On the other hand, resistance is also responsible for temperature increases in conductors which result in heat transfer away from those conductors into their surroundings – called energy losses, which, of course, are undesirable.

Natural resistance of any circuit can be modified by adding resistors. These are circuit components, which are manufactured to have specific values of resistance. By changing the resistance of a circuit using resistors, we can, for example, modify or limit the current flowing through that circuit.

Conductance (symbol: G) is the reciprocal of resistance: G=1/R. The SI unit of measurement for conductance is the siemens (symbol: S).

The SI unit of measurement of resistance is the ohm (symbol: Ω), named in honour of Georg Simon Ohm.
The ohm is defined as ‘the electrical resistance between two points along a conductor such that, when a constant potential difference of one volt is applied between those points, a current of one ampere results’.