Introduction:

There is no coherent formal definition which can be given for electricity that states ‘this is what electricity is’. In actuality it is a property of the physical world that is intrinsically present and which is examined and exploited instead of explained. Most attempts to describe it centre on its behaviour and its effects instead of its existence that must simply be accepted as part of nature. In electronic and electrical engineering it is essentially a source of energy that can be generated and transformed into other forms of energy like mechanical energy to do work or employed in its own right in electronic circuits to process electrical signals that usually signify information in some form or other.

Atoms:

All materials in physical world, both synthetic and natural, are made up of atoms. Our present knowledge and representation of the atom is mainly based on the work of Niels Bohr (1885-1962) a Danish physicist and Nobel Prize winner, whose model of atom is that of a nucleus at the centre as shown in figure below, surrounded by orbiting electrons. The three-dimensional orbits are categorized in groups termed as shells. The nucleus consists of two types of particle that collectively account for the weight of atom. The first of such is the neutron that possesses the property of weight however is neutral in the electrical respect in that, it possesses no electrical charge. Second is the proton that has similar weight as the neutron, however also possesses an electric charge that is nominated as positive by convention. The particles orbiting around the nucleus are electrons, and such have negligible mass as compared with the protons, but contain an equivalent and opposite charge which is nominated as negative. The atoms in their natural state have similar number of protons and electrons and are thus electrically neutral.

Electrons orbiting the nucleus acquire energy and as well rotate on their own axes while circling the nucleus in their orbits. This energy is different in different orbits situated at differing distances from the nucleus. At most two electrons in the same atom can encompass the same energy. Two electrons can engage the same orbit however when they do so they spin on their own axes in the opposite directions.

Individual atoms contain differing atomic numbers that is essentially the number of both protons and electrons which they hold. The Periodic Table, shown in table below, lists the known atoms in an ordered way according to different properties however primarily according to their atomic number.

Figure: The Bohr Model of the Atom

Electric Charge:

Electric charge is a phenomenon related with the particles in atoms. The particle is considered to encompass an electric charge whenever it reacts to the influence of electricity, like an electric field or the existence of other charged particles. The protons and electrons in atoms possess the property of charge with protons nominated as containing positive charge and electrons nominated as containing negative charge. A French physicist Charles de Coulomb (1736 – 1806), discovered most of the elementary laws of electrostatics that govern the behaviour of fixed charges. Among such are the law:

“Like charges repel each other whereas unlike charges attract each other”

Unit of the charge is Coulomb, termed after him and given the symbol, C.

The magnitude of an elemental charge on a single electron or proton is given the symbol q so that:

Charge on Proton q = +1.6 x 10 ^-19  C  ;   Charge on Electron  -q = -1.6 x 10 ^-19  C

The quantity of charge at certain point in an electrical circuit is generally designated by the symbol Q and hence:

Electric Charge = Q Coulombs (C)

Table: Periodic Table of the Elements

The charged bodies experience forces among them. When both bodies encompass similar type of charge, then the force is one of repulsion tending to attempt to move the bodies away from each other, when they are free to move. When the bodies encompass opposite types of charge, then force is one of attraction, tending to try to move the bodies nearer together. This is this force of attraction between electrons and protons which holds atoms altogether. The force F between two bodies having charges Q1 and Q2 and separated by a distance r is given by the Coulomb’s Law as:

Coulomb’s Law F = Ke [(Q₁Q₂)r²] N
Here ke = 8.98 x 109 Nm2/C2 is termed as Coulomb’s force constant. The positive force is one of repulsion between the charges whereas a negative force is one of attraction. Figure below shows the lines of force between particles containing like and unlike charges. In case of like charges it can be seen that, the forces tend to divert the charges away from each other, were they are free to move, whereas in the case of unlike charges they would tend to shift towards each other.

 Figure: The Forces between Like and Unlike Charged Particles

Energy Bands:

The electrons orbiting nucleus in an atom as well possess thermal energy, once the atom is at an ambient temperature bigger than the absolute temperature of 0OK. The energy that electrons can possess is quantised and can only contain certain permitted values. The energy possessed by an electron is as well determined by the radius at which the orbit it occupies lies from the nucleus of atom. As explained, only two electrons in an atom can comprise similar energy however in this case, spin on their own axes in the opposite direction. This is termed as Pauli’s Exclusion Principle in that; it excludes any two electrons in the same atom from having exactly similar energy features. This as well applies when atoms are in close proximity to each other as in molecules forming up a piece of material made from specific element. This signifies that corresponding electrons in closely neighbouring atoms should have slightly distinct energy levels. Whenever neighbouring atoms over a range of some hundreds of atoms are examined all of the corresponding electrons contain minute differences in the energy levels they inhabit. This gives mount to bands of energy levels instead of individual discrete shells, where each band is basically a continuum of energy levels extending over a finite range. In the discussion of solids, the outermost band of an element that comprises electrons at absolute zero temperature is termed to as the valence band and it is in this band, the electrons that partake in most chemical interactions inhabit. The next outer band that is free of electrons at absolute zero temperature is termed to as the conduction band. In certain materials, at temperatures higher than absolute zero, various electrons gain sufficient energy to be able to make a transition from the valence band to conduction band.

Figure below shows a representation of energy bands in materials of three various types namely: insulators, conductors and the semi-conductors. Insulators are characterised by a big energy gap between the valence and conduction bands. This signifies that at room temperature no electrons gain adequate energy to make a transition between bands and hence electrons remain firmly bonded to their atoms in valence band.

In case of conductors it can be seen that the valence and conduction bands overlap. This signifies that there is an abundant supply of free energy levels close to those engaged by electrons in the upper area of the valence band of metals. At room temperature electrons can simply move into the vacant levels in the conduction band. With this abundant supply of vacant energy levels to move between the external electrons of metals essentially break free of their parent atoms and become free charge carriers each containing the change –q defined formerly. Such free negatively-charged electrons can then simply be made to move beneath the influence of an electric field for instance. Popular materials employed as conductors in electrical equipment and wiring are: Copper (Cu), Silver (Ag), Nickel (Ni), Aluminium (Al), Gold (Au) and Iron (Fe). Lead (Pb) is one of the main components of solder which when melted is employed to join wires together electrically.

Semiconductors encompass an energy gap between the valence and conduction bands which is much lower than that of the insulators. As a result, at room temperature, a number of electrons can make the transition from valence band to the conduction band however this number is much smaller than from the case of conductors. The extent of conduction in semi-conductors can be controlled by doping the semi-conductor materials with impurities in the form of the other element from a neighbouring group in the Periodic Table. This procedure is employed in most active electronic devices and forms the basis of semiconductor industry. The most general semiconductors are Germanium (Ge) and Silicon (Si).

Figure: Energy Bands in Solid Materials

Potential and Voltage:
 
In the world, we live in ground, which is normally taken as somewhere deep in the earth, is considered as electrically neutral point to which all electrical consequences can be referenced. In realism, it can be argued that this point is really somewhere in the deep outer-space. Though, ground serves as a suitable reference to be considered as electrical ‘zero’ point. Anything which is electrically joined to ground is taken to be at zero potential. When a body has a quantity of positive charge present on it someway, like the proton in an atom, it has a potential that is higher than ground and is stated to be at a positive potential relative to ground. When a body consists of a quantity of negative charge, like the electron in an atom, it is stated to be at negative a potential relative to ground. It has been seen above that, forces exist between charged particles and forces are always related with energy or the ability to do work. When like-charged particles in close proximity are free to move, then the forces which exist between them will cause them to move away from one another. This is an indication which charge or a potential other than ‘zero’ or ‘ground’ is intrinsically related with energy. It is expressed in terms of voltage where this is essentially the energy per unit of electric charge.

Voltage = (Energy/Charge) volts (V) or 1 Volt = 1 Joule/1 Coulomb

Much often and particularly in the field of Electronic Engineering, the words voltage and potential are treated as interchangeable and both are evaluated in units of Volts named after an Italian physicist Alessandro Volta (1745-1827), who discovered electrolysis and also discovered the battery.

When two points have distinct voltages or potentials relative to ground such that one point is at a potential V1 whereas the other is at a potential V2, then we can define potential difference V between the two points as simply the difference among the two voltages or potentials as measured relative to ground. The potential difference will thus as well have units of Volts.

Potential difference V = V₁ – V₂ Volts

Potential Difference is specified as positive whenever the potential V₁ relative to ground is more positive or less negative than potential V₂ relative to ground. Thus, in practice, it can be either negative or positive depending on the polarities and magnitudes of the potentials V₁ and V₂.

Electric Field and Electromotive Force:

Whenever forces exist among bodies, the energy related with such forces is considered to act in a field. In case of forces related with charged particles or bodies the energy gives mount to an electric field. The lines of force among charged particles shown in Figure above essentially show the direction of electric field that exists between such charges by virtue of their electric potentials. The electric field can be seen as acting from a point of positive potential in the direction of a point of negative potential as shown by the direction of field lines. The electric field is thus thought of as acting all along a path. Its strength based on the magnitudes of the potentials among which it acts and the distance among the points at which such potentials are positioned so that:

Electric Field Strength E^→ = Potential Difference/Distance = (V/d) V/m

Whenever an electric field acts all along a path where charged particles are free to move it give increase to an influence on the mobile charge carriers termed as an electromotive force or emf. This force is not strictly the Newtonian physical or mechanical force but instead an electrical one and is really measured in Volts just as is the potential difference that gives rise to it. The electromotive force in an electric circuit should come from some source of electrical energy like a battery. The voltage between the terminals of battery is essentially the emf that it generates.

Current:

The energy bands shown in figure of energy bands of materials above for conductors point out overlap between the conduction and valence bands. In good conductors, there is an abundant supply of free energy levels in the conduction band and electrons at room temperature can simply make the energy transition to such free levels and can then move between adjacent atoms with easiness. Whenever charged particles which are free to move are positioned in an electric field, they experience an electromotive force as an outcome of the field. If an emf or electric field is applied across a piece of material that is made up of an element which is a good conductor, this will give mount to a continual flux of charged particles via the conductor as shown in figure below. This comprises a flow of electric current. The quantity of this mobile charge passing via a unit area of the material per unit time is termed to as the charge flux density, J.

Figure: Charge Flux Density in Conducting Material

Thus, not only the quantity of charge that flows, however as well the rate at which it flows via a conductor is of significance in determining the total electric current. The Electric current is defined as the quantity of charge flowing via a piece of conducting material per unit time and is evaluated in units of Amperes, named after the French physicist Andre-Marie Ampere (1775 - 1836). This unit is usually shortened to Amps in Electrical Engineering. When a quantity of charge, Q, flows via the conductor in a time, T, then the resultant current is given as:

Current = Charge/Time     I = Q/T Amperes (A)

The quantity of charge and direction of flux mainly depend on the strength and direction of electric field, magnitude and polarity of the charge on particles. In case of electricity and electric circuits the mobile particles that contribute to the current flow are electrons that are negatively charged. Unluckily, in the discoveries of Physics of 17th, 18th and 19th centuries most of the conventions concerning polarity and direction were already established prior to this was recognized. The direction on an electric filed is taken as acting from the point of more positive to that of more negative (or least positive) potential (that is, from plus to minus). Therefore, electrons being negatively charged particles flow in the direction opposite to that of electric field. Though, the direction of current flow is always particular as being in the direction of electric field. This is sometimes termed to as the direction of conventional current. The other basic principle of electric current is that it should always flow in a loop or circuit, therefore the term electric circuit. This is illustrated in figure shown below.

                                  
Figure: An Illustration of an Electric Circuit

This illustrates the electric circuit comprised of a battery and a light bulb. The battery gives the emf and the resultant electric field acts from its positive terminal in the direction towards its negative terminal and therefore around the loop made by the battery, conducting wire and light bulb inserted into this loop in between the sections of wire. It can be seen that the conventional current is taken as flowing in a clockwise direction around circuit, whereas in reality electrons are really travelling in the opposite anti-clockwise direction.

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