Guru
Introduction
We know that electrons move around the nucleus of an atom in different orbits. The electrons in the inner orbits (i.e., orbits close to the nucleus) are tightly bound to the nucleus. As we move away from the nucleus, this binding goes on decreasing so that electrons in the last orbit (called valence electrons) are quite loosely bound to the nucleus. In certain substances, especially metals (e.g. copper, aluminium etc.), the valence electrons are so weakly attached to their nuclei that they can be easily removed or detached. Such electrons are called free electrons.Those valence electrons which are very loosely attached to the nucleus of an atom are called free electrons.
The free electrons move at random from one atom to another in the material. Infact, they are so loosely attached that they do not know the atom to which they belong. It may be noted here that all valence electrons in a metal are not free electrons. It has been found that one atom of a metal can provide at the most one free electron. Since a small piece of metal has billions of atoms, one can expect a very large number of free electrons in metals. For instance, one cubic centimetre of copper has about `8.5times10^{22}` free electrons at room temperature.
- A substance which has a large number of free electrons at room temperature is called a conductor of electricity e.g. all metals. If a voltage source (e.g. a cell) is applied across the wire of a conductor material,free electrons readily flow through the wire, thus constituting electric current.The best conductors are silver, copper and gold in that order. Since copper is the least expensive out of these materials, it is widely used in electrical and electronic industries.
- A substance which has very few free electrons is called an insulator of electricity. If a voltage source is applied across the wire of insulator material, practically no current flows through the wire. Most substances including plastics, ceramics, rubber, paper and most liquids and gases fall in this category.Of course,there are many practical uses for insulators in the electrical and electronic industries including wire coatings, safety enclosures and power-line insulators.
- There is a third class of substances, called semi-conductors. As their name implies, they are neither conductors nor insulators. These substances have crystalline structure and contain very few free electrons at room temperature. Therefore, at room temperature, a semiconductor practically behaves as an insulator. However, if suitable controlled impurity is imparted to a semi-conductor, it is possible to provide controlled conductivity.Most common semi-conductors are silicon, germanium, carbon etc.However, silicon is the principal material and is widely used in the manufacture of electronic devices
(e.g. crystal diodes, transistors etc.) and integrated circuits.
Importance of Free Electrons
- Electrical Conductivity: As referred to earlier, loose electrons are answerable for the electrical conductivity of metals. This assets is fundamental to the operation of electrical wires, circuits, and all digital devices.
- Thermoelectricity: Free electrons also are essential in thermoelectric substances, in which they make contributions to the conversion of heat into energy. This technology is used in strength generation and waste warmth restoration.
- Semiconductors: In semiconductors, the conduct of loose electrons can be managed and modified to create digital gadgets like transistors, diodes, and incorporated circuits. These gadgets are the building blocks of modern electronics.
- Electronics and Computers: Free electrons permit the operation of digital devices together with smartphones, computer systems, and televisions. They are accountable for the drift of electric current that strategies records and performs numerous features.
- Superconductivity: In certain substances at extraordinarily low temperatures, loose electrons can shape pairs and circulate with out resistance. This phenomenon, known as superconductivity, has the potential to revolutionize energy transmission and garage technologies.
- Laser Technology: Free electrons are used inside the introduction of lasers, where their movements produce coherent mild amplification, which has a extensive range of applications in enterprise, medicinal drug, and studies.
The Electron
Since electrical engineering generally deals with tiny particles called electrons,these small particles require detailed study.We know that an electron is a negatively charged particle having negligible mass.Some of the important properties of an electron are :
- Charge on an electron, e = `1.602times10^{-19}` coulomb
- Mass of an electron, m = `9.0times10^{-31}` kg
- Radius of an electron, r =`1.9times10^{-15}` meter
The ratio e/m of an electron is `1.77times10^{-11}` coulombs/kg.This means that mass of an electron is very small as compared to its charge.It is due to this property of an electron that it is very mobile and is greatly influenced by electric or magnetic fields.
Energy of an Electron
Valence Electrons
The electrons in the outermost orbit of an atom are known as valence electrons.
The outermost orbit can have a maximum of 8 electrons i.e. the maximum number of valence electrons can be 8. The valence electrons determine the physical and chemical properties of a material. These electrons determine whether or not the material is chemically active; metal or non-metal or, a gas or solid. These electrons also determine the electrical properties of a material.
On the basis of electrical conductivity, materials are generally classified into conductors, insulators and semi-conductors.As a rough rule, one can determine the electrical behaviour of a material from the number of valence electrons as under:
- When the number of valence electrons of an atom is less than 4 (i.e. half of the maximum eight electrons), the material is usually a metal and a conductor. Examples are sodium, magnesium and aluminium which have 1, 2 and 3 valence electrons respectively.
- When the number of valence electrons of an atom is more than 4, the material is usually a non-metal and an insulator. Examples are nitrogen, sulphur and neon which have 5, 6 and 8 valence electrons respectively.
- When the number of valence electrons of an atom is 4 (i.e. exactly one-half of the maximum 8 electrons), the material has both metal and non-metal properties and is usually a semi-conductor. Examples are carbon, silicon and germanium.