Electron Theory PDF

UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only ELECTRON THEORY STRUCTURE OF MATTER Matter is defined as anything that occupies space and has mass. Thus,...

UNCONTROLLED COPY - FOR TRAINING PURPOSE ONLY Revision Service Will Not Be Provided To The Holder TRAINING MAN UAL For Traini ng Purpose Only ELECTRON THEORY STRUCTURE OF MATTER Matter is defined as anything that occupies space and has mass. Thus, the objects in the environment that we live in, such as air and water, are all various forms of matter. Even the aircraft that you maintain are also a form of matter. The law of matter states that matter cannot be created or destroyed. However, the characteristics of the matter can be changed. All matter consists of elementary particles, e.g. electrons, protons and neutrons. The forces that bind these particles together to create matter are the same forces that create electrical power. Every aircraft generator, alternator and battery and virtually all electrical components function according to the electron theory. The electron theory describes specifically the internal molecular forces of matter as they pertain to electrical power. The electron theory is therefore a vital foundation upon which to build an understanding of electricity and electronics. ATOMS The concept and meaning of an atom arise on the foundation of ancient Greek natural philosophers who started asking interesting questions as: What is stuff composed of? What is the structure of material objects? Is there a basic unit from which all objects are made? As early as 400 B.C., some Greek philosophers believed that mater could not be continuously broken down and divided indefinitely. Thus, there must be an existence of a basic unit or form that was indivisible and foundational to the bigger structure. They named these basic forms as atomos (Atomos in Greek means indivisible). This indivisible building block of which all matter was composed became known as the atom. Atoms are the smallest particles of an element that can exist, either alone or in combination with other atoms. An element is a single substance that cannot be separated into different substances except by nuclear disintegration. Common elements include iron, oxygen, aluminium, hydrogen, copper, lead, gold, silver, which are all listed in the periodic table. The smallest particle of any of these elements would still have that element’s properties. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 1 TRAINING MAN UAL For Traini ng Purpose Only PROTONS AND NEUTRONS Each atom consists of a nucleus containing positively charged protons and electrically neutral neutrons. These protons and neutrons are not removable easily. It would require some form of high-energy nuclear occurrence to disturb the nucleus and subsequently dislodge its positively charged protons. ~ ~ Structure of an Atom ELECTRONS Surrounding the vast space outside the nucleus and travelling at high speed in orbits are electrons, each of which is negatively charged and weakly bounded to the atom. Each electron weighs about 1/1845 as * much as a proton. The charges carried by the electrons and protons are equal but opposite in nature. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 2 TRAINING MAN UAL For Traini ng Purpose Only EXAMPLES The hydrogen atom is the simplest atom and has one electron and one proton while a helium atom has two electrons, two neutrons and two protons. Hydrogen Atom Helium Atom A lithium atom has three protons, three electrons and three neutrons. Lithium Atom B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 3 TRAINING MAN UAL For Traini ng Purpose Only MOLECULES AND COMPOUNDS When atoms bond together, they form a molecule. However, there are some molecules that could exist as single atoms. Two examples in aircraft maintenance are helium and argon. A molecule is the smallest particle; in which any compound can be divided and still retain its identity. If the molecule is further divided, atoms are formed. A compound is a chemical combination of two or more different elements, and the smallest particle of a compound is a molecule. A water molecule (H2O) contains two hydrogen atoms and one oxygen atom while a carbon dioxide molecule contains a carbon atom and two oxygen atoms. Water Molecules Different compounds have distinctly different properties. Materials are composed of atoms and molecules of these elements and compounds, thus providing different materials with different electrical properties. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 4 TRAINING MAN UAL For Traini ng Purpose Only IONS Positive electrical forces outside an atom tend to attract electrons from an atom’s outer ring, which results in an unbalanced electrostatic condition. The atom thus becomes charged and charged atoms are called ions. If an atom has an excess of electrons, it is said to be negatively charged and is called a negative ion. Conversely, if it has a shortage of electrons, it will be positively charged and is called a positive ion. Charged molecules are also called ions. Note that protons remain within the nucleus while electrons are added or removed from an atom, thus resulting in a positive ion or a negative ion. Examples of Molecule Ions B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 5 TRAINING MAN UAL For Traini ng Purpose Only ATOMIC STRUCTURE AND FREE ELECTRONS The path which an electron travels around the nucleus of an atom describes an imaginary sphere or shell. Simpler atoms like Hydrogen and Helium only has one shell but the more complex atoms like oxygen has a number of shells. The atomic structure of oxygen is illustrated in the following figure. Oxygen Atom When an atom has more than two electrons, it must have more than one shell as the first cell can only accommodate two electrons. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 6 TRAINING MAN UAL For Traini ng Purpose Only CONDUCTORS, SEMICONDUCTORS AND INSULATORS The atomic structure of a substance determines its conductivity. An element is a conductor, semiconductor or an insulator based on the number of electrons in the valence orbit (shell) of the material’s atoms. The valence orbit of any atom is the outermost shell of the atom. The electrons in this valence orbit are known as the valence electrons. All atoms desire to have their valence orbit full of electrons. The fewer number of valence electrons in an atom, the easier it will give up those electrons. Valence Electrons and Shell The electrons that move from one atom to another are called free electrons. They moved from the outer shell of one atom to the outer shell of another atom. However, the movement of these free electrons does not always result in electrical current flow. Under a power supply, for example a battery, a potential difference is created between the two ends of a conductor and the free electrons will then move in the same direction as the electrical current, thereby producing a useful electron flow. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 7 TRAINING MAN UAL For Traini ng Purpose Only CONDUCTORS Some elements, mainly metals, allow current to flow through them easily. They are known as conductors. These elements have fewer than half of their valence electrons and tend to readily accept the moving electrons of an electric current flow. Examples of the best conductors are gold and silver as their valence orbits contain only one electron each. However, we normally use cheaper alternative materials to reduce costs and increase workability. Common conductors used are copper and aluminium. The following figure illustrates the structure of a copper conductor. It must be noted electron movement can only occur for a conductor when there is an external force in addition to the molecular forces residing in the conductor’s atoms. In the case of aircraft, these external forces are usually supplied by the battery or the generator. An electric potential applied across a conducting material will cause electrons to become detached from their atoms and move (as they are negatively charged) towards the positive potential. As an electron leaves its atom, that atom loses part of its negative charge and becomes a positively charged ion. This makes it attractive to another electron which takes the place of the one just attracted away, and so on. Thus a flow of electrons moves, atom by atom towards the positive terminal, being replaced by an excess of electrons at the negative terminal. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 8 TRAINING MAN UAL For Traini ng Purpose Only SEMICONDUCTORS Materials with exactly half of their valence electrons are semiconductors. Semiconductors have very high resistance to current flow in their pure state. However, when some electrons are added or removed, the material will offer very low resistance to electric current flow. The two most common semiconductors are germanium and silicon, with 4 valence electrons in their valence orbits. The following figure shows the structure of atomic silicon. Atomic Structure of Silicon In a semiconductor the bond between the valence electrons and their nucleus is much stronger than in a conductor, thus lesser electrons are free to move when a potential is applied. When current does flow, the chances of a collision between the electrons moving due to the electric potential, and randomly moving electrons due to heat, is much less. When the semiconductor material is heated, it frees electrons that are previously held by their atoms. These electrons are then free to add to the current flow. Although the total current flow is much less than in a conductor, the amount by which current flow increases due to heat is proportionally greater. It is thus important to keep components made from semiconductor materials, such as transistors, cool. Otherwise an effect called thermal runaway can occur. This is when an increase in temperature causes an increase in current, which in turn causes a further increase in temperature, leading to increasing current. A process that can escalate until the material passes so much current it is destroyed. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 9 TRAINING MAN UAL For Traini ng Purpose Only INSULATORS Materials that have more than half of their valence electrons are insulators. Insulators will not easily accept extra electrons. Two of the best insulators are neon and helium. Diamond is also a good insulator. Due to cost and increased workability, common insulators we used include air, plastic, fibreglass and rubber. The following figure shows the structure of Diamond. Diamond An insulator has all its electrons tightly bonded to the nucleus and so it takes very large forces of either heat or potential to dislodge them. Insulators do not normally pass current. However, under extreme conditions such as high temperatures or with very high voltages applied, some insulating materials will conduct. In these circumstances the insulating material is said to have "Broken down" and usually the structure of the material is permanently damaged. B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 10 TRAINING MAN UAL For Traini ng Purpose Only In some insulators (glass for example) heating the material to a high temperature will vibrate the atoms so violently that it will shake free enough electrons for conduction to occur. Cooling the material once more stops conduction. In most insulators however, conduction in a normally insulating material, whether caused by excessive heat or by excessive voltage will permanently destroy the material. For this reason insulating materials for electrical insulators, each have a safe working limit quoted by the manufacturer using the material for both voltage and temperature. In general, atoms with four valence electrons are semiconductors; atoms with fewer than four valence electrons are conductors, while those with more than four valence electrons are insulators. A summary is presented in the following table. Conductor Semiconductor Insulator No of Valence Less than half Exactly half More than half Electrons Examples Gold, Silver, Germanium, Neon, Helium, Copper, Silicon air, plastic, Aluminium fibreglass, rubber, diamond B-M3 ELECTRICAL FUNDAMENTALS Electron Theory 11