Module 3: Electrical Fundamentals - Static Electricity and Conduction PDF

Module 3: Electrical Fundaments Topic 3.2: Static Electricity and Conduction  INTRODUCTION On completion of this topic you should be able to: 3.2.1 Describe static electricity and distribution of electrostatic charges. 3.2.2 Describe...

Module 3: Electrical Fundaments Topic 3.2: Static Electricity and Conduction  INTRODUCTION On completion of this topic you should be able to: 3.2.1 Describe static electricity and distribution of electrostatic charges. 3.2.2 Describe the laws of electrostatic attraction and repulsion. 3.2.3 State the units of charge and describe Coulomb’s Law. 3.2.4 Describe conduction of electricity in solids, liquids, gases and a vacuum. 30-03-2024 Slide No. 2  MATTER AND ELEMENTS Matter: Anything which occupies space and has mass Found in any one of 3 states: SOLID, LIQUID, and GASEOUS Element: Substance which cannot be reduced to a simpler substance (chemically) Matter comprised of all similar atoms Both made up of small particles – smallest of these particles is the Atom 30-03-2024 Slide No. 3  ATOM Smallest particle of an element that retains characteristics of that element. Comprised of: Protons – positively charged Neutrons – neutrally charged Electrons – negatively charged 30-03-2024 Slide No. 4  STATIC ELECTRICITY Matter – typically neutral charge – equal number of electrons and protons. 30-03-2024 Slide No. 5  STATIC ELECTRICITY Matter – typically neutral charge – equal number of electrons and protons. More electrons than protons – negative charge. 30-03-2024 Slide No. 6  STATIC ELECTRICITY Matter – typically neutral charge – equal number of electrons and protons. More electrons than protons – negative charge. More protons than electrons – positive charge. 30-03-2024 Slide No. 7  STATIC ELECTRICITY Matter – typically neutral charge – equal number of electrons and protons. More electrons than protons – negative charge. More protons than electrons – positive charge. Some atoms hold on to their electrons more tightly than others do. How tightly – determines its place in Triboelectric series. 30-03-2024 Slide No. 8  TRIBOELECTRIC SERIES Positive + Human hands (usually too moist, though) Very positive Rabbit Fur Glass Human hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Neutral Steel Neutral Wood Amber Hard rubber Nickel, Copper Brass, Silver Gold, Platinum Polyester Styrene (Styrofoam) Saran Wrap Polyurethane Polyethylene (like Scotch Tape) Polypropylene Negative – Vinyl (PVC) Silicon 30-03-2024 Teflon Very negative Slide No. 9  TRIBOELECTRIC EFFECT Major causes of static electricity: Friction Pressure Separation Process is called Triboelectric Effect – tribo means rubbing. Triboelectric Effect – major cause of static The greater the force of friction or pressure – the more static will be created. The faster the separation rate – the more static will be created. 30-03-2024 Slide No. 10  TRIBOELECTRIC EFFECT Magnitude of static charge is determined by: Material composition Applied forces Separation rate Relative humidity When humidity is low, higher static charges are generated. More noticeable in winter months and in dry climates. 30-03-2024 Slide No. 11  COULOMB’S LAW OF CHARGES Relationship between attracting or repelling charged bodies: First documented by French scientist – Charles A. Coulomb. Coulomb’s Law states: Charged bodies attract or repel each other with a force that is directly proportional to the product of their individual charges, and is inversely proportional to the square of the distance between them The amount of attracting or repelling force which acts between 2 electrically charged bodies in free space depends on 2 things: 1. Their charges (q1 and q2), and 2. The distance between them (d) 30-03-2024 Slide No. 12  COULOMB’S LAW For example: If distance between 2 objects with similar charges is doubled, force of repulsion equals? ¼ of original value If charges are doubled, force of repulsion equals? 4 times original value Coulomb’s law: Charged bodies attract or repel each other with a force that is: Directly proportional to the product of their charges, and Inversely proportional to (the square of) the distance between them 30-03-2024 Slide No. 13  COULOMB’S LAW Unit of electric charge – Coulomb. Symbol for Coulombs is C. Symbol for charge is Q. 1 Coulomb = 6.24 x 1018 electrons 6.24 x 1,000,000,000,000,000,000 30-03-2024 Slide No. 14  STATIC BUILD-UP Atom: A positively-charged nucleus surrounded by negatively-charged electrons. Rubber scrapes electrons from fur atoms. Only electrons transfer Negative charges moves 30-03-2024 Slide No. 15  ELECTROSTATIC ATTRACTION AND REPULSION With charged rod, neutral ball is attracted. Some electrons from negatively charged rod move onto neutral ball. Ball and rod are now both negatively charged. Ball is then repelled away from rod. 30-03-2024 Slide No. 16  ELECTROSTATIC ATTRACTION AND REPULSION Charged comb attracts neutral bits of paper. Charged comb attracts neutral water molecules. 30-03-2024 Slide No. 17  DISTRIBUTION OF ELECTROSTATIC CHARGES Charge on Charge on Charge on Metal Conductors Insulators Points Metal Ball Plastic Ball Excess charge on an Excess charge on surface Charge on insulating irregular metal shape of a metal of uniform materials doesn't accumulates at points. curvature spreads out. move easily. Lightning, lightning rods Aircraft wings. 30-03-2024 Slide No. 18  DISSIPATION OF ACCUMULATED CHARGES Excess charge accumulates at points – Trailing edge. Feature of charge accumulation at points is used to advantage on most aircraft. Unwanted charge during flight is dissipated to atmosphere by discharge wicks. Discharge wicks: Located where air friction can pick off the charge concentrated on them Designed and located to minimise aircraft radio interference 30-03-2024 Slide No. 19  MOVEMENT OF FREE ELECTRONS Copper Atom Note only 1 electron in valence (outermost) shell. Strand of copper wire comprised of billions of copper atoms. Electron shells cross one another – valence electrons easily passed on. Electricity is conducted through solids via movement of free electrons. 30-03-2024 Slide No. 20  CONDUCTION OF ELECTRICITY IN SOLIDS RELATIVE RESISTANCES OF Conduction of electricity in solids: SOME SOLID CONDUCTING MATERIALS Movement of free electrons Gauged against copper with a datum of 1 All metals are generally good conductors. Silver 0.9 Copper – most commonly used Copper 1.0 Gold 1.4 conductor. Aluminium 1.6 Carbon: Tungsten 3.2 Brass 4.4 Non-metal with significance as Platinum 5.8 conductor Iron 6.7 Tin 8.2 Self lubricating quality Steel 8.6 Lead 13.0 Suitable for rubbing electrical Mercury 55.0 connections: Carbon 2000 to 3000.0 Generator commutators Alternator slip rings 30-03-2024 Slide No. 21  CONDUCTION OF ELECTRICITY IN LIQUIDS When current is passed through a liquid – it creates ions. Ion – an atom of liquid or gas which has either gained or lost an electron. Electricity is conducted through liquids via movement of ions. 30-03-2024 Slide No. 22  CONDUCTION OF ELECTRICITY IN LIQUIDS -ve charged ions move to +ve electrode and +ve charged ions move to –ve. Charges they carry are given up at appropriate electrodes. Usually when this process occurs: Liquid will break down into its component parts, or In some cases, one of the metal electrodes will be eaten away 30-03-2024 Slide No. 23  CONDUCTION OF ELECTRICITY IN LIQUIDS Solution Conductivity Pure Water (distilled) non Tap Water poor Pool or Sea Water weak Soluble Salt Solution (NaCl) strong Strong Acids (HCL) strong Weak Acids weak Strong Bases strong Weak Bases (Ammonia) weak Molecular Compounds (sugar solution) non Electrolytes – liquids able to conduct ionically. Most electrolytes are acid, alkali or salt solutions. Liquids in pure state may be insulators – contaminated may become conductors: Distilled water is an insulator Tap water is a poor conductor Salt water is a weak conductor 30-03-2024 Slide No. 24  CONDUCTION OF ELECTRICITY IN GASES Normal state – gases are insulators. Ionised state – gases become conductors. Heat or high electrical potentials: Can dislodge electrons from atoms or molecules of a gas, or Cause electrons to move into, the atoms or molecules of a gas Ions are thus formed – gas is ionised – resistance of gas is markedly decreased. 30-03-2024 Slide No. 25  IONISATION OF GASES Ionisation occurs: Across the gap of spark plugs In fluorescent and gas discharge lamps In the air path of a lightning discharge 30-03-2024 Slide No. 26  THERMIONIC EMISSION EDISON EFFECT Thomas Edison discovered principle of thermionic emission. Edison looked to keep soot from clouding his incandescent light bulb: Placed a metal plate inside bulb along with normal filament Left a gap between filament and plate Placed a battery in series between plate (+ve) and filament (-ve) Discovered current flowed across gap between filament (-ve) and plate (+ve). Current flow did not occur when polarity reversed – thus electron flow found. 30-03-2024 Slide No. 27  THERMIONIC EMISSION EDISON EFFECT Solid Conductor – Cathode Metallic conductors contain many free electrons – not bound to atoms. These free electrons are in continuous motion. The higher the temperature of conductor – more agitated are the free electrons. Temperature attained where some free electrons "boil" from conductor's surface. Similar to steam leaving surface of boiling water. Heating a conductor to cause it to give off electrons – THERMIONIC EMISSION. 30-03-2024 Slide No. 28  CONDUCTION OF ELECTRICITY IN A VACUUM Conduction of current in a vacuum is more difficult to achieve. In a vacuum – no gas molecules can liberate free electrons. Cathode (negative electrode) must be forced to release electrons – achieved by: Either heating it up (thermionic emission) or Using a high voltage to strip electrons out of it Stream of electrons will pass across tube to anode – has a high +ve potential. (25,000 volts is not uncommon in TV sets). 30-03-2024 Slide No. 29  CONDUCTION OF ELECTRICITY IN A VACUUM Thermionic emission Electrons emitted from (cathode). Electrostatic attraction Electron stream to plate (anode). 30-03-2024 Slide No. 30  CONCLUSION Now that you have completed this topic, you should be able to: 3.2.1 Describe static electricity and distribution of electrostatic charges. 3.2.2 Describe the laws of electrostatic attraction and repulsion. 3.2.3 State the units of charge and describe Coulomb’s Law. 3.2.4 Describe conduction of electricity in solids, liquids, gases and a vacuum. 30-03-2024 Slide No. 31  This concludes: Module 3: Electrical Fundaments Topic 3.2: Static Electricity and Conduction