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Electricity (Part I) Electrostatics in This Lecture Electricity Matter Electric Charge Electric Charge

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Electricity (Part I) Electrostatics in This Lecture Electricity Matter Electric Charge Electric Charge In this lecture Electricity (Part I) Electricity Electric Charge Electrostatics Electrification Electrostatic Charge Electric Field Electrostatic Force Lines of Equipotential Electricity Matter • X-ray tube converts electric energy into electromagnetic energy • Matter has mass, form and energy equivalence. • Other devices convert electric energy into other forms of energy – Kinetic energy: • Matter may also have electric charge – Heat energy: – Chemical energy: Electric Charge Electric Charge • Comes in discrete quanta • Electrons are often free – Negative or positive to move form the outer – Electrons & Protons shell of one atom to – Equal in magnitude, opposite in sign another • Protons are fixed in the • In most cases the net charge of matter nucleus and are not free is neutral to move – Total negative charge balances total positive charge 1 Electrification Electrification • Material takes on net charge (+ve or –ve) • Electrification causes static (stationary) charge • Outer-shell electrons of some types of atoms are loosely bound and can easily be removed • Electrostatics is the study of • Removal of these electrons leaves the stationary electric charge material with a net positive charge • Created by contact, friction or induction Electrification Electrostatic Charge • Smallest unit of charge (electron) far Electrification occurs due to movement • Electrification occurs due to movement too small to be useful of negative charge • Units of electric charge: Coulombs (C) • Positive charge remains in material • – Charles Coulomb (1736-1806) – 1 Coulomb = 6.24 X 1018 electron charges • Transfer of electrons from one object • -16 to another causes the first object to – Electron charge = 1.6 X 10 C become positively and the second to become negatively charged Electrostatic Charge Example: An electrostatic charge is transferred • Unlike charges attract between two people after one has scuffed their feet across a nylon rug is of the order of one microcoulomb. • Like Charges repel How many electrons are transferred? 2 Electric Field Electric Field • What happens when we • An electric field is associated bring charges close with each electric charge together? • Field radiates outwards from • Similar charges +ve charges and into –ve charge • Fields in opposite direction • Charges repel each other Electric Field Electric Field • What happens when we • What happens when we bring charges close bring charges close together? together? • Like charges • Neutral objects N N • Fields in same direction • No Fields • Charges attract each • No attraction or repulsion other Q2 Electrostatic Force Coulomb’’s Law r • Governs magnitude of • Force between charges is due to electrostatic force electric field Q Q • Repulsive or attractive force known as = 1 2 F K 2 F = electrostatic force (Newtons) electrostatic force r Q = electrostatic charge (Coulombs) R = distance between charges 1 K = constant K = = 8.988 ×109 m 2C −2 • Uncharged particles have no field are 4πεo not affected by electric field • Force is directly proportional to product of electrostatic charges and inversely proportional to the square of the distance between them 3 Electric Field Point, p r Electric Field and Force • Charge Q placed in an electric field E • Magnitude of Electric Field • experiences the following force: at point, p experiences the following force: Q 1 E = electric field = × E = K Q = electrostatic charge (Coulombs) F Q E 2 R = distance between charges r K = constant • What are the units of Electric field? • Electric Field is directly proportional to the electrostatic charges and inversely proportional to the square of the distance between them Electric Potential Electric Potential • Work is done by the electric field if the • What is potential energy? electric force acting on the charge causes it to move from one point to another. – These two points differ in their electric potential. • Electric charges possess potential energy when placed in an electric field W V = Q • Units of electric potential • A charge of one Coulomb in a potential difference of 1 Volt is equivalent to 1 Joules Summary Practice Questions Electricity PAM2011: Lecture 5 Problem Sheet Solutions Electric Charge 1. If the distance between two charged bodies is doubled, what will be the change in Electric Charge electrostatic force? Electrification 2. A lightening bolt caries 50 Coulombs of charge. How many electrons is this? 3. What is the electrostatic charge of one electron? Electrostatic Charge 4. Two electrons are separated by 100 nm. What is the electrostatic force between them? Electric Field Is the force attractive or repulsive? Electric Field Electrostatic Force 5. mAs is a measure of what quantity? Lines of Equipotential 4.
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