Electricity & Magnetism
Lecture 1: Basic PhenomenaMethods of Charging
Todays TopicsWhy study electromagnetismSome electrostatic phenomenaChapter 21: [21.1, 21.2]
Properties of Charge
Elementary ElectrostaticsRubbing a balloon on a wool jumper makes the balloon attract your hairThe balloon is said to be charged or to have an electric chargeSimilarly glass rubbed with silk/fur will become chargedCharged glass will attract a charged balloonTwo charged balloons will repel each other
Experiments Show.Benjamin Franklin (1706-1790)Two types of chargeCharles Coulomb (1736-1806)Coulombs LawRobert Milikan (1868-1953)Quantisation (1909)
Properties of Charge: Two types of charge Arbitrarily named Positive (+ve) e.g. glassNegative (-ve) e.g. wax & rubber
Like Charges Repel
& Opposites Attract
Properties of Charge: Charge is QuantisedWhenever we measure the amount of charge we get a value that is an integer multiple of a unique number eq = N e
Properties of Charge: Charge is always conserved
Charge is never created or destroyedThe process of charging is really moving charge from one place to another
QuizWhy does your hair stand on end in a lightning storm?
QuizA: more than before rubbing?B: the same as before?C: less than before?
Rub a balloon on your hair, the balloon attracts you hair. Is the total amount of charge in the balloon and in your hair ...
Equipment and methods of Charging
The ElectroscopeUsed to detect and crudely measure charge
EarthingThe Earth is a practically limitless supply (or sink) of charge
Van de Graf Generator
Different Methods for ChargingFrictione.g. rubbing a balloon with woolConductione.g. touching an electroscope Inductione.g. balloon sticking to a wall
Charging by FrictionWhen two different insulators are rubbed together, electrons can be transferred from one insulator to the other. The body which has gained electrons has a negative charge and the one which lost electrons has a positive charge of equal magnitude.This process is called charging by friction.
Charging by Conduction
InductionBalloon on wallWall is neutral and an insulator+ve Charges move slightly towards balloon-ve Charges move slightly away from balloonWall is still neutral but surface has small residual charge, sufficient to hold baloon
Charging an Electroscope by Induction
Induction to Charge Object
QuizA: the water moves away from the rod B: the water doesnt moveC: the water moves towards the rodIf a positively charged rod is brought near a trickle of water the water moves towards it. What happens if we use a negatively charged rod?
Quiza: A is positiveb: A is negativec: A is neutrald: not enough information
Object A attracts object B. If we know that B is positively charged what can we say about A?
Experiments and applications of Static ElectricityMilikans Oil Drop experimentThe Van de Graaff Generator The electrostatic Painting Photocopies and Laser Printers
Summary: Lecture 1Equipment and TechniquesElectroscope Measures chargeEarthing Earth as a limitless supply or sink of chargeVan de Graaff Generator Generates chargeIntroduced the concept of charge
Properties of chargeTwo types +ve & -veLike repelOpposites attractCharge always conservedCharge quantisedMethods of chargingFrictionConductionInduction