# Electricity & Magnetism

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Electricity & Magnetism. Lecture 1: Basic Phenomena Methods of Charging. Today’s Topics. Why study electromagnetism Some electrostatic phenomena Chapter 21: [21.1, 21.2]. Lightning. Properties of Charge. Elementary Electrostatics. - PowerPoint PPT Presentation

### Text of Electricity & Magnetism

• Electricity & Magnetism

Lecture 1: Basic PhenomenaMethods of Charging

• Todays TopicsWhy study electromagnetismSome electrostatic phenomenaChapter 21: [21.1, 21.2]

• Lightning

• 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

ChargeIntegerFixed number

• 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

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