Todays Topics Why study electromagnetism Some electrostatic phenomena –Chapter 21: [21.1, 21.2]
Properties of Charge
Elementary Electrostatics Rubbing a balloon on a wool jumper makes the balloon attract your hair The balloon is said to be charged or to have an electric charge Similarly glass rubbed with silk/fur will become charged Charged glass will attract a charged balloon Two charged balloons will repel each other
Experiments Show…. Benjamin Franklin ( ) –Two types of charge Charles Coulomb ( ) –Coulombs Law Robert Milikan ( ) –Quantisation (1909)
Properties of Charge: Two types of charge Arbitrarily named –Positive (+ve) e.g. glass –Negative (-ve) e.g. wax & rubber
Like Charges Repel … +ve -ve
& Opposites Attract… +ve-ve+ve-ve
Properties of Charge: Charge is Quantised Whenever we measure the amount of charge we get a value that is an integer multiple of a unique number e q = N e Charge Integer Fixed number
Properties of Charge: Charge is always conserved –Charge is never created or destroyed –The process of charging is really moving charge from one place to another
Quiz Why does your hair stand on end in a lightning storm?
Quiz A: 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 Electroscope Used to detect and crudely measure charge
Earthing The Earth is a practically limitless supply (or sink) of charge
Van de Graf Generator
Different Methods for Charging Friction –e.g. rubbing a balloon with wool Conduction –e.g. touching an electroscope Induction –e.g. balloon sticking to a wall
Charging by Friction When 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
Induction Balloon on wall Wall is neutral and an insulator +ve Charges move slightly towards balloon -ve Charges move slightly away from balloon Wall is still neutral but surface has small residual charge, sufficient to hold baloon
Charging an Electroscope by Induction
Induction to Charge Object
Quiz A: the water moves away from the rod B: the water doesnt move C: the water moves towards the rod If 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?
Quiz a: A is positive b: A is negative c: A is neutral d: 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 Electricity Milikans Oil Drop experiment The Van de Graaff Generator The electrostatic Painting Photocopies and Laser Printers
Summary: Lecture 1 Equipment and Techniques –Electroscope Measures charge –Earthing Earth as a limitless supply or sink of charge –Van de Graaff Generator Generates charge Introduced the concept of charge Properties of charge –Two types +ve & -ve Like repel Opposites attract –Charge always conserved –Charge quantised Methods of charging –Friction –Conduction –Induction