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Electricity & Magnetism

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.

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Electricity & Magnetism

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  1. Electricity & Magnetism Lecture 1: Basic Phenomena Methods of Charging

  2. Today’s Topics • Why study electromagnetism • Some electrostatic phenomena • Chapter 21: [21.1, 21.2]

  3. Lightning

  4. Properties of Charge

  5. 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

  6. Experiments Show…. • Benjamin Franklin (1706-1790) • Two types of charge • Charles Coulomb (1736-1806) • Coulomb’s Law • Robert Milikan (1868-1953) • Quantisation (1909)

  7. Properties of Charge: Two types of charge • Arbitrarily named • Positive (+ve) e.g. glass • Negative (-ve) e.g. wax & rubber

  8. +ve +ve +ve +ve -ve -ve -ve -ve Like Charges Repel …

  9. +ve -ve +ve -ve & Opposites Attract…

  10. 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 Fixed number Integer Charge

  11. 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

  12. Quiz Why does your hair stand on end in a lightning storm?

  13. Quiz Rub a balloon on your hair, the balloon attracts you hair. Is the total amount of charge in the balloon and in your hair ... • A: more than before rubbing? • B: the same as before? • C: less than before?

  14. Equipment and methods of Charging

  15. The Electroscope Used to detect and crudely measure charge

  16. Earthing The Earth is a practically limitless supply (or sink) of charge

  17. Van de Graf Generator

  18. 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

  19. 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.

  20. Charging by Conduction

  21. Induction Balloon on wall Wall is neutral and an insulator Wall is still neutral but surface has small residual charge, sufficient to hold baloon +ve Charges move slightly towards balloon -ve Charges move slightly away from balloon

  22. Charging an Electroscope by Induction

  23. Induction to Charge Object

  24. Quiz 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? • A: the water moves away from the rod • B: the water doesn’t move • C: the water moves towards the rod

  25. Quiz Object A attracts object B. If we know that B is positively charged what can we say about A? • a: A is positive • b: A is negative • c: A is neutral • d: not enough information

  26. Experiments and applications of Static Electricity • Milikan’s Oil Drop experiment • The Van de Graaff Generator • The electrostatic Painting • Photocopies and Laser Printers

  27. 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 Summary: Lecture 1

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