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Electricity and Magnetism

Electricity and Magnetism. Physics 102 Professor Lee Carkner Lecture 8. Electricity and Magnetism. In E&M, we will deal with forces that depend upon charge Charged particles generate E&M forces Stationary charges exert an electric force E&M forces only affect charged particles. Forces.

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Electricity and Magnetism

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  1. Electricity and Magnetism Physics 102 Professor Lee Carkner Lecture 8

  2. Electricity and Magnetism • In E&M, we will deal with forces that depend upon charge • Charged particles generate E&M forces • Stationary charges exert an electric force • E&M forces only affect charged particles

  3. Forces • The electric force is much stronger than the gravitational force • Why don’t we feel this strong force? • Most things have roughly equal numbers of positive and negative particles • Like charges repel, opposite attract • Charges want to arrange themselves so there is no force • e.g. lightning, static electric shock

  4. Plus and Minus • The basic particle of negative charge is the electron • The basic particle of positive charge is the proton • They are bound in the nucleus • Adding electrons makes something negative • All E&M forces depend on what the electrons are doing

  5. Using Electricity • Why is the electrical force important? • Convert electrical energy into work • Convert electrical energy into heat and light • Convert electrical energy into sound • Electrical energy can be very finely controlled • Computer, internet

  6. Units of Charge • The unit of charge is the Coulomb (C) • The electron and the proton have equal and opposite charges: 1 e = 1.60 X10-19 C • Charge is represented by the variable q (or sometimes Q)

  7. Electric Force F = k q1 q2/r2 • Where: • k is the Coulomb constant (8.99 X 109 N m2/C2) • q1 and q2 are the two charges (in Coulombs) • You must assign a sign to F at the end • r is the distance between them (in meters)

  8. Direction of Forces • but, • to find the direction of the force we need to use the rule: opposites attract, like repel • n.b., the direction of the force does not come out of the equation, you have to find it yourself

  9. Making Electricity • Three ways to do this • Chemically • e.g. • Magnetically • e.g. • Physically • Four basic methods: friction, conduction, induction, polarization

  10. Triboelectricity • Rubbing will transfer electrons from one substance to the other by friction • Example: Glass rubbed with silk becomes positive, rubber rubbed with fur becomes negative

  11. Triboelectric Charging e- Rubber Fur Rubbing a piece of rubber with fur transfers the electrons from the fur to the rubber due to friction.

  12. How Does Charge Move? • Conductors • When you charge a conductor, the electrons will flow through it • Other types do not allow electrons to flow (e.g. glass, rubber) • Insulators • If you charge them, the charge stays put

  13. Conduction • Both end up with the same sign charge • Example: shocking your friend

  14. Conductive Charging Neutral Metal Charged Metal e- Both Metal Rods Now Charged Charge will move from one conductor to another

  15. Induction • attract the opposite sign charges to the near end • The whole conductor has no net charge, but each end does • Will always attract the original charged object

  16. Inductive Charging Induced Charge on Metal Charged Metal A charged conductor will split the charge on a near-by conductor

  17. Induction via Grounding • If you connect a conductor to the Earth, an endless amount of charge can flow from it to the ground • If you place a charged object near a grounded conductor it will repel the same sign charges to the ground • Charge is opposite that of the inductor

  18. Inductive Charging with Grounding Charged Metal Metal becomes positive Electrons pushed to ground A charged conductor will push out the same charge to the ground, leaving the other conductor with an opposite charge

  19. Polarization • You have charged the balloon but not the wall • The negative charge on the balloon attracts the positive charged parts of the molecules of the wall, polarizing it

  20. Polarization

  21. Origins of Electricity • Moving charges by rubbing has been known since ancient times • Benjamin Franklin proposed the terms positive and negative for the two types of charge

  22. Franklin’s Kite • Franklin demonstrated that lightning is a form of electricity • He flew a kite in a thunderstorm and saw that charge flowed down the string

  23. Next Time • Read: 16.5-16.7 • Homework: Ch 16: P 8, 12, 23, 24

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