10.1-10.2 Static Electricity & Xerographic Copiers

1. 10.1-10.2 Static Electricity & Xerographic Copiers

2. Ideas for today: • Charge and Static Electricity • Electric fields and electric potential • Photocopiers

3. Observations About Static Electricity • Static electricity builds up on non-conductive things • Objects with static charge either cling or repel • Static electricity can lead to shocks • Static electricity can make your hair stand up

4. Pith ball Electroscope Ben Franklin named charge as “positive” and “negative” protons, positrons,… Charge comes in fundamental units of 1.6 · 10-19 Coulombs + + + Charge is conserved! - - - electrons , muons,…

5. Van de Graaff Forces Between Charges Charge Dependence Distance Dependence F 1/r2 Like Charges Repel Opposite Charges Attract

6. Net Charge • Net charge is the sum of an object’s charges • Most objects have zero net charge (neutral) • Neutral objects contain equal + & – charges - - - + + + + + + + - + - - - - + + - + - - + - before after

7. Polarizing Objects: Balloons Click me

8. Clicker Question What would happen if the force between charges did not depend on distance? A. Static cling would get stronger. B. Static cling would get weaker. C. Static cling would disappear. D. Nothing would change in the world.

9. The Xerographic Process An terrific use for static electricity What’s up with that Corona Wire?

10. What do lightning rods do?

11. There is a different way to think about forces on charges… • Electric fields • Potential

12. Electric Fields Two views of charge forces: • Charge/Charge: • Charge 1 pushes directly on Charge 2 • Charge/Field/Charge: • Charge 1 creates an Electric Field • Electric Field pushes on Charge 2 Electric Fields are Real!

13. Electric Fields

14. Electric Fields Electric field lines • The magnitude of the electric field is proportional to the magnitude of the force on a positive test charge • The direction of the field is the direction of the force on a positive test charge

15. Voltage • Charge has electrostatic potential energy (EPE) • Voltage measures the EPE per unit of charge • Raising the voltage of a positive charge takes work • Lowering the voltage of a negative charge takes work • Voltage is measured in joules/coulomb or volts

16. Voltage

17. Voltage Gradients & Electric Fields • Objects accelerate to reduce potential energy • A charge • has electric potential energy proportional to voltage • so it accelerates toward lower voltage • but it also accelerates because of anelectric field • Voltagegradients are electric fields!

18. Clicker Question: If you separate two oppositely charged balloons, • the voltage of the positively-charged balloon does not change. • the voltage of the positively-charged balloon increases. • the voltage of the positively-charged balloon decreases. + - - + - + + - +

19. Charges In and Around Metals Faraday sphere • Charges can move inside metals • They move to minimize potential energies • They give the metal a uniform voltage • There is thus no electric field inside a metal • However, outside metals • Charges can’t move so easily • so voltages can vary with location • and there can thus be electric fields

20. Wimshurst generator Getting shocked Click me

21. Rapidly changing potential means very large electric fields / forces!

22. Corona Discharges Plasma globe • Outside a sharp or narrow charged metal • the electric field is very strong • charges are pushed onto passing air particles: corona discharge • Corona discharges can dissipate static electricity

23. Clicker question What is the purpose of a lightning rod? • To attract lightning to itself • To create a large electric field and dissipate charge from the building • To repel lightning from the building

24. The Xerographic Process An terrific use for static electricity What’s that photoconductor stuff?

25. Photoconductors Photoconductor • In the dark, a semiconductor is insulating • In the light, a semiconductor may conduct

26. Photocopier!

27. And laser printers…

28. For next class: Read Section 10.3 See you next class!