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# Ch 20 & 21 review electrostatics

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### Ch 20 & 21 review electrostatics

1. 1. Electrostatics Review Glencoe Ch 20 & 21Monday, May 9, 2011
2. 2. Electric Charge, Q Electric charge is conserved. When charging occurs, it is because charges are separated, not created. Charges can be positive or negative. Opposites attract and like charges repel. The fundamental charge was measured by Millikan and is the charge of an electron (or proton) and is 1.6E-19 Coulomb. The unit of charge is the Coulomb, C.Monday, May 9, 2011
3. 3. Inductors & Conductors Charges sit on the surface of inductors (e.g. plastics) and flow through conductors (e.g. metals). YOU are a conductor to ground (an infinite sink for electrons). When a charge is INDUCED on a surface, that means like charges were driven away and opposite charges were brought to surface:Monday, May 9, 2011
4. 4. Coulomb’s Law The force of attraction between two charges varies directly with the product of their charges and inversely with the square of the distance between them: Fe = k Q1Q2 / r2 k = 9E9 N m2 / C2Monday, May 9, 2011
5. 5. Coulomb’s Law Problem What is the net force acting on charge C below? C 2 µC 5 cm B 30o A 20o 2 µC -3 µC 7.5 cmMonday, May 9, 2011
6. 6. Coulomb’s Law Problem What is the net force acting on charge C below? C 2 µC 5 cm B 30o A 20o 2 µC -3 µC 7.5 cm Answer: 58.6 N @ -19 degMonday, May 9, 2011
7. 7. Electric Field Any charge creates an electric field. E = Fe / q (Units N/C) Draw field lines from + => - It takes work to set up an electric field. W = Fe * d = Ue. Electric potential difference is V = Ue / q or V = E * d.Monday, May 9, 2011
8. 8. Capacitors Capacitors store charge. C = Q / V (Unit = Farad) C = κεo A / d (εo = permittivity constant = 8.8 E -12 F / m)Monday, May 9, 2011
9. 9. Millikan’s Oil Drop Experiment http://www.youtube.com/watch?v=XMfYHag7LiwMonday, May 9, 2011
10. 10. Ch 21 Sample Problems How much work is done on a proton to move it from a negative plate to a positive plate 4.3 cm away if the field is 125 N/C?Monday, May 9, 2011
11. 11. Ch 21 Sample Problems How much work is done on a proton to move it from a negative plate to a positive plate 4.3 cm away if the field is 125 N/C? Answer: W = Fed Fe = Eq = 125 N/C * 1.6E-19C = 2 E-17N W = 2E-17N * 0.043m = 8.6E-19 JMonday, May 9, 2011