1. Chapter 21, Electric Charge, and electric Field

2. Charles Allison © 2000 21.1 Electric Charge, q or Q Charge comes in two types 1e = 1.6x10 -19 Coulombs (unit of charge) fundamental unit of charge Law of Conservation of Charge: The net charge on a closed system never changes. Charge can be transferred Charges are added algebraically +e-e

3. Charles Allison © 2000 21.1 Electric Charge Like charges Opposite charges Polarization +e -e repel attract 0e + + + - - - Charge separation in a neutral object

4. Charles Allison © 2000 Atom: Nucleus (small, massive, positive charge) Electron cloud (large, very low density, negative charge) 21 - 2 Electric Charge in the Atom

5. Charles Allison © 2000 Ways to charge things 1) Charge Insulators by rubbing 2) Charge Conductors by contact 0e -e +e Charge transfers -e 0e -e

6. Charles Allison © 2000 Polar molecule: neutral overall, but charge not evenly distributed 21 - 2 Electric Charge in the Atom

7. Charles Allison © 2000 21.3 Conductors and Insulators Conductors -Charges move easily -metals Insulators -Charges do NOT move easily -wood -glass -rubber

8. Charles Allison © 2000 Question Starting with a neutral rubber rod and piece of fur, you rub them together. If the rubber rod gets a charge of -7µC, what is the charge on the fur? A) There is not enough information to determine the answer B) -7 µC C) +7 µC D) -3.5 µC E) + 3.5 µC

9. Charles Allison © 2000 Question A metal sphere A has charge Q. Two other spheres, B and C, are identical to A except they have zero net charge. A touches B then the two spheres are separated. B touches C, then the two spheres are separated. What are the charges on each sphere. A) A=Q, B=0, C=0 B)A=Q/2, B=Q/2, C=Q/2 C) A=Q/3, B=Q/3, C=Q/3 D) A=Q/2, B=Q/4, C=Q/4 A B C

10. Charles Allison © 2000 Metal objects can be charged by conduction: 21-4 Induced Charge; the Electroscope

11. Charles Allison © 2000 They can also be charged by induction, either while connected to ground or not: 21-4 Induced Charge; the Electroscope

12. Charles Allison © 2000 Nonconductors won’t become charged by conduction or induction, but will experience charge separation: 21-4 Induced Charge; the Electroscope

13. Charles Allison © 2000 The electroscope can be used for detecting charge. 21-4 Induced Charge; the Electroscope

14. Charles Allison © 2000 The electroscope can be charged either by conduction or by induction. 21-4 Induced Charge; the Electroscope

15. Charles Allison © 2000 The charged electroscope can then be used to determine the sign of an unknown charge. 21-4 Induced Charge; the Electroscope

16. Charles Allison © 2000 Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them. 21-5 Coulomb’s Law

17. Charles Allison © 2000 Coulomb’s Law The magnitude of the electric force two charges exert on each other Direction: –always along a line between the two charges –Can be attractive or repulsive depending on the signs of the charges –Forces on the two charges are equal in magnitude and opposite in direction q1q1 q2q2 k = 8.99x10 +9 Nm 2 /C 2 = coulomb’s constant

18. Charles Allison © 2000 The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same. 21-5 Coulomb’s Law

19. Charles Allison © 2000 Unit of charge: coulomb, C. The proportionality constant in Coulomb’s law is then: k = 8.99 x 10 9 N·m 2 /C 2. Charges produced by rubbing are typically around a microcoulomb: 1 μC = 10 -6 C. 21-5 Coulomb’s Law

20. Charles Allison © 2000 21-5 Coulomb’s Law Example 21-2: Three charges in a line. Three charged particles are arranged in a line, as shown. Calculate the net electrostatic force on particle 3 (the -4.0 μC on the right) due to the other two charges.