1. Charles Allison © 2000 Chapter 21, Electric Charge, and electric Field

2. 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.

3. Charles Allison © 2000 Question An  particle with a charge +2e and a mass of 4m p is on a collision course with a proton with a charge +e and mass m p. The only force acting is the electrostatic force. Which of the following statements is true about the magnitudes of the forces on the two particles?  p Charge+2e+e Mass4m p mpmp A) F  = F p B) 2F  = F p C) 4F  = F p D) F  = 2F p E) F  = 4F p

4. Charles Allison © 2000 Question An  particle with a charge +2e and a mass of 4m p is on a collision course with a proton with a charge +e and mass m p. The only force acting is the electrostatic force. Which of the following statements is true about the accelerations of the two particles?  p Charge+2e+e Mass4m p mpmp A) a  = a p B) 2a  = a p C) 4a  = a p D) a  = 2a p E) a  = 4a p

5. Charles Allison © 2000 Problem 9 9.(II) What is the total charge of all the electrons in a 15-kg bar of gold? What is the net charge of the bar? (Gold has 79 electrons per atom and an atomic mass of 197 u.)

6. Charles Allison © 2000 21-5 Coulomb’s Law Example 21-3: Electric force using vector components. Calculate the net electrostatic force on charge Q 3 shown in the figure due to the charges Q 1 and Q 2.

7. Charles Allison © 2000 Problem 11 11.(II) Two positive point charges are a fixed distance apart. The sum of their charges is Q T.What charge must each have in order to (a) maximize the electric force between them, and (b) minimize it?

8. 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

9. 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

10. 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

11. 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

12. Charles Allison © 2000 21-6 The Electric Field Gravitational Field + -e Electric Field + + + –Units N/C –Direction of E is the direction a + test charge would go

13. Charles Allison © 2000 Problem 26

14. Charles Allison © 2000 21-6 The Electric Field Example 21-7: E at a point between two charges. Two point charges are separated by a distance of 10.0 cm. One has a charge of -25 μC and the other +50 μC. (a) Determine the direction and magnitude of the electric field at a point P between the two charges that is 2.0 cm from the negative charge. (b) If an electron (mass = 9.11 x 10 -31 kg) is placed at rest at P and then released, what will be its initial acceleration (direction and magnitude)?