1. 20.2 Electrical Forces

2. What was the first method of charging an object? Friction – rubbing electrons onto one object creating a negative charge and leaving an excess amount of protons on the other object creating a positive charge

3. Charging Method #2 Conduction  Conduction is charging an object by touching a charged object to a neutral object.  Example: Electroscope from yesterday, touching a live wire

4. Charging Method #3 Induction  Induction is charging an object by bringing a charged object near a neutral object, causing the charges inside the neutral object to move.  Example: Electroscope from yesterday

5. Charging Examples – which method?

7. Induction usually causes polarization  Polarization causes a difference in charge on two sides of an object Wall

8. How do we measure the size of a charge?  Charge is measured in Coulombs (C)  Represented by the letter q  Charge of an electron (fundamental charge) q = -1.6 x 10 -19 C (proton is the same but positive)

9. Electric Force  Attractive (opposite charges)  Repulsive (same charges)  Varies inversely with the square of the distance (1/d 2 )  Varies directly with the size of the each of the charges called q (q 1 x q 2 )  A Positive force is repulsive, A Negative force is attractive F = kq 1 q 2 d2d2 F = force K = coulomb’s constant = 9 x 10 9 N m 2 / C 2 q = charge of particles d = distance

10. Electric Force vs. Gravitational force  Both are field forces  Both are inverse proportional to the square of the distance  Can be added as vectors  Electric can be Attractive or Repulsive  Gravitational force is much weaker than Electric force

11. Example Problem #1 Two positive identical charges of 6.0 x 10 -6 C are separated by a distance of 0.50m. What is the force between the charges?

12. Example Problem #2 A negative charge of -6.0 x 10 -6 exerts an attractive force of -65n on a second charge 0.05 meters away. What is the size of the second charge?

13. Problem #3 What is the force on the middle charge? F 36 = 405n F -56 = 300n

14. Problem #4 How would you find the force on the charge on the left? F 36 = 405n F -35 = -54n

15. Problem #5 How would you find the force on the charge on the right? F 65 = 300n F -35 = -54n