Chapter 17 Electrostatics

Review

1. What is the basic law of electrostatics?

Like charges repel, unlike charges attract.

2. What is an electroscope? What is a proof plane?

Electroscope – a device for detecting charge.

Proof plane – a metal plate on an insulating handle, used to transfer charge.

3. What particles are moved when charging an object?

Only the negatively charged electrons.

4. Describe three methods of charging an object.

1. Stripping electrons by friction.

2. Charging by conduction. 3. Charging by induction.

5. Describe how to positively charge an object by induction.

1. Hold a negative object close to the object you want to charge. 2. Ground the object you want to charge. 3. Remove the ground. 4. Remove the negative object.

6. What property of an electrical conductor makes it a good conductor?

Loosely held electrons. The electrons are not held tightly in a crystal lattice.

7. What is the rule for the residual charge of an object charged by conduction? By induction?

The residual charge produced by conduction is always the same charge as the original charge.

The residual charge produced by induction is always the opposite charge as the original charge.

8. What is the charge of an electron?

-1.6 x 10-19

coulombs

9. List two differences in electrostatic and gravitational forces.

Electrostatic forces are much stronger than gravitational forces, and they can repel.

10. Two electrostatic charges of +30.0 C and -20.0 C are 5 m apart. Calculate the force between the two charges. Is this force attractive or repulsive?

F = k q1 q2 / d2

F = (9 x 109)(30 x 10-6)(20 x 10-6)/52

F = 0.216 NThe charges are opposite so the force is attractive.

11. Two electrostatic charges of -60.0 C and -40.0 C are 55 cm apart. Calculate the force between the two charges. Is this force attractive or repulsive?

F = k q1 q2 / d2

F = (9 x 109)(60 x 10-6)(40 x 10-6)/0.552

F = 71.4 NThe charges are the same so the force is repulsive.

12. Two charges are separated by 20 cm. If they are moved to a distance of 60 cm apart, how does the electrostatic force between them change?

20 cm to 60 cmF = k q1 q2 / d2

Triple the distance means 1/9th the force.

13. Draw the lines of force around single positive charge. Around a single negative charge.

14. Draw the lines of force between two equally charged negative charges. Between two equally charged positive charges.

15. Draw the lines of force between one positive and one negative charge that are equal in magnitude.

16. A charge of 20 C experiences a force of 10 Newtons. What is the strength of the electric field?

E = F/qE = 10 N/ 20 C E = 10 N/(20 x 10 -6 C)

E = 5 x 105 N/C

17. What is the force on a +12 C charge if placed in an electric field of 4000 N/C? What is the direction of the force?

E = F/q4000 N/C = F/ +12 C 4000 N/C = F/(12 x 10 -6 C)

F = 0.048 NIn the direction of the field.

18. What is the electric field strength at a point halfway between a +10 C charge and a +30 C charge that are separated by a distance of 4 meters?

E10 = kq/d2 E30 = kq/d2

E10 = (9 x 109)(+10 x 10-6)/22

E30 = (9 x 109)(+30 x 10-6)/22

E10 = 22500 N/C E30 = 67500 N/C

The net field is 67500 – 22500 = 45000 N/C directed away from the +30 C charge.

19. What force is applied to a +20 C charge when placed halfway between the two charges listed in problem 18?

F = qEF = (20 x 10-6)45000F = 0.9 NDirected away from the +30 C charge.

20. How would the answer to problem 19 be different if it were a -20 C charge?

Same magnitude force, but the opposite direction.

21. What is the strength of the electric field at a distance of 2 m from a +40 C charge? What is the direction of the field? In what direction would the field push a -20 C charge? What is the magnitude of this force?

E = kq/d2

E = (9 x 109)(+40 x 10-6)/22 E = 90000 N/C

The direction is away from the positive charge.

A -20 C charge would be pushed in the opposite direction of the field.F = qE F = (20 x 10-6)90000F = 1.8 N