Electrostatics

Read chapter 33.

1.

In the first instance we use B. Franklin's definition of negative charge and find that electrons were transferred from rabbit fur to amber in order to make the amber carry a negative charge. When the amber is brought near, electrons a repelled from the knob and into the leaves (making them repel). When the electroscope is grounded the electrons are repelled from the knob into the ground instead. When the ground is opened the electrons are unable to return when the amber is removed, so that when the electrons redistribute they demonstrate a net positive charge remaining.

2. Use Coulomb's Law to determine the amount of force between two electrons 25 nm apart (1 nm = 1 x 10^-9 m).

F = kq1q2÷d² = 3.69 x 10^-13 N

3. What happens to the force between two charged objects in each of the following events:

• The distance between them doubles (1/4th the force)
• The distance between them is halved (4 times the force)
• The magnitude of one charge is tripled (3 times the force)
• The magnitude of one charge is tripled while the magnitude of the other is halved (force x 1.5)
• The magnitude of each charge is tripled while the distance between them is halved (force x 9/4)

Metric conversions

4. How many meters in a kilometer? 1000 m

5. How many centigrams in a kilogram? 100,000 cg

6. How many kilocoulombs in a centicoulomb?.00001 kC

7. Convert 476 cm to meters. To kilometers. 4.76 m, .00476 km

8. Convert .0087 kg to grams. To centigrams. 8.7 g, 870 cg

9. How many milligrams in 76.8 kg? 76,800,000 mg

Electric fields

10. Directly east of the point charge? East

11. Directly north of the point charge? North

12. Directly behind of the point charge? Back (towards "behind")

13. Directly underneath the point charge? Down

A negative charge that occupies a small point in space (considered a "point charge") is studied by placing a test charge in various places around it. What is the direction of the field at a point:

14. Directly east of the point charge? west

15. Directly north of the point charge? south

16. Directly behind of the point charge? ahead

17. Directly underneath the point charge? up

18. What direction does a proton placed between the plates travel?

repelled by positive and attracted by negative it moves down towards the positive

19. What direction does an electron placed between the plates travel?

repelled by negative and attracted by positive it moves up towards the negative

20. An electron is about 1/2000th the mass of a proton. Which experiences more acceleration? Why?

Coulomb's law shows equal force against each in the direction of the motion. Equal force against 1/2000th of the mass means 2000 times as much acceleration.

21. How much work is required to move a 3 C charge from a 6V potential to the ground (0 V)?

W = q ∆V = 3 C • 6 V = 18 J (note that a Volt is the same as a Joule/Coulomb)

22. How much work is required to move an alpha particle (2 protons and 2 neutrons) across a 50,000 V potential?

q = 2 • 1.6x10^-19 = 3.2x10^-19 C so work is 50,000 V • 3.2x10^-19 C = 1.6x10^-14 J