***Note about bridges. After Tuesday I will approach the top ten noodle bridge builders and twist arms to participate in the next round of qualifying for the IIT contest. Anyone else who strongly desires to continue participating is invited -- consider only that the bundle of wood we give you does come at a price and that if you accept it you have committed yourself to continued participation.

Electrostatics

Read chapter 33.

1. The following series of events occurs:

• A piece of amber is charged by rubbing it with a piece of rabbit fur
• The amber is brought near, but not touching an initially neutral electroscope (the leaves are seen to diverge)
• The electroscope is grounded without moving the amber away (the leaves are seen to converge)
• The ground is removed
• The amber is moved to a distance (the leaves once again diverge)

Explain, by tracing the motion of electrons into or out of the leaves and into and out of the knob, what is happening in each of the stages above. What is the final charge causing the leaves to diverge at the end of the sequence of events?

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

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

• The distance between them doubles
• The distance between them is halved
• The magnitude of one charge is tripled
• The magnitude of one charge is tripled while the magnitude of the other is halved
• The magnitude of each charge is tripled while the distance between them is halved

Metric conversions

(This week we'll assess on any conversion from centi, milli, kilo to and from base units and from each other.)

4. How many meters in a kilometer?

5. How many centigrams in a kilogram?

6. How many kilocoulombs in a centicoulomb?

7. Convert 476 cm to meters. To kilometers.

8. Convert .0087 kg to grams. To centigramss.

9. How many milligrams in 76.8 kg?

Electric fields

A positive 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:

10. Directly east of the point charge?

11. Directly north of the point charge?

12. Directly behind of the point charge?

13. Directly underneath the point charge?

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?

15. Directly north of the point charge?

16. Directly behind of the point charge?

17. Directly underneath the point charge?

Two capacitor plates are given equal magnitudes of opposite charges and position near each other and parallel to each other. The positive plate is position vertically above the negative plate.

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

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

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

The work done to move an electric charge in an electric field has to do not with the change in position of the charge, but with the change in potential (∆V) that it experiences. Work equals the product of the magnitude of the charge and the change and potential that it experiences (W = q∆V).

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

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