Some Old (Looks like a lot, but it's mostly review and pretty easy problems -- looks a lot like Friday's assessment *hint*:

Metric (show work):

1. Convert 32 km to meters

32 km • 1000 m/km = 32000 m

2. Convert 1,234 mm to meters

1,234 mm • 1 m/1000 mm = 1.234 m

3. Convert 127 cJ to kJ

127 cJ • 1 J/100 cJ • 1 kj/1000 j = .00127 J

Electrostatics Lab

3. Imagine you are given the following: a piece of PVC (which behaves like black plastic or amber) and a piece of acetate plastic (which behaves as rabbit fur), a rod of unknown material and piece of waxed paper. The task is to determine the charge that the waxed paper leaves on the rod. What charge is now on the electroscope?

Fur gives amber a negative charge, so acetate gives PVC a negative charge. This then transfers electrons to the electroscope, making it negative.

4. You rub the PVC with acetate then touch it to an electroscope. The leaves of the electroscope diverge and stay diverged when the PVC is put away. You then rub the unknown material with waxed paper and bring it near but not touching the electroscope. The leaves get closer together (slightly). What charge is on the unknown rod? How do you know? Describe the motion of electrons in the electroscope in each phase of the experiment.

Since the leaves get closer together, the rod must be opposite the leaves. The rod is therefore positive.

When the PVC is rubbed with acetate, electrons transfer from the acetate to the rod. When the PVC touches the electroscope some of those excess electrons transfer to the electroscope including the leaves, which having like negative charges diverge as a result of the repulsion experienced by like charged objects. Finally, when the unknown rod is rubbed with waxed paper, electrons must transfer from the rod to the waxed paper, we know this because the reason for the leaves to move together is for electrons to leave them -- which they will only do to respond to a positive rod.

Electrically charged particles

5. What particles make up a proton? What is the charge on each of those particles and on the proton (in terms of multiples of the elementary charge).

2 up and 1 down quark (2•2/3 + 1 • -1/3 = +1)

6. What particles make up a neutron? What is the charge on each of those particles and on the proton (in terms of multiples of the elementary charge).

2 down and 1 up quark (2•-1/3 + 1•2/3 = 0)

7. Discuss the composition of an ion in terms of the number of protons and electrons present.

An ion consists of one or more atoms that carries an electric charge -- the ion has one proton more or less than it has electrons.

8. Discuss how polarization makes possible the attraction of a neutral (always neutral -- it never acquires a net charge) object to a charge rod that has either positive or negative character.

The neutral object becomes polarized in one of two ways. Most easily, if it is partly conductive, electrons simply move towards a positive or away from a negative creating a large dipole across the object. Additionally, if the object is not a conductor, individual dipoles within the material can be made to align with their positive and negative ends pointing the same way to produce a net dipole for the object.

Coulomb's Law

9. What happens to the force between two particles when the distance between them is:

10a. doubled

one fourth

10b. tripled

one-ninth

10c. increased by a factor of 2.7

reduced to .137 of the original

11. What happens to the force between two charged particles when the charge of one is doubled?

doubled

12. What happens to the force between two charged particles when the charge of one is doubled and the charge of the other is taken times 2.5?

multiplied by 5

13. What happens to the force between two charged particles when the charge of each is multiplied by 5 and the distance between them is divided by 3.2?

256 times as much (5•5 /(1/3.2²) = 256

Ohm's Law

14. What current does 6 V push through an 8 Ω wire?

I = 6/8 = .75 A

15. What potential is required to produce 2 A in a 12 Ω wire?

∆V = IR = 2A •12 Ω = 24 V

16. What resistance gets .5 A of current under the influence of 5 V of potential?

R = ∆V/I = 5 V / .5A = 10 Ω

17. What is the symbol and unit of electric potential?

commonly called voltage: V measured in volts (V)

18. What is the symbol and unit of electric current?

commonly called amperage: I measured in Amperes (A)

19. What is the symbol and unit of electric resistance?

R measured in Ohms (Ω)

Resistor Networks

20. What is the resistance where a 1 Ω, 2 Ω and 3 Ω resistance are placed in series? Draw a schematic for the circuit in your notes.

1 + 2 + 3 = 6 Ω

21. What is the resistance where a 1 Ω, 2Ω and a 3 Ω resistance are placed in parallel? Draw a schematic for the circuit in your notes.

1/1 + 1/2 + 1/3 = 1/R

R = .545 Ω