### Summer 2010 Physics 102 Hour Exam 1(23 questions)

The grading button and a description of the scoring criteria are at the bottom of this page. Basic questions are marked by a single star *. More difficult questions are marked by two stars **. The most challenging questions are marked by three stars ***.

True-false questions are worth 2 points each, three-choice multiple choice questions are worth 3 points each, five-choice multiple choice questions are worth 6 points each. The maximum possible score is 95. The exam period was 75 minutes; the mean score was 66.4 the median was 66. Click here to see page1 page2 page3 of the formula sheet that came with the exam.

· A physics 102 light bulb acts just like a resistor: its resistance is constant, independent of the current flowing through the light bulb. The bulb's brightness increases with increasing current.

· A reminder about prefixes: p (pico) = 10-12; n (nano) = 10-9; μ (micro) = 10-6; m (milli) = 10-3; k (kilo) = 10+3; M or Meg (mega) = 10+6; G or Gig (giga) = 10+9.

### QUESTION 1*

An electrically neutral metal sphere hangs near a van de Graaff generator. The generator develops a large negative charge, but assume there are no sparks.

Does the generator attract the neutral hanging sphere?

(a)   Yes, the van de Graaff generator attracts the hanging metal sphere.
(b)   No, the van de Graaff generator does not attract the hanging metal sphere.

### QUESTION 2*

You try to use a van de Graaff generator to charge, by induction, a nearby metal sphere, which is connected by wire to the ground. What do the negative charges on the generator do to the nearby sphere?

(a)   They induce negative charge on the grounded sphere.
(b)   They induce positive charge on the grounded sphere.
(c)   They cannot induce a charge on the neighboring sphere as long as that sphere is grounded.

### QUESTION 3*

This question and the next two pertain to the following situation:

Positive charges, each equal to = 4 nC, are located at (x = -5 m, y = 12 m) and (x = 5 m, y = 12 m); and a negative charge q = -4 nC is located at (x = 0, y = 12 m).

At the origin, the x-component of the electric field Ex equals:

(a)   0.14 N/C
(b)   0.086 N/C
(c)   0
(d)   -0.086 N/C
(e)   -0.14 N/C

### QUESTION 4**

At the origin, the y-component of the electric field, Ey, equals:

(a)   0.14 N/C
(b)   0.086 N/C
(c)   0
(d)   -0.086 N/C
(e)   -0.14 N/C

### QUESTION 5**

How much work was done to assemble these 3 charges? Assume they started infinitely far away from each other.

(a)   43.2 nJ
(b)   10.1 nJ
(c)   0
(d)   -10.1 nJ
(e)   -43.2 nJ

### QUESTION 6**

This question and the next one pertain to the following situation:

A positive charge q1 = 8 nC is located at the origin. A second positive charge, q2 = 5 nC, is located on the x-axis, at x = 10 cm.

There is only one point on the x-axis at which the net electric field equals zero. Where is that point located? Find the formula which that location (labeled by x) obeys. (All lengths are in cm.)

(a)   x < 0
(b)   0 < x < 5
(c)   x = 5
(d)   5 < x < 10
(e)   x > 10

### QUESTION 7*

Calculate the electric potential at the point x = 15 cm on the x-axis.

(a)   148 V
(b)   212 V
(c)   1380 V

### QUESTION 8**

This question and the next two pertain to this situation.

the circuit shown, the battery voltage ε = 9 volts, and the capacitors C1 = 6 μF, C2 = 27 μF and C4 = 15 μF.

As of now, we are not told the value of C3. Nevertheless, what can we say about Q2 (the charge on C2) versus Q3 (the charge on C3)?

(a)   Q2 is greater than Q3.
(b)   Q2 is not greater than Q3.
(c)   We need to know the value of C3 before we can answer this question.

### QUESTION 9*

Assume now that C3 = 18 μF. How much charge flowed through the battery as the circuit charged up? (Assume all the capacitors were initially uncharged.)

(a)   54 μC
(b)   68 μC
(c)   144 μC
(d)   188 μC
(e)   297 μC

### QUESTION 10**

A dielectric (κ = 1.5) is inserted inside C4. Assume that the dielectric fills completely the space between the plates. What is the charge on that capacitor with the dielectric in place? (You should once again assume C3 = 18 μF.)

(a)   54 μC
(b)   81 μC
(c)   146 μC

### QUESTION 11**

Compare the capacitance of circuit A (CA) to the capacitance of circuit B (CB). How do they compare?

(a)   CA = CB
(b)   CA > CB
(c)   CA < CB

### QUESTION 12**

As in lecture: a charged capacitor is connected to an electroscope, which is a pair of hanging gold strips intended to detect charge.

As the plates of the capacitor are pulled apart, what happens to the hanging gold strips?

(a)   They collapse (fall together).
(b)   They fly apart.

### QUESTION 13**

Calculate the resistance of this circuit.

(a)   3 Ω
(b)   4.8 Ω
(c)   7.2 Ω
(d)   12 Ω
(e)   20 Ω

### QUESTION 14**

This question and next two pertain to the following situation:

In this circuit, you are given:

R1 = 15 Ω
R2 = 20 Ω
R3 = 20 Ω
R4 = 5 Ω
R5 = 35 Ω

You are also told that a current I1 = 0.4 A flows downward through R1.

Calculate the battery voltage.

(a)   2 V
(b)   3 V
(c)   4 V
(d)   5 V
(e)   6 V

### QUESTION 15*

Calculate the total resistance equivalent to the five resistors shown.

(a)   7.5 Ω
(b)   22 Ω
(c)   25 Ω
(d)   30 Ω
(e)   95 Ω

### QUESTION 16**

Replace the original (unknown) battery with a 12 volt source. Calculate the voltage across R4 with this new battery in place.

(a)   1 V
(b)   4 V
(c)   8 V

### QUESTION 17**

This question and next two pertain to the following situation:

These three questions refer to three different experiments conducted with the circuit shown. In each, assume the capacitor starts uncharged.

Close both switches at t = 0. After a long time has elapsed, what is the current through the 12 Ω resistor?

(a)   0
(b)   0.5 A
(c)   1.25 A

### QUESTION 18**

Again, assume the capacitor starts without charge. Leave open S2 but close S1. After a long time has elapsed, open S1 and then close S2. At the instant at which S2 closes, what initial current flows through the 18 Ω resistor?

(a)   0
(b)   0.5 A
(c)   0.83 A
(d)   1.25 A
(e)   2.08 A

### QUESTION 19**

Once again, assume the capacitor starts uncharged. Leave S2 open. At t = 0, close S1. What is the charge on the capacitor at t = 5 μs?

(a)   7.05 μC
(b)   6.10 μC
(c)   4.14 μC
(d)   3.15 μC
(e)   2.10 μC

### QUESTION 20***

This question and next three pertain to the following diagram:

True or false: resistors R2 and R3 can be combined into a single resistor because they are in series.

(T)   True
(F)   False

### QUESTION 21**

True or false: resistors R5 and R6 can be combined into a single resistor because they are in parallel.

(T)   True
(F)   False

### QUESTION 22**

Which one of the following equations is true?

(a)   I4 = I3 + I2
(b)   I1 + I2 = I3 + I4
(c)   I1 + I2 = I4 - I3

### QUESTION 23*

Which one of the following equations is false?

(a)   -V2 - I2R4 + I3R5 = 0
(b)   I3R5 + I4R6 - V3 = 0
(c)   V3 + I4R6 - V2 - I2R4 = 0

Scoring of True-False (T,F) Questions:
• If you mark the correct answer, you get 2 points.
• If you mark the wrong answer or if you mark neither answer, you get 0 points.
Scoring of Multiple Choice I (a,b,c) Questions:
• If you mark the correct answer, you get 3 points.
• If you mark a wrong answer or if you mark none of the answers, you get 0 points.
Scoring of Multiple Choice II (a,b,c,d,e) Questions:
• If you mark one answer and it is the correct answer, you get 6 points.
• If you mark two answers and one of them is the correct answer, you get 3 points.
• If you mark three answers and one of them is the correct answer, you get 2 points.
• If you do not mark the correct answer, or if you mark more than three answers, or if you mark none of the answers, you get 0 points.