### Summer 2007 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 87. The exam period was 75 minutes; the mean score was 47.3 the median was 46. Click here to see 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***

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

A charge Q1 = +2 μC is fixed at (0,+3); a second charge Q2 = -2 μC is fixed at (0,-3). There are initially no charges at points P (+3,0), A (+3,2), and B (5,0), which are labeled for the sake of reference.

Points B and P lie along the same equipotential curve.

(T)   True
(F)   False

### QUESTION 2**

To cancel any horizontal (x-direction) component of the electric field at point P, a third particle is put at point B. What can we say about that third particle?

(a)   A particle put at point B must be a positive charge: QB > 0.
(b)   A particle put at point B must be a negative charge: QB < 0.
(c)   A particle put at point B must be uncharged: QB = 0.

### QUESTION 3*

In addition to the original charges Q1 and Q2, a third charge Q3 = +4 μC is put at point A. What is the magnitude of the force experienced by a +6 μC charge located 185 meters away from the origin of this diagram? The following answers are approximate.

(a)   6 × 10-6 N
(b)   9 × 10-6 N
(c)   1.5 × 10-5 N

### QUESTION 4***

Two charges are fixed on the x-axis: Q1 = -4 μC is located at x = 0, and Q2 = +9 μC is located at x = 5 m.

A third charge will be brought onto the x-axis from infinity. To which of the following points on the x-axis could we bring that charge without doing any net work?

(a)   x = 4 m
(b)   x = 2 m
(c)   x = 1 m
(d)   x = -4 m
(e)   Need to know the value of the charge being brought from infinity.

### QUESTION 5**

To move a charge from a region of low electric potential to higher electric potential, one must necessarily do work (W > 0).

(T)   True
(F)   False

### QUESTION 6**

The following statements refer to an empty, hollow metal cylinder between the plates of a charged capacitor. Which one of them is true?

(a)   The potential inside that cylinder must be zero.
(b)   The electric field inside that cylinder must be zero.
(c)   Both (a) and (b) are true.

### QUESTION 7**

Two resistors are constructed from the same material. The first is a cylinder of length 1.5 cm and diameter 0.8 mm. The second is a cylinder of length 2.2 cm and diameter 0.5 mm. If the resistance of the first is called R1 and that of the second, R2 , find the ratio R1/R2.

(a)   0.27
(b)   0.43
(c)   0.68

### QUESTION 8*

This and the next two questions pertain to the circuit shown below:

Find the rate at which the battery delivers energy to this circuit.

(a)   2.33 W
(b)   4.66 W
(c)   5.63 W
(d)   15.9 W
(e)   18.0 W

### QUESTION 9**

Find the current through the 20 Ω resistor.

(a)   0.27 A
(b)   0.41 A
(c)   0.65 A
(d)   0.68 A
(e)   0.79 A

### QUESTION 10**

Suppose you were to replace the 40 Ω with an 80 Ω resistor. What effect would that have on your answer to the previous question?

(a)   The current through the 20 Ω resistor would decrease.
(b)   The current through the 20 Ω resistor would not change.
(c)   The current through the 20 Ω resistor would increase.

### QUESTION 11**

This and the next question pertain to the following situation:

In this circuit, the resistors are R1 = 14 Ω, R2 = 25 Ω, R3 = 8 Ω, R4 = 30 Ω, and R5 = 20 Ω. What is the resistance between points A and B?

(a)   4.3 Ω
(b)   9.8 Ω
(c)   10.9 Ω
(d)   19.5 Ω
(e)   24.2 Ω

### QUESTION 12*

The two terminals of an unknown battery are connected to points A and B. As a result, a current I = 8 mA flows through R1. What power is dissipated in R2?

(a)   0.0009 W
(b)   0.0016 W
(c)   0.02 W

### QUESTION 13*

This and the next question pertain to the following situation:

Four capacitors are connected, as shown, to an unspecified battery.

What is the total capacitance of the four capacitors, connected as shown?

(a)   5.4 μF
(b)   12 μF
(c)   27 μF
(d)   33 μF
(e)   71 μF

### QUESTION 14***

Suppose the voltage across the 18 μF capacitor is 10 volts. What is the charge on the 24 μF capacitor?

(a)   240 μC
(b)   360 μC
(c)   720 μC

### QUESTION 15***

This and the next three questions pertain to the following situation:

The 1.5 μF capacitor in this circuit is initially uncharged. The resistances are as shown in the diagram, but the battery voltage is unknown. The switch is then set to position A. After a very long time, the voltage across the capacitor is 18 volts.

What is the battery voltage?

(a)   18 V
(b)   24 V
(c)   27 V

### QUESTION 16**

After having waited a very long time, the switch is rapidly set to position B. At that instant, at what initial rate does charge flow off the top plate of the capacitor?

(a)   15 mA
(b)   30 mA
(c)   45 mA

### QUESTION 17***

Suppose that instead of setting the switch to position B, we had set the switch to position C. At what rate is energy dissipated in the 1200 Ω resistor 0.003 seconds after the switch is set to position C? (Assume there are 18 volts across the capacitor as the switch is set to C.)

(a)   2.4 mW
(b)   4.8 mW
(c)   9.6 mW
(d)   270 mW
(e)   540 mW

### QUESTION 18***

Once the capacitor has charged to 18 volts, you can either set the switch to position B or C. In which case is more energy converted to heat?

(a)   More is converted to heat if the switch is set to B.
(b)   More is converted to heat if the switch is set to C.
(c)   The same amount of energy is converted to heat in each case.

### QUESTION 19***

This and the next question pertain to the the following figure.

Two capacitors are connected to a battery, as shown. Assume initially that C1 = 6 μF, C2 = 18 μF, and Vb = 12 volts. There will be some resulting voltage across the second (C2 = 18 μF) capacitor. Now fill the region between the plates of that second capacitor with a dielectric of constant 1.67.

What happens to the voltage across that second capacitor as a result of inserting the dielectric? Compare that voltage after inserting the dielectric with the voltage before.

(a)   It decreases by two volts.
(b)   It decreases by one volt.
(c)   It does not change.
(d)   It increases by one volt.
(e)   It increases by two volts.

### QUESTION 20**

Which one of the following would have the same effect on its capacitance as inserting the specified dielectric between the plates of that capacitor?

(a)   making the plate separation 5/3 of its original value
(b)   making the plate separation 3/5 of its original value

### QUESTION 21***

This and the next two question refer to the figure below.

Referring to the above circuit, which of the following equations or statements is correct?

(a)   I1 + I2 = I3
(b)   I1 + I2 + I3 = 0
(c)   Neither the equation in (a) nor the equation in (b) is correct.

### QUESTION 22*

Referring to the above circuit, which of the following equations or statements is correct?

(a)   V4 - I3R5 = 0
(b)   V4 + I3R5 = 0
(c)   Neither the equation in (a) nor the equation in (b) is correct.

### QUESTION 23***

Referring to the above circuit, which one of the following equations is correct?

(a)   V1 - I1R1 + I1R2 - V2 = 0
(b)   V1 - I1R1 - I1R2 - V2 = 0
(c)   V1 - I1R1 + I2R2 - V2 = 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.