### Summer 2011 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 92. The exam period was 75 minutes; the mean score was 55.1 the median was 55. Click here to see page1 page2 page3 of the formula sheet that came with the exam.

• 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*

A metal sphere is pulled by an electric force toward a negatively-charged Van de Graaff generator. True or False: the metal sphere must have a net positive charge on it.

(T)   True
(F)   False

### QUESTION 2***

As in lecture, a capacitor charged to 200 volts (then disconnected from the voltage source) can be connected to two light bulbs to make them glow briefly. The experiment is run twice, once with the bulbs in series, and once with the bulbs in parallel. How do the outcomes of these trials compare?

(a)   When the bulbs are in parallel, they glow more dimly, but for a longer time than the series.

(b)   When the bulbs are in series, they glow dimly and extinguish more quickly than when in parallel.

(c)   In each trial, the same amount of energy is dissipated by the bulbs.

### QUESTION 3**

Two capacitors, connected in parallel, are charged to a battery, which is then removed. Pull apart the two plates of one of the capacitors. What can we say about the voltage on the other capacitor while the plates of the first are being pulled apart?

(a)   It stays the same because the battery has been removed.
(b)   It increases because the charge in the circuit increases.
(c)   It increases because the charge on the first capacitor decreases.

### QUESTION 4*

This question and next two pertain to the following situation:

A positive charge Q1= +4 μC is located on the x-axis at x = -4 m. A negative charge Q2 = -12 μC is at the origin (x = 0, y = 0).

What is the y-component of the electric field at y = 5 on the y-axis?

(a)   5000 N/C
(b)   -3400 N/C
(c)   -3600 N/C
(d)   -5000 N/C
(e)   -16000 N/C

### QUESTION 5**

At which one of the following locations on the x-axis does the electric potential = 0?

(a)   x = -3 m
(b)   x = -2 m
(c)   x = -1 m

### QUESTION 6**

How much work must one do to bring a third charge Q3 = +9 μC from infinitely far away to the point x = 0, y = 5 m?

(a)   +0.03 J
(b)   -0.03 J
(c)   -0.14 J
(d)   -0.24 J
(e)   -0.48 J

### QUESTION 7***

This question and next two pertain to the following situation:

Three charges (Q1 = Q2 = +4 μC, Q3 = +6 μC) are located at the vertices of an equilateral triangle of side 70 cm.

What is the magnitude of the force exerted by Q1 and Q2 on Q3?

(a)   0.31 N
(b)   0.44 N
(c)   0.53 N
(d)   0.76 N
(e)   0.88 N

### QUESTION 8*

How much work was done to assemble these three charges?

(a)   0.82 J
(b)   1.18 J
(c)   820 J
(d)   180000 J
(e)   257000 J

### QUESTION 9*

In this series of problems, the three charges are all positive. Suppose the three charges were all negative (Q1 = Q2 = -4 μC, Q3 = -6 μC). How would this affect the work done to assemble the system of three charges?

(a)   In this case, the work done to assemble the charges would be the same as in the previous question.

(b)   In this case, the work done to assemble the charges would be the negative of the previous answer.

### QUESTION 10*

This question and next three pertain to the following situation:

In the circuit shown, the battery voltage ε = 20 volts, and the capacitors C1 = 70 μF, C2 = 30 μF, C3 = 21 μF, and C4 = 28 μF.

Calculate the total capacitance of this network of capacitors.

(a)   33 μF
(b)   49 μF
(c)   81 μF
(d)   100 μF
(e)   149 μF

### QUESTION 11**

Calculate the energy stored in capacitor C2.

(a)   2.94 mJ
(b)   4.2 mJ
(c)   12 mJ

### QUESTION 12***

For this and the following question, assume a dielectric of constant κ = 1.5 fills the region between the plates of capacitor C4. How would the presence of this dielectric affect the voltage across that capacitor? Calculate
Vwith - Vwithout
the difference between the voltage across C4 with and without the dielectric.

(a)   -1.9 V
(b)   -0.95 V
(c)   0
(d)   0.95 V
(e)   1.9 V

### QUESTION 13**

What effect does the dielectric (κ = 1.5) inserted inside C4 have on the charge stored on C1?

(a)   The dielectric in C4 has no effect on the charge stored in C1.
(b)   The dielectric in C4 reduced the charge stored in C1.
(c)   The dielectric in C4 increased the charge stored in C1.

### QUESTION 14**

This question and next three pertain to the following situation:

In this circuit, you are given R1 = 10 Ω, R2 = 36 Ω, R3 = 9 Ω, R4 = 36 Ω and R5 = 10 Ω. Note the arrows defining the currents I2 and I4 through resistors through R2 and R4, respectively. You are not told the battery voltage.

True or false: resistors R2 and R4 are in parallel.

(T)   True
(F)   False

### QUESTION 15*

True or false: resistors R1 and R5 are in series.

(T)   True
(F)   False

### QUESTION 16*

What total resistance is equivalent to the five resistors in this circuit?

(a)   4.48 Ω
(b)   40 Ω
(c)   60 Ω
(d)   81 Ω
(e)   101 Ω

### QUESTION 17**

Suppose I2 = 100 mA. What does I4 equal?

(a)   60 mA
(b)   80 mA
(c)   100 mA
(d)   125 mA
(e)   167 mA

### QUESTION 18**

This question and next two pertain to the following situation:

The circuit shown contains a capacitor which is initially uncharged.

Close switch S at t = 0. What current initially flows into the capacitor?

(a)   0.133 A
(b)   0.2 A
(c)   0.4 A

### QUESTION 19***

After a long time (during which the switch S stays closed), what is the charge on the capacitor?

(a)   2.4 μC
(b)   4.8 μC
(c)   8.8 μC
(d)   12.1 μC
(e)   13.2 μC

### QUESTION 20***

Having left the switch S closed for a long time, the switch is now opened. Calculate the current flowing through the 30Ω resistor 8 μs after having opened that switch.

(a)   0
(b)   0.118 A
(c)   0.139 A
(d)   0.177 A
(e)   0.2 A

### QUESTION 21***

This question and next two pertain to the circuit shown at right.

Is the equation I1 + I2 + I3 = 0 true or false?

(T)   True
(F)   False

### QUESTION 22*

Is the equation -V1 = + I1R1 - I2R2 true or false?

(T)   True
(F)   False

### QUESTION 23*

Is the equation I4R4 - I1R1 + V1 + V2 + I3R3 + I4R5 - V3 = 0 true or false?

(T)   True
(F)   False

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.