### Fall 2005 Physics 102 Hour Exam 1(27 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 115. The exam period was 90 minutes; the mean score was 85.9; the median was 87. 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*

In the circuit shown below, doubling the battery voltage will double the circuit's time constant.

(T)   True
(F)   False

### QUESTION 2**

The electric field between two parallel plates of a capacitor remains constant during the charging process.

(T)   True
(F)   False

### QUESTION 3*

Which of these schematic figures best represents the electric field due to two spheres of opposite charge?

(a)
(b)
(c)

### QUESTION 4*

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

A network of capacitors is shown in the figure below. All capacitors are identical, with capacitance C = 3 μF. A battery maintains a potential difference of 2V across the network. Two of the 3 μF capacitors have been labeled, for identification in the problems.

What is the equivalent capacitance of the network?

(a)   Ceq = 1.8 μF
(b)   Ceq = 2.4 μF
(c)   Ceq = 3.6 μF
(d)   Ceq = 4.8 μF
(e)   Ceq = 6.0 μF

### QUESTION 5**

Capacitor C1 holds charge Q1 while capacitor C2 holds charge Q2. How do the charges Q1 and Q2 compare?

(a)   Q1 = Q2
(b)   Q1 > Q2
(c)   Q1 < Q2

### QUESTION 6*

A dielectric material with dielectric constant κ = 5.5 is inserted between the plates of each of the capacitors while the network remains connected to the battery. How does the charge Q1 on capacitor C1 change after the insertion of the dielectrics?

(a)   Q1 increases.
(b)   Q1 decreases.
(c)   Q1 remains the same.

### QUESTION 7**

In this circuit the voltage at the point labeled A is approximately:

(a)   7.38 V
(b)   14.22 V
(c)   17.78 V
(d)   21.33 V
(e)   24.62 V

### QUESTION 8*

In the circuit shown below, voltmeters indicate the voltage drops across the resistors.

(a)   3 V
(b)   7 V
(c)   15 V

### QUESTION 9**

This and the next question both refer to the circuit diagram and graphs shown below.

The capacitor is initially uncharged. At time t1 the switch is closed; at t2 the switch is opened. The time interval t2 - t1 is roughly the same size as RC for this circuit. Which graph best represents the current I flowing in the circuit?

(a)   graph A
(b)   graph B
(c)   graph C
(d)   graph D
(e)   graph E

### QUESTION 10**

The capacitor is initially uncharged. At time t1 the switch is closed; at t2 the switch is opened. The time interval t2 - t1 is roughly the same size as RC for this circuit. Which graph best represents the voltage VC on the capacitor?

(a)   graph A
(b)   graph B
(c)   graph C
(d)   graph D
(e)   graph E

### QUESTION 11*

This and the next question both refer to this situation:

The illustrated circuit contains a battery, a 75 watt bulb, and a 25 watt bulb. (The bulbs consume the specified power when connected to a 100 V battery.)

Which one of the following statements is correct?

(a)   The 75 watt bulb has larger resistance than the 25 watt bulb.
(b)   The 75 watt bulb has smaller resistance than the 25 watt bulb.
(c)   The resistances of both bulbs are the same.

### QUESTION 12*

What is the equivalent resistance of the circuit?

(a)   100 Ω
(b)   75 Ω
(c)   50 Ω
(d)   25 Ω
(e)   15 Ω

### QUESTION 13*

The system of charges shown below was assembled by bringing the (widely separated) charges from infinity to the positions shown. Calculate the work done to assemble this system.

(a)   0 J
(b)   -0.0255 J
(c)   +0.0975 J
(d)   +0.036 J
(e)   +0.108 J

### QUESTION 14*

Two +4 μC charges are placed as shown in the figure below. Note that both charges are positive.

What is the difference in the electric potential at points A and B, namely V(B) - V(A)?

(a)   0 V
(b)   +9000 V
(c)   +48000 V
(d)   -9000 V
(e)   -48000 V

### QUESTION 15*

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

A point charge of -2 μC is placed 1 m to the right of a neutral conducting sphere of radius 0.25 m.(Note that only one of the figures (a), (b), (c) is correct.)

Which one of the charge distributions shown in the figure best describes the charge distribution on the surface of the conductor?

(a)
(b)
(c)

### QUESTION 16*

The force between the conducting sphere and the -2 μC charge is

(a)   zero.
(b)   attractive.
(c)   repulsive.

### QUESTION 17**

Two points on the surface of the conducting sphere are labeled P1 and P2.The electric potential difference between the two points, V(P2) - V(P1), is

(a)   positive.
(b)   zero.
(c)   negative.

### QUESTION 18*

In the circuit shown below, I1 = 5 A and I2 = 3 A. The value of I3 is

(a)   2 A.
(b)   8 A.
(c)   15 A.

### QUESTION 19*

Take note of the orientation of the batteries in the following circuit. Voltmeters indicate the voltage drops across the resistors.

The current I is

(a)   5 A
(b)   15 A
(c)   60 A

### QUESTION 20**

For times earlier than t = 0 the switch in the illustrated circuit is open and the capacitor is uncharged. At t = 0 the switch is closed so that the capacitor begins charging.

At what time does the capacitor volatage VC reach 90% of the battery voltage Vbattery?

(a)   0.10 × RC
(b)   0.31 × RC
(c)   2.30 × RC
(d)   2.72 × RC
(e)   10.0 × RC

### QUESTION 21**

In the following circuit, what is the value of I3?

(a)   0.93 A
(b)   4.01 A
(c)   1.76 A
(d)   2.87 A
(e)   1.56 A

### QUESTION 22*

Two parallel plate capacitors are constructed of the same materials and have the geometry as illustrated below. The first, with capacitance C1, has plates with dimensions W and L separated by a distance d. The second, with capacitance C2, has plates with dimensions W and 3L separated by a distance 3d. Which of the following is correct?

(a)   C1 > C2
(b)   C1 = C2
(c)   C1 < C2

### QUESTION 23*

Switch S1 is closed and switch S2 is open. What single, equivalent resistor could replace all of the resistors in the circuit?

(a)   20 Ω
(b)    12 Ω
(c)   6 Ω
(d)    4 Ω
(e)    3 Ω

### QUESTION 24**

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

Two charges of +2 μC and -2 μC are separated by a distance of 1 m. The point P is equidistant to the two charges and 0.5 m above the line joining the charges, as shown in the figure below.

What is the magnitude of the force on a +10 μC charge placed at point P?

(a)   0.0 N
(b)   0.127 N
(c)   0.509 N
(d)   1.2 N
(e)   0.254 N

### QUESTION 25*

What is the direction of the force on a +10 μC charge placed at point P?

(a)   to the right
(b)   to the left
(c)   up

### QUESTION 26**

Imagine that the +2 μC, -2 μC charges are replaced with +4 μC, -4 μC charges. How does the force on a +10 μC charge at point P change?

(a)   It remains the same.
(b)   It becomes twice as large.
(c)   It becomes four times larger.

### QUESTION 27**

Two +4 μC charges are held fixed at the positions shown in the figure. Note that both charges are positive. Calculate the change in potential energy U(C) - U(A) of a 1 μC charge that is moved from A to C.

(a)   -0.036 J
(b)   0.072 J
(c)   0 J
(d)   0.144 J
(e)   -0.0191 J

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.