### Summer 2001 Physics 102 Hour Exam 1(21 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 ***.

This exam consists of 21 questions; 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 105. When the exam was given, the mean was 75.0; the median was 75. Click here to see the formula sheet that came with the exam.

### QUESTION 1**

Two uncharged metal spheres, L and M, are in contact. A negatively charged rod is brought close to, but not touching L as shown. The two spheres are slightly separated and the rod is then withdrawn. As a result, the spheres are charged:

(a)   both neutral
(b)   both positive
(c)   both negative
(d)   L negative and M positive
(e)   L positive and M negative

### QUESTION 2*

Three charges of equal magnitude are positioned as shown, with Q3 equidistant from Q1 and Q2. Q1 and Q3 are positive charges; Q2 is negative. What direction is the force on charge Q3?

(a)   left
(b)   right
(c)   up
(d)   down
(e)   the force is zero

### QUESTION 3*

The units of an electric field intensity are:

(a)   N C2
(b)   C/N
(c)   N
(d)   N/C
(e)   N/m2

### QUESTION 4**

This and the following three questions concern the same physical situation.

Four charges are fixed at the corners of a square of sides 4 m as shown.

What is the electric potential at the center of the square?

(a)   - 46.63 N-m/C
(b)   - 19.1 N-m/C
(c)   0 N-m/C
(d)   + 19.1 N-m/C
(e)   + 46.3 N-m/C

### QUESTION 5**

What is the magnitude of the electric field at the center of the square?

(a)   7.0 N/C
(b)   13.5 N/C
(c)   18.7 N/C
(d)   27.5 N/C
(e)   42.0 N/C

### QUESTION 6*

What is the direction of the force on charge Q1 due to charges Q2, Q3, and Q4 ?

(a)   along the vertical
(b)   along the horizontal
(c)   along the diagonal

### QUESTION 7***

What is the magnitude of the force on charge Q1 due to charges Q2, Q3, and Q4 ?

(a)   0.42 × 10-8 N
(b)   0.59 × 10-8 N
(c)   1.19 × 10-8 N
(d)   1.93 × 10-8 N
(e)   2.45 × 10-8 N

### QUESTION 8***

The charge on a glass rod which has been rubbed with silk is called ‘positive'

(a)   by arbitrary convention.
(b)   because the proton charge is positive.
(c)   because like charges repel.
(d)   because glass is an insulator.
(e)   because silk is conducting.

### QUESTION 9*

This and the following two questions concern the same physical situation.

A small particle with charge +15e (e = 1.6x10-19 C) is located at the origin, a distance of 5 nm from a second particle of charge –30e.

What is the potential energy of the two-charge system?

(a)   -6.90 × 10-9 J
(b)   -8.98 × 10-12 J
(c)   -3.33 × 10-15 J
(d)   -2.07 × 10-17 J
(e)   -4.15 × 10-19 J

### QUESTION 10**

At what distance from the +15e particle along the x-axis is the electric potential equal to zero?

(a)   - 2.50 nm
(b)   2.89 nm
(c)   3.71 nm
(d)   7.35 nm
(e)   - 5.00 nm

### QUESTION 11***

A third particle of charge –5e is placed on the x-axis at the location –5 nm. How much work is required to move this particle to the position d = +5/sqrt(3) nm on the y-axis?

(a)   0 J
(b)   6.65 × 10-19 J
(c)   9.56 × 10-19 J
(d)   1.24 × 10-18 J
(e)   5.18 × 10-18 J

### QUESTION 12*

This and the following two questions concern the same physical situation.

Four capacitors are connected as shown below. A 15 V battery is centered in the circuit. Assume that all of the capacitors are fully charged and that no current is flowing.

What is the charge on one of the plates of the 4 mF capacitor?

(a)   Q4 = 15 mC
(b)   Q4 = 30 mC
(c)   Q4 = 60 mC

### QUESTION 13*

Compute the total stored energy in the capacitors.

(a)   Utot = 0.054 × 10-3 J
(b)   Utot = 0.420 × 10-3 J
(c)   Utot = 0.875 × 10-3 J
(d)   Utot = 1.125 × 10-3 J
(e)   Utot = 2.660 × 10-3 J

### QUESTION 14**

If you place a dielectric with k = 7.5 between the plates of the 1 mF capacitor (at the top), the charge on the other three (unmodified) capacitors would

(a)   increase.
(b)   decrease.
(c)   remain the same.

### QUESTION 15**

Compute the equivalent capacitance between the points A and B of the following network of capacitors.

(a)   Ceq = C0 / 4
(b)   Ceq = C0 / 2
(c)   Ceq = C0
(d)   Ceq = 2C0
(e)   Ceq = 5C0

### QUESTION 16*

This and the following two questions concern the same physical situation.

An isolated parallel-plate capacitor has area A = 3.0 × 10-4 m2 and plate separation d = 2 × 10-5 m. The charge on each plate has magnitude Q = 11 nC. The capacitor is initially filled with air (k = 1).

Compute the capacitence of the capacitor.

(a)   C = 2.31 × 10-9 F
(b)   C = 4.32 × 10-8 F
(c)   C = 9.06 × 10-6 F
(d)   C = 0.4 × 10-9 F
(e)   C = 0.133 × 10-9 F

### QUESTION 17*

Compute the energy stored in the capacitor.

(a)   U = 2.31 × 10-8 J
(b)   U = 4.32 × 10-8 J
(c)   U = 9.06 × 10-8 J
(d)   U = 1.51 × 10-7 J
(e)   U = 4.56 × 10-7 J

### QUESTION 18**

The distance between the plates is now increased to twice its present value. The potential difference between the plates

(a)   increases by a factor of 2.
(b)   decreases by a factor of 2.
(c)   remains the same.

### QUESTION 19**

Two capacitors, one of capacitance C and the other of capacitance 2C are connected to a battery as shown below. Compare Q2 the charge on the positive plate of capacitor 2C with Q1 the charge on the positive plate of capacitor C.

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

### QUESTION 20*

This and the following question concern the same physical situation.

An electronic circuit is constructed as shown in the figure. The switch S is open.

The effective resistance of the circuit is

(a)   95 W
(b)   80 W
(c)   35 W

### QUESTION 21**

The ammeter reads 2.0 A. If S is closed, the ammeter reading will

(a)   increase.
(b)   decrease.
(c)   remain the same.

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.