### Spring 2003 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 ***.

This exam consists of 27 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 104. When the exam was given, the mean was 67.1; the median was 68. Click here to see the formula sheet that came with the exam.

### QUESTION 1***

This and the next question pertain to the following situation.

A negative charge Q1 = -q is placed at the center of a hollow metal sphere. A positive charge Q2 = +q is placed outside the sphere as shown. The sphere is ungrounded.

How do the magnitudes of the forces on the charges compare?

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

### QUESTION 2***

If the sphere is grounded, how does the force on Q2 change?

(a)   does not change
(b)   increases
(c)   decreases

### QUESTION 3***

Given the configuration of charges shown at right, what is the work done by the electric force to bring a charge of –q from infinitely far away to point X?

q = 1.2 μC and the square has sides of length 2 cm.

(a)   +0.838 J
(b)   +0.350 J
(c)   0
(d)   -0.350 J
(e)   -0.838 J

### QUESTION 4*

This and the next four questions pertain to the following situation.

Two 5 μC charges are placed at (-3, 0) and (3, 0) as shown. The grid spacing is in meters.

Calculate the magnitude of the electric force on the charge at (-3,0) due to the other charge?

(a)   F = 1.80 × 10-2 N
(b)   F = 6.25 × 10-3 N
(c)   F = 9.99 × 10-4 N

### QUESTION 5*

What is the electric potential at point A?

(a)   3.00 × 104 V
(b)   1.62 × 104 V
(c)   -1.51 × 104 V

### QUESTION 6*

What is the x-component of the electric field at point B?

(a)   -5.65 × 103 V/m
(b)   -3.34 × 103 V/m
(c)   0
(d)   +3.34 × 103 V/m
(e)   +5.65 × 103 V/m

### QUESTION 7*

Calculate the y-component of the electric field at point B.

(a)   -5.65 × 103 V/m
(b)   -3.84 × 103 V/m
(c)   0
(d)   +3.84 × 103 V/m
(e)   +5.65 × 103 V/m

### QUESTION 8***

What is the work done by the electric force when a +1 μC charge is moved along the y axis from A to C?

(a)   -2.65 × 10-2 J
(b)   -1.20 × 10-2 J
(c)   0
(d)   +1.20 × 10-2 J
(e)   +2.65 × 10-2 J

### QUESTION 9**

This and the next question pertain to the following situation.

A region in space contains a uniform electric field of Ex = 0 and Ey = 10 V/m. At t = 0 a particle of mass M = 0.0015 kg and charge Q = +530 μC enters the region with a speed of 8 m/s headed downward. Neglect gravity.

Find the y position of the particle at time t = 3 s.

(a)   y = -53.3 m
(b)   y = -26.6 m
(c)   y = -25.0 m
(d)   y = -8.10 m
(e)   y = -3.21 m

### QUESTION 10**

While the particle travels down through the electric field its potential energy

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

### QUESTION 11**

The image at right shows two regions with electric fields as labeled. Which point has the highest electric potential? Assume there are no other fields.

(a)   A
(b)   B
(c)   C

### QUESTION 12**

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

Four parallel plate capacitors are connected to a battery as shown below.

Compare Q3 the charge on capacitor C3 with Q2 the charge on capacitor C2.

(a)   Q3 > Q2
(b)   Q3 = Q2
(c)   Q3 < Q2

### QUESTION 13*

Calculate Ceq, the equivalent capacitance of the 4 capacitors in the circuit.

(a)   Ceq = 7.88 mF
(b)   Ceq = 18.1 mF
(c)   Ceq = 25.6 mF

### QUESTION 14***

Calculate U4, the energy stored in capacitor C4.

(a)   U4 = 13.2 mJ
(b)   U4 = 23.2 mJ
(c)   U4 = 43.2 mJ
(d)   U4 = 63.2 mJ
(e)   U4 = 93.2 mJ

### QUESTION 15**

When a dielectric material with dielectric constant κ = 1.8 is inserted between the plates of all of the capacitors, Q3, the charge on capacitor C3

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

### QUESTION 16**

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

In the circuit shown, the switch has been open for a long time so that the capacitor is uncharged.

What is the current through the battery immediately after the switch is closed?

(a)   I(0+) = V/(3R)
(b)   I(0+) = V/(2R)
(c)   I(0+) = V/R

### QUESTION 17*

What is the current through the battery after the switch has been closed a long time?

(a)   I(∞) = 0
(b)   I(∞) = V/(3R)
(c)   I(∞) = V/(2R)

### QUESTION 18***

What is the charge on the capacitor after the switch has been closed for a long time?

(a)   Q(∞) = CV/3
(b)   Q(∞) = 2CV/3
(c)   Q(∞) = CV

### QUESTION 19**

After being closed for a long time, the switch is opened. How long does it take for the charge on the capacitor to drop to 1/2 of the charge it had just before the switch was opened? Assume that V = 50 volts, C = 2 μF, and R = 20 Ω.

(a)   t = 13.8 μs
(b)   t = 27.7 μs
(c)   t = 55.5 μs
(d)   t = 88.1 μs
(e)   t = 113.2 μs

### QUESTION 20**

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

Twelve identical 10 ohm resistors are connected to a 12 volt battery as shown here.

Resistors R1 and R2 are in

(a)   series.
(b)   parallel.
(c)   neither

### QUESTION 21**

Calculate Ib the current through the battery.

(a)   Ib = 0.6 amps
(b)   Ib = 1.2 amps
(c)   Ib = 2.4 amps
(d)   Ib = 4.4 amps
(e)   Ib = 6.8 amps

### QUESTION 22*

Calculate P1 the power dissipated by resistor R1.

(a)   P1 = 7.2 W
(b)   P1 = 14.4 W
(c)   P1 = 28.8 W

### QUESTION 23***

What is VAB = VA - VB, the voltage difference between the points marked A and B?

(a)   VAB = 4 volts
(b)   VAB = 8 volts
(c)   VAB = 12 volts

### QUESTION 24*

This and the next question pertain to the following situation.

Two parallel plate capacitors are constructed as shown below. The first has plates with dimension W and L separated by a distance d. The second has plates with dimension W and 2L separated by a distance 2d.

Compare the capacitance of the two capacitors.

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

### QUESTION 25**

The two capacitors are now connected to a battery as shown schematically to the right. Compare |E1| the magnitude of the electric field between the plates of capacitor C1 with |E2| the magnitude of the electric field between the plates of capacitor C2.

(a)   |E1| > |E2|
(b)   |E1| = E2|
(c)   |E1| < E2|

### QUESTION 26**

This and the next question pertain to the following situation.

Four wires and three batteries are connected as shown here.

Calculate VAB = VA - VB, the potential difference between the points marked A and B.

(a)   20 V
(b)   30 V
(c)   40 V

### QUESTION 27*

Which equation is a valid Kirchhoff’s voltage law for the circuit?

(a)   I2R2 - ε1 + I1R1 + ε2 - ε3 = 0
(b)   I2R2 + ε1 + I1R1 - ε2 + ε3 = 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.