### Spring 2001 Physics 102 Hour Exam 1(31 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 31 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 128. When the exam was given, the mean was 93.8; the median was 97. Click here to see the formula sheet that came with the exam.

### QUESTION 1**

A positive point charge q is placed above an uncharged, conducting plate as shown to the right. What is the electric force on q ?

(a)   away from the plate
(b)   toward the plate
(c)   zero

### QUESTION 2*

This and the next question are about the following situation:

The figure to the right shows the electric field lines for two charges. The arrows have been intentionally omitted.

Compare |QA|, the magnitude of charge A, with QB the magnitude of charge B.

(a)   |QA| < |QB|
(b)   |QA| = |QB|
(c)   |QA| > |QB|

### QUESTION 3*

Charge A and charge B have the same sign.

(T)   True
(F)   False

### QUESTION 4**

A parallel plate capacitor is charged by connecting it to a 5 Volt battery. The battery is then removed. If the plates of the capacitor are now allowed to come closer together, the potential difference V across the capacitor will

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

### QUESTION 5*

This and the next question are about the following situation:

Four point charges are arranged in this configuration. All four are equal in magnitude but a and c are negative while b and d are positive.

In which direction does the electric force on charge c point due to all other charges?

(a)   up
(b)   down
(c)   zero

### QUESTION 6**

In which direction does the electric force on charge a point due to all other charges?

(a)   up
(b)   down
(c)   zero

### QUESTION 7**

This and the next question are about the following situation:

Compare VA the electric potential at point A with VB the electric potential at point B.

(a)   VA < VB
(b)   VA = VB
(c)   VA > VB

### QUESTION 8**

When a negative charge is moved from point B to point A, the electric field does

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

### QUESTION 9**

This and the next two questions are about the following situation:

Three charges are placed located as shown in the figure to the right. The grid spacing is in meters.

Calculate the x-component of the net force on the bottom charge at (0,-3) due to the two charges on the x-axis.

(a)   Fx = -4.66 × 10-2 N
(b)   Fx = -3.50 × 10-2 N
(c)   Fx = 0
(d)   Fx = +3.50 × 10-2 N
(e)   Fx = +4.66 × 10-2 N

### QUESTION 10*

Calculate the y-component of the net force on the bottom charge due to the two other charges.

(a)   Fx = -4.66 × 10-2 N
(b)   Fx = -3.50 × 10-2
(c)   Fx = 0
(d)   Fx = +3.50 × 10-2 N
(e)   Fx = +4.66 × 10-2 N

### QUESTION 11*

Calculate the electric potential at the position (0,+3) marked with an X due to all three charges. Assume that potential is zero at infinity.

(a)   -4.05 × 104 V
(b)   -1.35 × 104 V
(c)   0
(d)   +1.35 × 104 V
(e)   +4.05 × 104 V

### QUESTION 12*

This and the next three questions are about the following situation:

A capacitor array is shown in the figure to the right. Each capacitor is identical with capacitance C = 4µF. An unknown potential difference V is maintained by a power supply.

What is the equivalent capacitance of the network?

(a)   Ceq = 2.18 µF
(b)   Ceq = 4.22 µF
(c)   Ceq = 5.91 µF
(d)   Ceq = 9.63 µF
(e)   Ceq = 25.6 µF

### QUESTION 13*

If the charge on the capacitor marked #1 at the bottom of the array is determined to be 21 µC, what is the unknown potential V?

(a)   1.43 V
(b)   4.22 V
(c)   6.88 V
(d)   9.63 V
(e)   18.1 V

### QUESTION 14***

If a dielectric material with k = 8.3 is inserted between the plates of all the capacitors, the voltage across each capacitor

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

### QUESTION 15**

With the dielectric still in place, a new battery is inserted in the place of the old one. The new battery maintains a potential difference of 40 V across the array. How much energy is stored in the capacitors after they are fully charged?

(a)   Utot = 3.11 mJ
(b)   Utot = 10.6 mJ
(c)   Utot = 14.5 mJ

### QUESTION 16*

This and the next two questions are about the following situation:

How much work did it require to assemble the charge distribution shown at the right?

(a)   W = -67.2 nJ
(b)   W = -13.9 nJ
(c)   W = +48.8 nJ
(d)   W = +76.2 nJ
(e)   W = +89.3 nJ

### QUESTION 17*

What is the potential at the origin (if we define the potential at infinity to be 0)?

(a)   V(0,0) = -5.90 V
(b)   V(0,0) = -4.33 V
(c)   V(0,0) = +0.15 V
(d)   V(0,0) = +2.22 V
(e)   V(0,0) = +3.02 V

### QUESTION 18*

Which arrow best represents the direction of the net electric field at the origin?

(a)
(b)
(c)

### QUESTION 19**

This and the next three questions are about the following situation:

In this circuit, 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+) = 0
(b)   I(0+) = V/(2R)
(c)   I(0+) = V/R

### QUESTION 20**

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

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

### QUESTION 21***

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

(a)   Q(¥) = CV/2
(b)   Q(¥) = CV
(c)   Q(¥) = 2CV

### QUESTION 22**

After being closed for a long time, the switch is opened. How long does it take before the charge on the capacitor drops to 1/2 of the charge it has when the switch is opened?

(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 23*

Which one of the following is a valid Kirchhoff voltage equation for the dotted outside loop?

(a)   -E1 + I1R1 + E4 - I4R4 = 0
(b)   -E1 + I1R1 - E4 + I4R4 = 0
(c)   -E1 + I1R1 - E4 - I4R4 = 0

### QUESTION 24*

Which one of the following is a valid Kirchhoff current equation for the above circuit.

(a)   I1 + I2 + I3 + I4 = 0
(b)   I1 - I2 + I3 + I4 = 0
(c)   I1 + I2 - I3 - I4 = 0

### QUESTION 25**

Let E1 = 10 V, E2 = 15 V, R1 = 10 W, and R2 = 20 W. What is the magnitude of the potential difference between points A and B?

(a)   |VAB| = 25 volts
(b)   |VAB| = 18 volts
(c)   |VAB| = 15 volts
(d)   |VAB| = 10 volts
(e)   |VAB| = 5 volts

### QUESTION 26**

This and the next three questions are about the following situation:

Compare I2, the current through resistor 2, with I4, the current through resistor 4.

(a)   I2 > I4
(b)   I2 = I4
(c)   I2 < I4

### QUESTION 27**

Compare V1, the voltage across resistor 1, with V5, the voltage across resistor 5.

(a)   V1 > V5
(b)   V1 = V5
(c)   V1 < V5

### QUESTION 28*

How much power is delivered by the battery?

(a)   P = 3.92 W
(b)   P = 5.32 W
(c)   P = 11.9 W
(d)   P = 19.5 W
(e)   P = 24.8 W

### QUESTION 29**

What is I2, the current through resistor #2?

(a)   I2 = 0.216 A
(b)   I2 = 0.388 A
(c)   I2 = 0.459 A
(d)   I2 = 0.521 A
(e)   I2 = 0.692 A

### QUESTION 30*

This and the next question are about the following situation:

This figure should look familiar from your discussion section work! Simplify the system of capacitors and determine the total electrical energy stored.

(a)   Utot = 1.01 × 10-4 J
(b)   Utot = 2.98 × 10-4 J
(c)   Utot = 4.18 × 10-4 J
(d)   Utot = 5.39 × 10-4 J
(e)   Utot = 6.86 × 10-4 J

### QUESTION 31**

The voltage across the 8 µF capacitor is

(a)   9.52 V
(b)   11.8 V
(c)   14.1 V

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.