Spring 2005 Physics 102 Hour Exam 1
(28 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 126. The exam period was 90 minutes; the mean score was 69.0; the median was 66. Click here to see the formula sheet that came with the exam.


QUESTION 1***

Two charges are fixed at (-4, 0) and (+4, 0) as shown in the figure: q1 = +2 μC and q2 = -2 μC.

What is the magnitude of the electric field at point P [with coordinate (0, 2)] due to charges q1 and q2?

(a)   1798 N/C
(b)   1608 N/C
(c)   899 N/C
(d)   804 N/C
(e)   0 N/C


QUESTION 2*

This and the following three questions all refer to the figure at right.

Three charges are fixed at (-4,0), (+4,0), and (0,-1) as shown in the figure: q1 = +2 μC, q2 = -2 μC, and q3 = +0.5 μC.

What is the direction of the electric field at point P due to all three charges?

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


QUESTION 3*

What is the electric potential at point P due to all three charges?

(a)   11038 J/C
(b)   8041 J/C
(c)   4495 J/C
(d)   2997 J/C
(e)   1498 J/C


QUESTION 4***

What is the magnitude of the force exerted on charge q3 due to the other two point charges?

(a)   1.026 × 10-3 N
(b)   1.058 × 10-3 N
(c)   2.052 × 10-3 N
(d)   2.116 × 10-3 N
(e)   2.554 × 10-3 N


QUESTION 5***

How much work is required to move charge q1 to infinity while holding the other charges fixed in place?

(a)   2.31 × 10-3 J
(b)   4.49 × 10-3 J
(c)   6.67 × 10-3 J
(d)   8.99 × 10-3 J
(e)   0 J


QUESTION 6**

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

A hollow conducting cylinder is held fixed between two large parallel conducting planes that are also fixed in position. The left plane contains positive charge while the right plane contains an equal amount of negative charge. The total charge on the conducting cylinder is zero. Note that only one of the diagrams A, B, C, and D is correct.

Which figure shows the correct pattern for field lines?

(a)   A
(b)   B
(c)   C
(d)   D
(e)   B and D


QUESTION 7**

Which one of the following statements is true?

(a)   The charge distribution is uniformly zero everywhere on the cylinder.
(b)   There is a net negative charge on the left half of the cylinder and a net positive charge on the right half of the cylinder.
(c)   There is a net positive charge on the left half of the cylinder and a net negative charge on the right half of the cylinder.


QUESTION 8*

If a positive point charge is placed at the center of the cylinder, in which direction will it move?

(a)   to the left
(b)   to the right
(c)   The charge will not move.


QUESTION 9**

A neutral conducting sphere is placed midway between two fixed unequal point charges q1 = +4 μC and q2 = -2 μC. The sphere is free to move. In which direction do you expect it to move?

(a)   to the left
(b)   to the right
(c)   The sphere will not move.


QUESTION 10*

This and the next question both refer to the figure at right.

The battery supplies 15 W of power to the circuit. What is the value of R ?

(a)   R = 10 Ω
(b)   R = 15 Ω
(c)   R = 30 Ω
(d)   R = 45 Ω
(e)   R = 60 Ω


QUESTION 11**

Imagine that the values of all three resistors are doubled. The power provided by the battery to the circuit would

(a)   double.
(b)   remain the same.
(c)   halve.


QUESTION 12**

A doped semiconductor has a resistivity of ρ =10-3 Ωm. If a 1 micrometer thick slab of this material is to be cut into a resistor equaling 100 Ω, what dimensions should be chosen? (Note: the length-to-width-to-thickness ratio of the slab shown in the figure is not necessarily similar to the correct answer.)

(a)   w = 10 μm and L = 1 μm
(b)   w = 1 μm and L = 1 μm
(c)   w = 1 μm and L = 100 μm
(d)   w = 0.1 μm and L = 100 μm
(e)   w = 100 μm and L = 1 μm


QUESTION 13**

What is the resistance from point A to point B in this circuit?

(a)   5 Ω
(b)   10 Ω
(c)   20 Ω
(d)   40 Ω
(e)   80 Ω


QUESTION 14***

This and the following question refer to this figure:

Five capacitors are hooked across a 10 V battery as shown. What is the potential difference, VA-VB?

(a)   0.91 V
(b)   1.00 V
(c)   1.11 V
(d)   5.00 V
(e)   6.00 V


QUESTION 15*

How much energy is stored in the capacitors?

(a)   10.0 μJ
(b)   25.0 μJ
(c)   33.3 μJ
(d)   50.0 μJ
(e)   250 μJ


QUESTION 16**

This and the next question pertain to the following situation:

An isolated parallel plate capacitor contains charge ±10 μC on its plates, corresponding to a potential difference of 10 V. The initial separation between the plates is d. Later, the distance between the plates is increased by a factor of two while the charge on each plate remains unchanged.

What is the voltage between the two plates after the separation becomes 2d ?

(a)   2.5 V
(b)   5 V
(c)   10 V
(d)   20 V
(e)   40 V


QUESTION 17**

The energy stored in the charged capacitor with plate separation 2d is

(a)   greater than in the charged capacitor with plate separation d.
(b)   the same as in the charged capacitor with plate separation d.
(c)   less than in the charged capacitor with plate separation d.


QUESTION 18***

Two isolated parallel plates capacitor contain charge ±10 μC on their plates. Both capacitors have plate separation d. The plates of the capacitor drawn at the left have area A, while the plates of the capacitor on the right have area 2A.

The potential difference between the plates of the left capacitor is 10 V. Which one of these statements is true?

(a)   The energy stored by the capacitor on the right is half that stored by the capacitor on the left.
(b)   The energy stored by the capacitor on the right is the same as that stored by the capacitor on the left.
(c)   The energy stored by the capacitor on the right is twice that stored by the capacitor on the left.


QUESTION 19**

Two otherwise identical parallel plate capacitors, with area A = 1 m2 and plate separation d = 1 cm are charged to a voltage VB = 1 V. In the first capacitor the plates are separated by empty space. In the second capacitor a dielectric with κ = 2 completely fills the volume between the plates. The energy and charge of the capacitors is U1 and Q1 for case 1, and U2 and Q2 for case 2. Which one of the following statements is true?

(a)   U1 = U2 and Q1 = Q2
(b)   U1 = 2U2 and 2Q1 = Q2
(c)   2U1 = U2 and 2Q1 = Q2
(d)   U1 = 2U2 and Q1 = 2Q2
(e)   2U1 = U2 and Q1 = 2Q2


QUESTION 20***

After the switch is closed (so that current flows through the resistor R1), the bulb’s brightness

(a)   decreases since the current from the battery splits between the resistor and the bulb.

(b)   remains the same since the battery maintains a constant voltage across the bulb.

(c)   increases since the battery drives more power into the circuit after the switch closes.


QUESTION 21***

After the switch is closed (so that current flows through the resistor), the battery’s current

(a)   decreases since the current passes in parallel through both the resistor and the bulb.

(b)   remains the same since the battery maintains a constant voltage across the bulb.

(c)   increases since the battery drives more power into the circuit after the switch closes.


QUESTION 22*

In the circuit shown above, the switch is initially closed so that the capacitor is fully charged. At time t = 0 the switch is opened, disconnecting the battery. An instant after the switch is opened, the brightness of the bulb

(a)   decreases noticeably since no current flows from the battery through the bulb.

(b)   is almost unchanged since voltage across the bulb is now provided by the capacitor.

(c)   increases noticeably since the capacitor is now free to discharge through the bulb.


QUESTION 23**

In the circuit shown above, the switch is initially closed so that the capacitor is fully charged. At time t = 0 the switch is opened, disconnecting the battery. At time T the current Ibulb has fallen to half the value it had immediately after the switch was opened. The value of T is

(a)   < RbulbC1
(b)   = RbulbC1
(c)   > RbulbC1


QUESTION 24**

In the circuit shown above, the switch is initially closed so that the capacitor is fully charged. At time t = 0 the switch is opened, disconnecting the battery. At time T the energy U stored in the capacitor has fallen to one quarter the value it had immediately after the switch was opened. The value of T is

(a)   > RbulbC1
(b)   = RbulbC1
(c)   < RbulbC1


QUESTION 25***

In the circuit shown here, the switch is initially open. At time t = 0 the switch is closed. After the switch is closed, 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 26*

Referring to the circuit above, which one of the following equations is correct?

(a)    Vbattery 1 - Vbattery 2 = I3R3
(b)   I1R1 + I2R2 + I3R3 = I1Vbattery 1 + I2Vbattery 2
(c)   I1 + I2 = I3


QUESTION 27***

In the circuit shown at right, the switch is initially open. At time t = 0 the switch is closed. Which of the above graphs best represents the voltage at point A as a function of time?

(a)   
(b)   
(c)   


QUESTION 28***

Two similar circuits are shown in the above figure. Note that the battery voltages and capacitances in the two circuits are different. For times t < 0, switch s1 is closed and switch s2 is open in both circuits. At t = 0, s1 is opened and s2 is closed in both circuits. The time for which the voltages at the points labeled A in the circuits will be the same is

(a)    t = 0
(b)    t = 6.93 × 10-3 seconds
(c)    t = 13.86 × 10-3 seconds
(d)    t = 15.00 × 10-3 seconds
(e)   The voltages are never the same for finite values of t.