Spring 2011 Physics 102 Hour Exam 1(26 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 114. The exam period was 90 minutes; the mean score was 71.7 the median was 77. Click here to see page1 page2 page3 of the formula sheet that came with the exam.

• 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**

A spherical conductor with net charge zero is held equidistant from two fixed charges of equal magnitude and opposite sign, as in the given figure. The conductor is then released. Which statement best describes the motion of the conductor?

(a)   The conductor moves away from point A.
(b)   The conductor does not move.
(c)   The conductor moves toward point A.

QUESTION 2*

A small metal ball of mass 3 grams is charged with 10 μC. A constant uniform electric field is generated in order to suspend the ball in air. What is the minimum field required to achieve this?

(a)   300 N/C
(b)   2940 N/C
(c)   5600 N/C

QUESTION 3***

Two dipoles are formed by gluing point charges of magnitude q = 325 nC and of opposite sign to each of two insulating rods of length d = 1.25 cm.

The dipoles are arranged so that the four point charges which compose the dipoles alternate to form a square of side length d = 1.25 cm, as in the figure.

What is the magnitude and direction of the electrostatic force on the bottom dipole (i.e., the sum of the forces felt by each charge which the dipole comprises?)

(a)   12.2 N, towards the other dipole
(b)   7.87 N, towards the other dipole
(c)   0 N
(d)   7.87 N, away from the other dipole
(e)   12.2 N, away from the other dipole

QUESTION 4***

Two charges are in the configuration indicated here. The first charge (Q1 = -1 μC) sits at the origin. The second charge (Q2 = 1 μC) sits at (x, y) = (1 m, 1 m). What is the magnitude of the electric field at the midpoint between the two charges?

(a)   0 N/C
(b)   1.3 × 104 N/C
(c)   1.8 × 104 N/C
(d)   2.5 × 104 N/C
(e)   3.6 × 104 N/C

QUESTION 5*

Consider the hydrogen atom as an electron (m = 9.1 × 10-31 kg, q = -1.6 × 10-19 C) revolving around a proton (m = 1.67 × 10-27 kg, q = 1.6 × 10- 19 C) in a circular orbit. If the radius of the orbit is 5 × 10- 11 m, what is the speed of the electron as it orbits the nucleus? Recall that the centripetal acceleration of an object in a circular orbit is ac = v2/r.

(a)   The electron is stationary.
(b)   1.9 × 105 m/s
(c)   2.2 × 106 m/s
(d)   3.4 × 106 m/s
(e)   3 × 108 m/s

QUESTION 6*

A particle with charge Q = 2 μC and mass m = 3 grams is initially at rest when it is placed in a constant uniform electric field E = 100 N/C. Assuming there are not any other forces acting on the particle, what is the speed of the particle after 10 seconds?

(a)   0.33 m/s
(b)   0.66 m/s
(c)   0.99 m/s

QUESTION 7*

Three identical capacitors are connected in series. Their equivalent capacitance is 5 picofarads (pF). If these capacitors are detached and reconnected in parallel, what will be the equivalent capacitance of this new system?

(a)   1.67 pF
(b)   5 pF
(c)   15 pF
(d)   45 pF
(e)   90 pF

QUESTION 8**

This question and next one pertain to the following situation:

Two parallel plates separated by a small distance d are given a charge Q to form a capacitor. The capacitor is not connected to a battery. The distance between the two plates is then doubled. Which one of the following is correct?

(a)   The electric field doubles, but not the voltage across the plates nor the charge on the plates.
(b)   The electric field and the voltage double, but not the charge on the plates.
(c)   The voltage doubles, but not the electric field or the charge on the plates.

QUESTION 9**

Next a dielectric is inserted between the charged parallel plates. Which one of the following statements on the potential energy of the capacitor is true?

(a)   The potential energy decreases.
(b)   The potential energy remains the same.
(c)   The potential energy increases.

QUESTION 10**

This question and next one pertain to the following situation:

Four positive charges, each with charge +Q, are placed at the corners of a square five centimeters on a side. The work done by you to assemble these charges is 100 J.

What must be the charge Q on each particle?

(a)   5.2 μC
(b)   6.2 μC
(c)   10.1 μC
(d)   12.8 μC
(e)   14.4 μC

QUESTION 11*

If these charges had all been -Q, that is, equal in magnitude but opposite in sign, what would have been the work done by you to assemble these charges?

(a)   -100 J
(b)   0 J
(c)   100 J

QUESTION 12**

A circuit contains a network of capacitors as shown.

What is the total energy stored in the capacitors in the system?

(a)   1.9 ×10-6 J
(b)   2.6 ×10-6 J
(c)   1.1 ×10-5 J

QUESTION 13**

This question and next one pertain to the following situation:

Three charges are placed on the corners of a square with sides of length 5 cm as shown.

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

(a)   -14363 V
(b)   -509 V
(c)   -255 V
(d)   420 V
(e)   1525 V

QUESTION 14*

If a charge were now placed at the upper right corner, what size charge would cause the potential at the center to equal zero?

(a)   -2 nC
(b)   -1 nC
(c)   0 nC
(d)   1 nC
(e)   2 nC

QUESTION 15*

With the switch S open, what is the current through resistance R1.

(a)
(b)
(c)

QUESTION 16***

If the current through R1 is denoted by I1(open) when the switch S is open and I1(closed) when it is closed, which one of the following statements is true? Consider R3 to be finite and non-zero.

(a)   I1(open)  <  I1(closed)
(b)   I1(open)  =  I1(closed)
(c)   I1(open)  >  I1(closed)

QUESTION 17***

If ε = 12 V, R1 = 3 Ω, R2 = 5 Ω and R3 = 2.5 Ω, what is the current through resistance R2 when the switch is closed?

(a)   4.43 A
(b)   3.12 A
(c)   2.61 A
(d)   1.20 A
(e)   0.86 A

QUESTION 18*

This question and next one pertain to the following situation:

A resistor of 5 Ω with the dimensions given above is in a circuit with a battery and light bulb.

If the power dissipated in the resistor is 5 W, what is the voltage across the resistor?

(a)   5 V
(b)   6 V
(c)   9 V

QUESTION 19*

If all the dimensions (height, width, and length) of the resistor are doubled, what will the resistance of the resistor be?

(a)   13 Ω
(b)   10 Ω
(c)   2.5 Ω

QUESTION 20**

Compute the equivalent resistance between point A and point B.

R1 = R2 = R3 = R4 = 4 Ω.

(a)   1.7 Ω
(b)   1.5 Ω
(c)   2.4 Ω
(d)   8.0 Ω
(e)   10.2 Ω

QUESTION 21*

This question and next one pertain to the following situation:

Which one of the following equations describing this circuit is incorrect?

(a)   ε1 - I1R1 - I3R3 + ε3 - I1R4 = 0
(b)   ε2 - I2R2 - I3R3 + ε3 = 0
(c)   I1 = I2 + I3

QUESTION 22***

What is the magnitude of I3?

(a)   1.06 A
(b)   2.50 A
(c)   3.33 A
(d)   4.70 A
(e)   6.23 A

QUESTION 23***

Initially the switch is open (no current flowing) and the capacitor is uncharged. At time 3 ms after the switch was closed, the current passing through the capacitor was found to be 2 mA. If R = 1.8 kΩ, what is the value of C in this configuration?

(a)   15.8 mF
(b)   10.2 mF
(c)   3.3 mF

QUESTION 24*

This question and next two pertain to the following situation:

The circuit above contains three capacitors, a resistor, battery and switch. The circuit switch starts in position A. There is no initial charge on the capacitors.

ΔV = 12.0 V, R = 200 Ω, C1 = 10.0 μF, C2 = C3 = 20.0 μF.

At t = 0, the switch is flipped to position B.

What is the charge on the capacitor C1 at t = 0 just after the switch is closed?

(a)   0 μC
(b)   80 μC
(c)   120 μC

QUESTION 25*

What is the current passing though the resistor at t = 0 just after the switch is closed?

(a)   0 mA
(b)   15.0 mA
(c)   30.0 mA
(d)   40.0 mA
(e)   60.0 mA

QUESTION 26***

What is the magnitude of charge on each plate of C1 after a long time (t)?

(a)   0 μC
(b)   48.0 μC
(c)   72.0 μC
(d)   96.0 μC
(e)   144 μC

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.