### Fall 2009 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 124. The exam period was 90 minutes; the mean score was 86.3 the median was 88. 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 uniform electric field throughout a region is pointing straight and to the right. If an electron is released from rest within the field, which way will it move?

(a)   right
(b)   left
(c)   down

### QUESTION 2*

Resistor R1 has a length L = 1 m and a cross section A = 1 cm2. Resistor R2 has a length L = 2 m and a cross section A = 3 cm2. Both resistors are made of the same material.

What is the ratio of the resistances R1 / R2 ?

(a)   4/9
(b)   2/3
(c)   1
(d)   3/2
(e)   9/4

### QUESTION 3**

This question and the next one pertain to the following situation:

Two spheres of equal mass are suspended by light-weight strings of length 15 cm each. When a charge of -4 nC is put on each sphere, each string makes an angle of 30° with the vertical in equilibrium.

What is the mass of each sphere?

(a)   3.39 × 10-7 kg
(b)   1.69 × 10-7 kg
(c)   1.13 × 10-6 kg
(d)   2.82 × 102 kg
(e)   1.13 × 103 kg

### QUESTION 4**

If an electric field pointing up is applied to the charges, the angle between the strings will

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

### QUESTION 5*

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

Four capacitors are connected as shown below. A 12 V battery is centered in the circuit. Assume that every capacitor is fully charged and no current is flowing.

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

(a)   Q4 = 0.33μC
(b)   Q4 = 3μC
(c)   Q4 = 48μC

### QUESTION 6*

Compute the total stored energy in the capacitors.

(a)   Utot = 7.80 × 10-5 J
(b)   Utot = 1.56 × 10-4 J
(c)   Utot = 9.36 × 10-4 J
(d)   Utot = 1.87 × 10-3 J
(e)   Utot = 3.74 × 10-3 J

### QUESTION 7*

If a dielectric with κ = 7.5 is inserted between the plates of the 3 μF capacitor, the charge on the other three (unmodified) capacitors would

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

### QUESTION 8***

This question and next one pertain to the following situation:

A simple circuit consists of a battery (ε = 10 V), a resistor (R = 12 Ω), and a capacitor (C = 0.2 μF) in series. If the capacitor is uncharged at t = 0, how long will it take for the potential difference across the capacitor to reach 6 V?

(a)   0.8 μs
(b)   1.2 μs
(c)   2.2 μs
(d)   3.5 μs
(e)   4.6 μs

### QUESTION 9**

Which of the graphs below represent the current through the circuit as a function of time

(a)
(b)
(c)

### QUESTION 10*

You want to connect two light bulbs to a battery. You can either connect the bulbs in series or in parallel. In which configuration will the bulbs be brightest?

(a)   When the bulbs are connected in parallel they will be the brightest.
(b)   When the bulbs are connected in series they will be the brightest.
(c)   The brightness will be the same regardless of whether they are in series or parallel.

### QUESTION 11**

For this circuit, which one of the following equations is NOT correct?

(a)   +V1 - I1R1+ I2R2 -V2 = 0
(b)   -V3 + V1 - I1R1 - I3R3 = 0
(c)   +V2 - I2R2 + I3R3 - V3 = 0

### QUESTION 12**

This question and the next one pertain to this situation.

What is the work done by you to assemble the charges?

(a)   -5.91 × 10-2 J
(b)   -2.39 × 10-2 J
(c)   -0.239 × 10-2 J
(d)   2.39 × 10-2 J
(e)   5.91 × 10-2 J

### QUESTION 13***

What is the magnitude of the electric field at point A?

(a)   -3110 N/C
(b)   2540 N/C
(c)   3310 N/C
(d)   4880 N/C
(e)   7080 N/C

### QUESTION 14**

This question and next one pertain to the following situation:

What is the value of I3?

(a)   .17 A
(b)   .33 A
(c)   .5 A
(d)   .83 A
(e)   1 A

### QUESTION 15**

If ε2 is replaced by a 1 μF capacitor, what will be the value of I2 after a long time?

(a)   .3 A
(b)   .97 A
(c)   1.33 A
(d)   2.57 A
(e)   3 A

### QUESTION 16*

If the area of the plates of a parallel plate capacitor is doubled while the distance between the plates is halved, the capacitance will

(a)   decrease by a factor of four.
(b)   decrease by a factor of two.
(c)   remain unchanged.
(d)   increase by a factor of two.
(e)   increase by a factor of four.

### QUESTION 17**

Consider a series RC circuit. Initially, the capacitor has no material between the plates. If a dielectric with κ = 2.4 is inserted between the plates of the capacitor, how must the resistance of the circuit be changed to maintain the same charging rate?

(a)   R' = 0.173R
(b)   R' = 0.417R
(c)   R' = 2.4R
(d)   R' = R
(e)   R' = 5.76R

### QUESTION 18**

In this system, seven identical negative charges are placed at equal distances from point A. What is the direction of the electric field at point A?

(a)   north
(b)   south
(c)   east
(d)   west
(e)   out of the page

### QUESTION 19*

In this picture, all the resistors are identical. If R = 1 Ω, what is Req?

(a)   0.067 Ω
(b)   0.438 Ω
(c)   2.28 Ω
(d)   4.67 Ω
(e)   15 Ω

### QUESTION 20*

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

A neuron can be modeled as a parallel plate capacitor, with the membrane acting as a dielectric and ions as charges on the plates. The membrane has a dielectric constant κ = 7.0 and a thickness of d = 6 nm.

In its resting state, the inside of the cell is at a potential of -85 mV relative to the outside.

What is the capacitance of a 1 (μm)2 (10-12 m2) area of membrane?

(a)   0.003 pF
(b)   0.01 pF
(c)   0.3 pF
(d)   4.5 pF
(e)   9.5 pF

### QUESTION 21*

When a neuron fires (called an "action potential"), Na+ ions (charge q = +1.6 × 10-19 C) are free to move across the membrane. In which direction do the Na+ ions move?

(a)   into the cell
(b)   out of the cell

### QUESTION 22**

When a neuron fires, the potential inside the cell changes from -85 mV to +60 mV. How many Na+ ions must move across a 1 (μm)2 area of membrane to establish this potential?

(a)   1.0 × 102
(b)   2.3 × 103
(c)   4.4 × 103
(d)   9.1 × 103
(e)   1.8 × 105

### QUESTION 23*

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

Shown here is a map of equipotential lines for two charges, Q1 and Q2. Potential values are accurate and given in volts.

What is the sign on the charge Q1?

(a)   positive
(b)   negative

### QUESTION 24*

Which one of the following statements about the magnitudes of the charges is true?

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

### QUESTION 25**

What is the external work W (work done by you) required to move a 1 μC charge from point A to point B?

(a)   W = -6 μJ
(b)   W = -2 μJ
(c)   W = 0 μJ
(d)   W = 2 μJ
(e)   W = 6 μJ

### QUESTION 26***

In circuit A, there are two light bulbs in series. In circuit B, there is only one light bulb. They both have the same battery. What is the ratio of the power output by one bulb in circuit A to the power output by one bulb in circuit B, P1bulb-A / P1bulb-B? (Treat the bulbs like resistors.)

(a)   1/4
(b)   1/2
(c)   1
(d)   2
(e)   4

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.