### Spring 2009 Physics 102 Hour Exam 1(29 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 150. The exam period was 90 minutes; the mean score was 116.4 the median was 118. 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*

This and the next question pertain to the following situation:

A circuit consists of a resistor and a battery.

What happens to the current in the resistor when the voltage is doubled and the resistance remains the same?

(a)   It is doubled.
(c)   It is halved.
(d)   It is quartered.
(e)   It remains the same.

### QUESTION 2*

What happens to the power dissipated in the resistor when the resistance is quadrupled and the voltage remains constant?

(a)   It is doubled.
(c)   It is halved.
(d)   It is quartered.
(e)   It remains the same.

### QUESTION 3*

Which of the following affects the resistance of a resistor?
I) the material from which it is made
II) the length of the resistor
III) the diameter of the resistor
(a)   I only
(b)   II only
(c)   I and II only
(d)   I and III only
(e)   I and II and III

### QUESTION 4*

Two configurations of resistors in parallel are shown below. The resistors are all identical. What is the ratio of the equivalent resistance RB of configuration B to the equivalent resistance RA of configuration A?

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

### QUESTION 5***

How much heat is produced in a 5 Ω resistor in 10 s when a current of 2 A flows through it?

(a)   2 J
(b)   10 J
(c)   20 J
(d)   100 J
(e)   200 J

### QUESTION 6*

Two configurations of capacitors are shown below. The capacitors are all identical. What is the ratio of the equivalent capacitance CB of configuration B to the equivalent capacitance CA configuration A?

(a)   CB / CA = 1/4
(b)   CB / CA = 1/2
(c)   CB / CA = 1
(d)   CB / CA = 2
(e)   CB / CA = 4

### QUESTION 7**

A dielectric is inserted into a capacitor while the charge on it is kept constant. What happens to the potential difference and the stored energy?

(a)   The potential difference decreases and the stored energy increases.
(b)   Both the potential difference and the stored energy increase.
(c)   The potential difference increases and the stored energy decreases.
(d)   Both the potential difference and the stored energy decrease.
(e)   Both the potential difference and the stored energy remain the same.

### QUESTION 8*

This question and next three pertain to the following situation:

In the circuit shown below, the battery voltage is 9 V, the resistor R1 = 20 Ω and the capacitor C = 1 μF. The capacitor is initially charged with the polarity shown in the picture and with a voltage of 18 V. Switches S1 and S2 are initially open.

What is the initial charge on the capacitor?

(a)   11 C
(b)   1.8 × 10-5 C
(c)   3.2 × 10-3 C
(d)   1.5 × 10-5 C
(e)   4.1 × 10-9 C

### QUESTION 9*

Switch S2 is suddenly closed. What is the current through the 20 Ω resistor immediately afterward?

(a)   0.2 A
(b)   0.9 A
(c)   1.0 A
(d)   0.5 A
(e)   0.3 A

### QUESTION 10*

When will the current through the 20 Ω resistor reach 1/e of its initial value?

(a)   2.0 × 10-5 s
(b)   1.0 s
(c)   3.2 × 10-6 s
(d)   5.4 × 10-9 s
(e)   10 s

### QUESTION 11*

Once the current through the 20 Ω resistor reaches 1/e of its initial value, switch S1 is suddenly closed (switch S2 remains closed). What is the current through the 20 Ω resistor a long time later?

(a)   2.0 A
(b)   0.23 A
(c)   0.45 A
(d)   20 A
(e)   0.15 A

### QUESTION 12*

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

In the circuit shown below, the battery voltage is ε, the resistors R1 = R3 = R and R2 = R4 = 2R.

What is the equivalent resistance for this configuration of resistors?

(a)   5R
(b)   R / 10
(c)   3R / 2
(d)   6R / 5
(e)   R

### QUESTION 13*

What is the total power dissipated by resistors R3 and R4?

(a)   ε2 / 10R
(b)   ε2 / R
(c)   ε2 / 5R
(d)   ε2 / 20R
(e)   ε2 / 3R

### QUESTION 14*

The battery voltage ε is 9 V and the current through the battery is 2 A. What is R?

(a)   3 Ω
(b)   1 Ω
(c)   10 Ω
(d)   2 Ω
(e)   0.1 Ω

### QUESTION 15*

Which one of the following choices is correct regarding the potential difference VAB = VA - VB between point A and point B?

(a)   VAB > 0
(b)   VAB < 0
(c)   VAB = 0

### QUESTION 16**

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

In a cathode ray tube (CRT), a beam of electrons is deflected vertically by a constant electric field between parallel plates of length L = 2 cm, separated a distance d = 1 cm. The electrons (charge q = -1.6 ×10-19 C, and mass me = 9.11 × 10-31 kg), travel at a velocity v = 3 × 106m/s along the horizontal. You may neglect the effect of gravity on the electrons.

What is the horizontal velocity component of the electrons after passing through the parallel plates?

(a)   1 × 106 m/s
(b)   3 × 106 m/s
(c)   5 × 106 m/s
(d)   7 × 106 m/s
(e)   9 × 106 m/s

### QUESTION 17***

What is the vertical velocity component of the electrons after passing through the parallel plates?

(a)   1 × 106 m/s
(b)   3 × 106 m/s
(c)   5 × 106 m/s
(d)   7 × 106 m/s
(e)   9 × 106 m/s

### QUESTION 18**

If the beam of electrons were replaced with protons (charge q = +1.6 × 10-19C, and mass mp = 1836me) and everything else kept the same, which of the following statements would be true about the magnitude and direction of deflection?

(a)   The beam would deflect the same amount, in the opposite direction.
(b)   The beam would deflect more, in the opposite direction.
(c)   The beam would deflect more, in the same direction.
(d)   The beam would deflect less, in the opposite direction.
(e)   The beam would deflect less, in the same direction.

### QUESTION 19**

This and the next question pertain to the following situation:

A hollow, conducting cylinder is placed between two long, oppositely charged parallel plates. Consider the following diagrams.

Which diagram best represents the equipotential lines for this situation?

(a)
(b)
(c)
(d)
(e)

### QUESTION 20*

A negative point charge is placed at the center of the cylinder. In which direction does it move?

(a)   toward the positive electrode
(b)   toward the negative electrode
(c)   It does not move.

### QUESTION 21*

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 = 6.0 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)   0.0015 pF
(b)   0.01 pF
(c)   0.3 pF
(d)   4.7 pF
(e)   7.5 pF

### QUESTION 22*

When a neuron fires (called an "action potential"), Na+ ions (charge q = +1.6 × 10-19C) 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 23**

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-μm2 area of membrane to establish this potential?

(a)   3.7 × 102
(b)   1.3 × 103
(c)   5.4 × 103
(d)   9.1 × 103
(e)   1.2 × 105

### QUESTION 24*

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

A single charge of unknown sign and magnitude is located at the origin. A graph of the electrical potential a distance r away from this charge is shown below. We assume that the potential at infinity is zero. The graph is accurately prepared so you can read meaningful values from scales on the axes.

The sign of the charge is

(a)   positive
(b)   negative

### QUESTION 25*

The magnitude of the charge is:

(a)   |Q| = 0.22 nC
(b)   |Q| = 5.55 nC
(c)   |Q| = 50.0 nC

### QUESTION 26**

If you moved a -3 nC charge from an initial distance of r = 1 m to a final distance of r = 0.5 m, how much work must you do?

(a)   W = 0.15 μJ
(b)   W = 0.80 μJ
(c)   W = 1.33 μJ
(d)   W = 8.66 μJ
(e)   W = 25.0 μJ

### QUESTION 27*

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

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 28***

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

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

### QUESTION 29**

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 = -4 μJ
(b)   W = -2 μJ
(c)   W = 0 μJ
(d)   W = 2 μJ
(e)   W = 4 μJ

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.