### Summer 2008 Physics 102 Hour Exam 2(22 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 82. The exam period was 60 minutes; the mean score was 45.8; the median was 46. 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 picture depicts an aluminum ring falling through a magnetic field which points into the page and which gets gradually stronger at lower heights.

In which direction does an induced current flow?

(a)   clockwise
(b)   counter clockwise

### QUESTION 2**

Suppose instead the ring were falling through a magnetic field that got gradually weaker at lower heights. How would this experiment differ from the previous question?

(a)   The direction of magnetic force on the ring would reverse, but not the direction of electric current.

(b)   The current direction would reverse, but not the direction of magnetic force.

(c)   Both current and magnetic force would reverse direction.

### QUESTION 3**

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

Consider two parallel wires, one carrying current I1 = 5.8 A to the right, and at a distance L = 5 cm below it, another carrying I2 = 2.6 A to the right.

What is the magnetic field a distance 10 cm above the top wire?

(a)   1.5 × 10-5 T, out of page
(b)   8.1 × 10-6 T, out of page
(c)   8.1 × 10-6 T, into page
(d)   1.16 × 10-5 T, into page
(e)   1.5 × 10-5 T, into page

### QUESTION 4**

What is the net force exerted by the bottom wire on a 6 cm segment of the top wire?

(a)   1.04 × 10-5 N, upward along page
(b)   3.6 × 10-6 N, upward along page
(c)   0
(d)   3.6 × 10-6 N, downward along page
(e)   1.04 × 10-5 N, downward along page

### QUESTION 5***

An electron initially moves rightward along a line parallel to the wires and midway between them. (The dashed line shows the initial velocity of that electron.) In which direction is it accelerating at that instant?

(a)   upward along page
(b)   downward along page
(c)   out of page
(d)   into page
(e)   it experiences no force

### QUESTION 6***

This and the next three questions pertain to the the following situation.

A proton (q = +1.6 × 10-19 C, m = 1.673 × 10-27 kg) travels in a clockwise circle within a solenoid, along the dashed line in the figure. The solenoid consists of 500 turns of wire, and carries a current of 0.4 A, producing a magnetic field of 0.0022 T within the solenoid.

In which of these two directions could the magnetic field be pointing?

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

### QUESTION 7*

How long is the solenoid? Calculate the length along its axis (the direction perpendicular to the page).

(a)   11.4 cm
(b)   15.4 cm
(c)   19.4 cm

### QUESTION 8**

As viewed in the diagram, in which direction does the current in the loops of the solenoid flow?

(a)   the current flows clockwise
(b)   the current flows counter-clockwise.

### QUESTION 9*

The charge traverses a circular path of radius 0.4 cm. How fast is it moving?

(a)   27 m/s
(b)   84 m/s
(c)   270 m/s
(d)   840 m/s
(e)   2700 m/s

### QUESTION 10***

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

The primary coil of a transformer, consisting of 180 turns of wire, is connected to a voltage source of some kind. The secondary of that transformer, consisting of 450 turns of wire, is connected to an 82 Ω resistor. The diagram might not accurately depict those values.

In which of these two situations is more energy dissipated, per second, in the resistor?

(a)   a 12 volt battery is connected to the primary
(b)   a voltage source with Vrms = 6 volts is connected to the primary

### QUESTION 11***

Connect an alternating voltage source Vmax = 25 volts to the primary. What rms current flows in the primary?

(a)   0.54 A
(b)   0.76 A
(c)   1.35 A

### QUESTION 12***

Compare the following values of power: (i) P1 equals the average power dissipated in the 82 Ω resistor when it is connected, as shown, to the secondary of this transformer. (ii) P2 is the power dissipated in this resistor when it is connected directly to the 25 volt source, leaving the transformer out of the circuit entirely. What can we say about the relationship between these two quantities?

(a)   P2 = 0.16 P1
(b)   P2 = 0.4 P1
(c)   P2 = P1
(d)   P2 = 2.5 P1
(e)   P2 = 6.25 P1

### QUESTION 13**

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

A bar is free to slide along a pair of metal tracks separated by a distance of L = 25 cm. (You may ignore friction between the bar and the tracks.) At one end, the tracks are connected electrically by a resistor R = 4.7 Ω. You should assume the metal bar completes an electric circuit. You push the bar with a constant 0.18 N force, under which the bar initially accelerates, but eventually reaches equilibrium, ultimately moving at v = 4.6 meters per second. An unknown magnetic field points into the page.

All the following questions assume the bar has reached equilibrium and is therefore moving at a constant velocity.

What is the strength of the magnetic field?

(a)   1.7 T
(b)   2.9 T
(c)   4.4 T

### QUESTION 14**

At what rate is energy dissipated in the resistor?

(a)   0.18 W
(b)   0.23 W
(c)   0.27 W
(d)   0.34 W
(e)   0.83 W

### QUESTION 15**

Assuming the field points into the page, and assuming you pull the bar to the right, in what direction does the current flow through the bar?

(a)   upward
(b)   downward

### QUESTION 16*

This and the next three questions pertain to the the following situation.

In this RLC circuit, R = 270 Ω, L = 10 mH, and C = 0.7 μF. A function generator, whose maximum voltage and operating frequency can be adjusted, applies an alternating voltage.

Suppose the maximum current Imax = 12 mA when f = 410 Hz. What average power is dissipated in the circuit?

(a)   0.019 W
(b)   0.026 W
(c)   0.039 W

### QUESTION 17**

Leaving its frequency at 410 Hz, adjust the function generator so that the peak generator voltage is 14 volts. What is the rms voltage across the inductor?

(a)   0.43 V
(b)   6.37 V
(c)   9.90 V

### QUESTION 18**

Imagine the resistor were a light bulb. Leaving the maximum generator voltage fixed at 14 volts, vary the function generator frequency f. At what frequency does the bulb glow most brightly?

(a)   1240 Hz
(b)   1900 Hz
(c)   12400 Hz

### QUESTION 19**

Which of these phasor diagrams most accurately depicts this circuit when it operates at 410 Hz?

(a)
(b)
(c)

### QUESTION 20**

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

Fifteen turns of wire are wound in a circle of radius 3.5 cm. The wire has a total resistance of 1.8 Ω. By connecting the loop to a regulated motor, you can get it to spin at a constant angular frequency ω =14 rad/sec in a magnetic field of strength B = 0.9 T. Define θ to be the angle between the magnetic field and a vector perpendicular to the plane of the loop. (In the picture, the loop rotates around an axis perpendicular to the page.)

What current flows in the loop when θ = 0 ?

(a)   0
(b)   0.027 A
(c)   0.404 A

### QUESTION 21***

What current flows when the magnetic flux Φ = 0?

(a)   0
(b)   0.027 A
(c)   0.404 A

### QUESTION 22***

What torque τ must the motor exert (to keep ω constant) when θ = 30° ?

(a)   5 × 10-3 Nm
(b)   7 × 10-3 Nm
(c)   1.0 × 10-2 Nm
(d)   1.4 × 10-2 Nm
(e)   2.1 × 10-2 Nm

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.