### Summer 2010 Physics 102 Hour Exam 2(25 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 87; the mean score was 65.9; the median was 69. The exam period was 75 minutes. 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 question and the next three pertain to the the following situation.

The diagram, not necessarily drawn to scale, shows two chambers, the first with a magnetic field whose absolute value B1 = 0.028 T and the second with a magnetic field of absolute value B2 = 0.092 T. These fields are perpendicular to the page.

A particle with a mass m = 4.4 × 10-27 kg enters the first chamber in its lower left corner, moving with an initial speed v = 105 m/s directed upward (in the positive y-direction). It is moving due rightward (in the positive x-direction) as it passes into the second chamber, and moving straight upward as it exits the second chamber.

What is true regarding W1, the work done on the charge by the magnetic field in the first chamber?

(a)   W1 < 0
(b)   W1 = 0
(c)   W1 > 0

### QUESTION 2*

In the first chamber, the radius of the charge's trajectory is R1 = 4.9 cm. What is the magnitude (absolute value) of the charge?

(a)   9.8 × 10-20 C
(b)   2.0 × 10-19 C
(c)   3.2 × 10-19 C

### QUESTION 3*

Assuming the charge is positive, what is the direction of the magnetic field in the first chamber? (The "first chamber" is the one on the left in the diagram.)

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

### QUESTION 4**

Find L, the length of the chambers in the y-direction.

(a)   6.4 cm
(b)   9.8 cm
(c)   21 cm

### QUESTION 5**

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

The diagram shows two wires perpendicular to the page. At x = - 6 cm on the x-axis, a wire carries a current I1 = 2.2 A into the page; at x = 6 cm on the x-axis, a wire carries a current I2 = 2.2 A out of the page. The questions below will refer to a point A at y = 16 cm on the y-axis, and a point B at x = 10 cm on the x-axis.

What is the direction of the magnetic field at point A?

(a)   to the right (positive x-direction)
(b)   to the left (negative x-direction)
(c)   upward along the page (positive y-direction)
(d)   downward along the page (negative y-direction)
(e)   perpendicular to the page

### QUESTION 6***

A positive charge moves toward the wires at point B (its velocity is in the negative-x direction). In what direction does the net force on that charge act?

(a)   into the page
(b)   out of the page
(c)   neither of the above; the force vector lies in the plane of the page

### QUESTION 7*

Calculate the force that the first wire exerts on a 23 cm long segment of the second wire:

(a)   4.4 × 10-7 N
(b)   8.4 × 10-7 N
(c)   1.9 × 10-6 N
(d)   3.7 × 10-6 N
(e)   8.0 × 10-6 N

### QUESTION 8*

True of False: the wires attract each other.

(T)   True
(F)   False

### QUESTION 9*

This question and the next deal with the situation described below.

A transformer 840 turns of wire in its primary and 660 turns of wire in its secondary. A voltage Vrms = 120 volts is applied to the primary. A current Irms = 0.16 A flows into the primary. A light bulb -- acting like an ideal resistor -- is attached to the secondary.

What is the resistance of the light bulb?

(a)   460 Ω
(b)   90 Ω
(c)   750 Ω
(d)   950 Ω
(e)   1200 Ω

### QUESTION 10**

You have super-fast vision. Nothing escapes your notice! You watch that light bulb continuously while it is connected to the secondary of the transformer (whose primary remains connected to the power source). Does that light bulb ever have to go dark?

(a)   Yes, sometimes the bulb has to go dark.
(b)   No, there is no reason that the bulb ever has to go dark.

### QUESTION 11***

You drop a coin of solid copper between the poles of a strong magnet. What effect does the magnet have?

(a)   The magnet slows the coin as it enters the field and speeds it back up as it leaves.
(b)   The magnet slows the coin as it enters and slows the coin as it leaves.
(c)   The magnet has no noticeable effect on copper.

### QUESTION 12*

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

A bar is free to slide along a pair of metal tracks separated by a distance of L = 19 cm. (You may ignore friction between the bar and the tracks.) At one end, the tracks are connected electrically by a resistor R = 0.27 Ω. You should assume the metal bar completes an electric circuit. An unknown magnetic field B points out of the page. The bar moves at 3.4 m/s, resulting in an electromotive force of 0.055 volts across the resistor.

What magnetic force acts on the sliding bar?

(a)   3.3 × 10-3 N
(b)   0.87 N
(c)   0.17 N

### QUESTION 13*

Find the magnitude of the magnetic field.

(a)   0.023 T
(b)   0.046 T
(c)   0.085 T

### QUESTION 14*

In which direction does the induced current flow?

(a)   clockwise (left to right through the resistor)
(b)   counter-clockwise (right to left through the resistor)

### QUESTION 15*

This phasor diagram refers to a particular RLC circuit.

True or False: at the instant shown, the voltage across the capacitor is decreasing (becoming more negative).

(T)   True
(F)   False

### QUESTION 16***

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

The diagram shows a single loop of wire spinning in the presence of a magnetic field B = 0.6 T. The width w = 18 cm, the length L = 15 cm, and the loop is rotating at 26 revolutions per second. You should assume that the loop has a total resistance of 6.8 Ω.

At the instant shown, the segment marked "L" is moving toward the back of the page. (It will soon be behind the vertical dashed axis around which the loop is rotating.) At this instant shown, which way is the induced current in that segment flowing?

(a)   It is flowing upward in the segment marked "L".
(b)   It is flowing downward in the segment marked "L".
(c)   At the instant shown, no current flows in the segment marked "L".

### QUESTION 17**

This loop rotates at a constant rate. As it does, you measure the torque required to keep it moving at that constant rate. What maximum value of torque would be required?

(a)   0 Nm
(b)   1.0 × 10-3 Nm
(c)   6.3 × 10-3 Nm
(d)   0.23 Nm
(e)   0.39 Nm

### QUESTION 18**

True or False: in this loop, the flux and the electromotive force reach their peaks at the same time.

(T)   True
(F)   False

### QUESTION 19*

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

In the RLC circuit shown, R = 140 Ω, XL = 30 Ω and XC = 110 Ω. A function generator applies an alternating voltage of frequency f = 240 Hz and peak voltage Vmax = 11 V.

What rms current Irms is delivered to the above circuit?

(a)   0.042 A
(b)   0.048 A
(c)   0.059 A
(d)   0.068 A
(e)   0.084 A

### QUESTION 20*

Compare (i) the times at which the voltage across the capacitor reaches its peak (its maximum positive value) to (ii) the times at which the voltage across the inductor reaches its peak (maximum positive value). How do they compare?

(a)   These peaks occur at the same time.
(b)   These peaks are separated by one half cycle.
(c)   Neither of the above is true for this circuit.

### QUESTION 21*

At what rate, on average, does this circuit consume power?

(a)   0.33 W
(b)   0.38 W
(c)   0.43 W

### QUESTION 22*

At what frequency would the power delivered to this circuit be a maximum?

(a)   240 Hz
(b)   460 Hz
(c)   600 Hz
(d)   880 Hz
(e)   1200 Hz

### QUESTION 23*

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

A solenoid connected to a battery produces a magnetic field of 0.004 T. The solenoid has 300 turns of wire along an axis of length 10 cm and diameter of 3 cm. The picture shows a typical solenoid, before the battery has been attached.

How much current must flow for this coil to produce the required magnetic field?

(a)   0.01 A
(b)   0.1 A
(c)   1 A

### QUESTION 24*

Suppose the current is varied so that the magnetic field produced by the solenoid rises from 0 to 0.001 T in 5 ms. What electromotive force does a single loop of wire in the solenoid experience?

(a)   1.4 × 10-4 V
(b)   5.6 × 10-3 V
(c)   7.1 × 10-3 V

### QUESTION 25**

Using the same "form" (cylinder around which the coil is wound), you create another solenoid with twice as much wire as you used previously. How does the inductance of the new solenoid compare with the inductance of the original one?

(a)   The inductance of both will be the same.
(b)   The new solenoid has twice the inductance of the original one.
(c)   The new solenoid has four times the inductance of the original one.

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.