### Fall 2002 Physics 102 Hour Exam 2(31 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 ***.

This exam consists of 31 questions; 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 94. When the exam was given, the mean was 73.6; the median was 70. Click here to see page1 page2 of the formula sheet that came with the exam.

### QUESTION 1**

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

A rectangular loop with dimensions a = 4 cm and b = 5 cm lies in the x-y plane. It consists of 5 turns of wire carrying a current of 3.7 amps with the direction of current flow indicated in the figure. The loop is in a uniform magnetic field of strength 3.5 × 104 T pointing along a direction lying in the yz plane and making an angle of 30° from the y axis.

What is the magnetic flux through one turn of the wire making the loop?

(a)   3.5 × 10-7 T-m2
(b)   6.1 × 10-7 T-m2
(c)   7.0 × 10-7 T-m2

### QUESTION 2**

What is the magnitude of the torque on the loop?

(a)   6.48 × 10-6 N-m
(b)   1.12 × 10-5 N-m
(c)   1.30 × 10-5 N-m

### QUESTION 3**

About which axis does the torque try to rotate the loop?

(a)   x-axis
(b)   y-axis
(c)   z-axis

### QUESTION 4*

What is the magnitude of the force on the left side of the loop labeled a in the figure?

(a)   1.30 × 10-4 N
(b)   2.24 × 10-4 N
(c)   2.59 × 10-4 N

### QUESTION 5**

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

At t = 0, a rectangular single-loop coil of resistance R = 5 W and dimensions W = 2 cm and L = 10 cm enters a region of constant magnetic field B = 2.5 T directed into the paper. The length of the region containing the magnetic field is LB = 5 cm as shown. The coil is observed to move at a constant velocity v = 2 cm/s to the right.

What is the force required at time t = 0.5 s to maintain this velocity?

(a)   0
(b)   1.0 × 10-5 N
(c)   4.0 × 10-5 N

### QUESTION 6***

What is the force required at time t = 3 s to maintain this velocity?

(a)   0
(b)   1.0 × 10-5 N
(c)   4.0 × 10-5 N

### QUESTION 7**

What is the power dissipation in the coil at time t = 2.0 s?

(a)   0
(b)   1.0 × 10-7 W
(c)   2.0 × 10-7 W
(d)   4.0 × 10-7 W
(e)   8.0 × 10-7 W

### QUESTION 8**

In which direction will the induced current flow through the loop at time t = 2.0 s?

(a)   clockwise
(b)   counterclockwise

### QUESTION 9*

This and the following question pertain to the following situation.

An electron (charge = -1.67 × 10-19 C, mass = 9.11 × 10-31 kg) with velocity v = 4.6 × 106 m/s to the right enters a box 0.5 meters wide with a uniform magnetic field pointing into the page.

What is the direction of the magnetic force on the negatively charged particle when it first enters the box?

(a)   downward
(b)   upward

### QUESTION 10*

What is the minimum magnetic field B necessary to keep the particle from hitting the far side of the box?

(a)   6.28 × 10-6 T
(b)   1.26 × 10-5 T
(c)   5.02 × 10-5 T

### QUESTION 11**

Which element in this AC circuit has the largest peak voltage across it?

(a)   R
(b)   L
(c)   C
(d)   generator
(e)   none of the above

### QUESTION 12*

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

Two parallel wires, each with length L = 5 m, are separated by a distance of 0.05 m. Wire 1 carries a current I1 = 15 A, and wire 2 carries a current I2 = 25 A, both in the same direction.

What is the magnitude of the magnetic field due to wire 1 at the center of wire 2 ?

(a)   6.0 × 10-5 T
(b)   0.9 × 10-4 T
(c)   1.5 × 10-4 T

### QUESTION 13*

What is the magnetic force acting on wire 2.

(a)   4.5 × 10-3 N
(b)   7.5 × 10-3 N
(c)   1.25 × 10-2 N

### QUESTION 14*

Is the force between the two wires attractive or repulsive?

(a)   attractive
(b)   repulsive

### QUESTION 15*

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

A solenoid with cross sectional area A = 4 cm2 is 120 cm long and has 2500 turns. The magnetic field at the center of the solenoid is 5 × 10-3 T pointing into the page. A square wire loop 1.5 cm on each side is fixed inside the solenoid as shown.

What is the direction of the current through the coils of the solenoid?

(a)   clockwise
(b)   counterclockwise

### QUESTION 16*

What is the magnitude of the current through the coils of the solenoid?

(a)   1.28 A
(b)   1.59 A
(c)   1.91 A

### QUESTION 17**

The current in the solenoid is uniformly decreased to 1/2 its previous value over 5 seconds. What is the magnitude of the induced emf in the square loop?

(a)   1.1 × 10-7 V
(b)   7.5 × 10-7 V
(c)   1.8 × 10-3 V

### QUESTION 18**

This and the following question pertain to the following situation.

A conducting wire loop is being manually rotated, at constant w = 30 rad/s. At the instant shown, side AC is coming out of the page, and side BD is going into the page. There is a constant magnetic field B = 0.4 T pointing to the right as shown and the loop has resistance 5 W.

At the instant shown, the induced current in the loop will flow

(a)   clockwise.
(b)   counterclockwise.
(c)   No current is induced.

### QUESTION 19**

What is the induced current at the moment the loop has rotated 45° from the position shown?

(a)   0 A
(b)   3.4 A
(c)   4.8 A

### QUESTION 20**

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

A generator is hooked up to a transformer with NP = 300, NS = 1500, as shown (but not to scale). The generator produces voltage V(t) = (200 V) sin (2pt) across the primary of the transformer.

What is the period of the oscillating voltage in the secondary circuit?

(a)   0.159 s
(b)   1 s
(c)   6.28 s

### QUESTION 21*

What is the rms voltage across the secondary?

(a)   28.3 V
(b)   40 V
(c)   707 V

### QUESTION 22**

Compare Ip, the peak current through the primary with Is, the peak current through the secondary.

(a)   Ip > Is
(b)   Ip = Is
(c)   Ip < Is

### QUESTION 23*

The tuner of a radio involves an RLC circuit with a fixed value inductor but with a variable capacitor. After listening to the station 107.1 MHz for a while, you now tune in to another radio station at 97.3 MHz. Does the value of the capacitor in the radio increase or decrease as you change the station?

(a)   C decreases.
(b)   C increases.

### QUESTION 24*

This and the following question pertain to the following situation.

Initially unpolarized light is incident on a series of two linear polarizers.

If the initial intensity of the incident (unpolarized) light is I0, what is the intensity I2 transmitted by the stack?

(a)   I2 = 0.015 I0
(b)   I2 = 0.0301 I0
(c)   I2 = 0.48 I0

### QUESTION 25***

If the magnetic field amplitude after passing through the first polarizer is B1, then what is the amplitude after passing through the second polarizer?

(a)   B2 = 0.0251 B1
(b)   B2 = 0.1736 B1
(c)   B2 = B1

### QUESTION 26*

This and the following question pertain to the following situation.

Electromagnetic waves of frequency 4 × 106 Hz and average energy density 4 × 10-6 J/m3 are incident upon a panel of area 10 m2.

What is the wavelength of these waves?

(a)   1.33 × 10-2 m
(b)   75 m
(c)   1.2 × 1015 m

### QUESTION 27*

How much energy is transmitted through the panel in a period of 10 s?

(a)   4 × 10-4 J
(b)   1.2 × 105 J
(c)   3 × 1015 J

### QUESTION 28*

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

The phasor diagram shown gives information on a particular series RLC circuit. It is only partially labeled, but you should know from examining it the proper quantity that is referred to by each number. The maximum voltage supplied by an AC power source of unknown frequency is 100 volts.

What is the total impedance?

(a)   1 W
(b)   3 W
(c)   5 W

### QUESTION 29*

What is the peak voltage across the capacitor?

(a)   20 V
(b)   60 V
(c)   100 V

### QUESTION 30**

What is the average power delivered by the generator?

(a)   600 W
(b)   1000 W
(c)   3333 W

### QUESTION 31*

The voltage across the generator is _________ the current through it?

(b)   in phase with
(c)   lagging

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.