### Spring 2002 Physics 102 Hour Exam 2(30 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 30 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 75. Click here to see page1 page2 of the formula sheet that came with the exam.

### QUESTION 1*

A point-like charge Q = +6 C is placed at the end of the second-hand (L = 1 cm) of a watch, as shown in the figure.

What is the direction of the magnetic field at the center of the watch due to this moving charge?

(a)   into the page
(b)   out of the page
(c)   depends on the position of the charge

### QUESTION 2*

Two carts, each carrying two circular loops, are positioned on a oval rail-track as shown in the figure. In the loops currents I circulate in opposite directions as shown by the arrows.

If the carts are set free to move, then (assume there is no friction!)

(a)   they will attract each other and stick together.

(b)   they will first repel each other, move in opposite directions, and finally stick together on the opposite side of the track.

(c)   they will repel each other, move in opposite directions, until they come close on the opposite side of the track and then bounce back again.

### QUESTION 3*

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

Two particles, A (mA = 1.7 × 10-27 kg, QA = 1.6 × 10-19 C) and B (mB = 3.4 × 10-27 kg, QB = -1.6 × 10-19 C) enter a box with uniform magnetic field B = 5 T. Particle A enters with velocity vA = 106 m/s and follows trajectory A.

What is the direction of the magnetic field inside the box?

(a)   into the page
(b)   out of the page
(c)   up
(d)   down
(e)   right

### QUESTION 4*

Particle B also enters with velocity vB = 106 m/s. Which trajectory will it follow?

(a)   trajectory 1
(b)   trajectory 2
(c)   trajectory 3

### QUESTION 5*

The distance L, from entry to the point where trajectory A exit the spectrometer is

(a)   L = 1.25 x 10-3 m
(b)   L = 3.75 x 10-3 m
(c)   L = 4.25 x 10-3 m

### QUESTION 6**

This and the following question pertain to the following situation.

A conducting ring, of resistance R = 3 W , is held at h = 3 m above a rectangular region where a constant magnetic field is directed into the page, as shown in the figure. At t = 0 the ring is left to fall through the magnetic field, at t = tin it enters the magnetic field, at t = tout it exits the magnetic field, and finally at t = tend reaches h = 0.

The presence of the magnetic field causes the ring to fall from height h to height 0

(a)   faster.
(b)   at the same rate.
(c)   slower.

### QUESTION 7**

Which one of the following graphs best describes the behaviour of the induced current in the ring? (A positive current means that it is flowing counter-clockwise in the ring).

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

### QUESTION 8*

This and the following question pertain to the following situation.

A rectangular current loop is located near a long, straight wire that carries a current of 8 A flowing to the right (see the figure). The current in the loop Iloop = 30 A is flowing clockwise.

The top wire of the loop is attracted to the long straight wire.

(T)   True
(F)   False

### QUESTION 9**

The magnitude of the net force on the entire loop is

(a)   |F| = 0 N
(b)   |F| = 8.4 × 10-5 N
(c)   |F| = 12.6 × 10-5 N

### QUESTION 10*

This and the following question pertain to the following situation.

Two long concentric solenoids are pictured. The inner solenoid contains n1 = 120 turns/meter of wire, has a radius R1 = 1.5 cm and carries a current I1 = 5 A that (alone) generates a magnetic field into the page at the center of the solenoid. The outer solenoid contains n2 = 150 turns/meter of wire and has a radius R2 = 2.5 cm and carries an unknown current I2. The magnetic field at the common center of the coils is zero.

What is the direction of the current in the outer coil?

(a)   clockwise
(b)   counter-clockwise

### QUESTION 11**

What is I2, the current in the outer coil?

(a)   I2 = 3.0 A
(b)   I2 = 4.0 A
(c)   I2 = 5.0 A
(d)   I2 = 6.3 A
(e)   I2 = 8.3 A

### QUESTION 12**

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

Two fixed conductors are connected by a resistor R = 20 W . The two fixed conductors are separated by L = 1.5 m. A moving conductor is pushed along them (without friction) at a constant speed v = 6 m/s. A 3 T magnetic field (shown by the black dots in the figure) points out of the page.

In which direction does the current flow through the resistor?

(a)   to the right
(b)   to the left

### QUESTION 13*

What force must be applied to the bar to keep it moving at 6 m/s?

(a)   4 N
(b)   6 N
(c)   7.5 N

### QUESTION 14*

If the 20 ohm resistor was replaced by a 100 ohm resistor, the net force needed to keep the bar moving at 6 m/s would

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

### QUESTION 15**

This and the following question pertain to the following situation.

In the figure is shown a square loop (L = 2 cm) of wire in edge view, which is free to rotate around an axis O (perpendicular to the page). The loop consists of 8 loops of wire and the normal to the loops forms an angle of 30°. An external battery (not shown) causes a current I = 3.0 amps to flow around the loop. The external magnetic field directed down (B = 0.7 T) creates a clockwise torque on the current loop.

What is the direction of the current in wire A?

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

### QUESTION 16*

What is the magnitude of the torque?

(a)   3.4 × 10-3 N m
(b)   8.4 × 10-3 N m
(c)   12.6 × 10-3 N m

### QUESTION 17***

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

A rectangular loop is being rotated at a constant angular velocity w via an external means. There is no battery or power supply connected to the loop. At the instant shown, the loop is lying flat on the page, as shown in the figure below. The direction of the rotation is such that the top segment of the loop is coming out of the page and the bottom segment of the loop is going into the page. A constant magnetic field of 0.5 T is coming out of the page. The rectangular loop is connected to a circuit that contains a light bulb with resistance of 10 W.

At the instant shown, what can you say about the direction of the induced current at point a?

(a)   There is no current flowing.
(b)   There is current flowing to the right.
(c)   There is current flowing to the left.

### QUESTION 18**

If w is doubled, what will happen to the peak current through the light bulb?

(a)   It will not change.
(b)   It will increase by a factor of two.
(c)   It will increase by a factor of four.

### QUESTION 19*

What is the maximum value of the current flowing through the light bulb if w = 377 rad/sec?

(a)   0.028 A
(b)   0.079 A
(c)   0.092 A

### QUESTION 20*

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

The phasor diagram to the right 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 by each number, given in ohms, in the drawing. The maximum voltage supplied by an AC power source of unknown frequency is 100 volts.

If the capacitance is 3.2 µF, what is the inductance?

(a)   0.67 mH
(b)   0.80 mH
(c)   0.93 mH

### QUESTION 21*

What is the the total impedance?

(a)   16 W
(b)   20 W
(c)   40 W

### QUESTION 22*

What is the maximum voltage across the inductor?

(a)   63 V
(b)   77 V
(c)   83 V

### QUESTION 23**

Is the voltage across the generator leading or lagging the current through it?

(a)   It lags the current.
(c)   It is in phase with the current.

### QUESTION 24**

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

A polarized (in z direction) electromagnetic wave is incident at a series of three linear polarizers, each with polarization angles as shown in the figure. The incident wave has an average intensity of 2000 W/m2.

What is the rms magnitude of the magnetic field after the first polarizer?

(a)   2.9 × 10-6 T
(b)   4.5 × 10-6 T
(c)   6.5 × 10-6 T

### QUESTION 25**

Which direction does the magnetic field point after the third polarizer?

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

### QUESTION 26*

What is the intensity I3 transmitted by the stack of polarizers?

(a)   475 W/m2
(b)   796 W/m2
(c)   1130 W/m2

### QUESTION 27**

If a detector with a circular area of radius 3 cm is placed before the first polarizer, how much energy is incident on the detector over 5 seconds?

(a)   28 J
(b)   38 J
(c)   55 J

### QUESTION 28*

The figure to the right illustrates a plane electromagnetic wave. Compare the magnitudes of the z-component of the electric field at point b and c.

(a)   Eb = Ec
(b)   Eb > Ec
(c)   Eb < Ec

### QUESTION 29*

A local music station broadcasts at a frequency f. If the wavelength of this FM wave is 2.96 m, what is the frequency f ?

(a)   101.5 MHz
(b)   105.3 MHz
(c)   107.1 MHz

### QUESTION 30*

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

(a)   It decreases.
(b)   It increases.

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.