### Spring 2001 Physics 102 Hour Exam 2(32 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 32 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 112. When the exam was given, the mean was 89.4; the median was 91. Click here to see page1 page2 of the formula sheet that came with the exam.

### QUESTION 1*

This and the next question are about the following situation:

A square loop of wire carries a current I. Current is flowing in a counterclockwise direction as shown.

What is the direction of the magnetic field at A?

(a)   into the page
(b)   out of the page
(c)   the B field is zero

### QUESTION 2*

What is the direction of the magnetic field at B?

(a)   into the page
(b)   out of the page
(c)   the B field is zero

### QUESTION 3*

This and the next two questions are about the following situation:

Two square loops of wire are oriented in a uniform magnetic field as shown. Both loops are attached to and free to rotate about a pivot rod. The current which flows in each loop is provided by power supplies which are not shown.

The current in the outer loop is Ib = 5 A. The length of side b = 0.4 m and the uniform magnetic field strength is B = 2 T. What is the maximum torque on the outer loop?

(a)   0.8 N m
(b)   1.6 N m
(c)   2.4 N m
(d)   3.2 N m
(e)   4.0 N m

### QUESTION 4*

If the current in each loop flows counterclockwise, the loops will rotate such that the

(a)   left side comes out of the page and the right side goes into the page.
(b)   right side comes out of the page and the left side goes into the page.

### QUESTION 5*

Length b is twice the length of a (b = 2a) and the current in b is half the current in a ( Ib = ½ Ia ). Compare the maximum torque on loop a, to the maximum torque on loop b.

(a)   ta < tb
(b)   ta = tb
(c)   ta > tb

### QUESTION 6*

This and the next two questions are about the following situation:

Two positively charged particles enter a region of uniform magnetic field B. The B field is oriented perpendicular to the page. The paths of the two charged particles are shown.

Which direction is the B field pointing?

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

### QUESTION 7*

If the two particles have the same mass and initial velocity which one has the greater charge?

(a)   particle a
(b)   particle b

### QUESTION 8*

Let B = 1.5 T, mb = 3.0 x 10 –27 kg, vb = 2 x 107 m/s and qb = 1.6 x 10 –19 C. What is the length L?

(a)   L = 0.33 m
(b)   L = 0.50 m
(c)   L = 1.50 m
(d)   L = 2.67 m
(e)   L = 7.50 m

### QUESTION 9**

Two identical bar magnets are dropped from the same height. Magnet B passes through a tube wrapped with a coil of wire which is connected to a light bulb. Which one hits the ground first?

(a)   magnet A
(b)   magnet B
(c)   they hit at the same time

### QUESTION 10*

This and the next four questions are about the following situation:

A long solenoid with 3333 turns per meter carries a current in the direction of the arrow. Its magnitude as a function of time is given by the expression

I(t) = 3 t + 15 amps

A rectangular loop with resistance 15 W surrounds the solenoid.

At time t = 0, the magnetic field at the center of the solenoid is:

(a)   B = 0.015 mT
(b)   B = 30.1 mT
(c)   B = 62.8 mT

### QUESTION 11*

This field points

(a)   into the shown open end.
(b)   out of the shown open end.

### QUESTION 12***

The magnetic flux through the rectangular loop at t = 0 is

(a)   F = 0.79 × 10-4 T m2
(b)   F = 2.26 × 10-4 T m2
(c)   F = 4.52 × 10-4 T m2

### QUESTION 13***

The current in the rectangular loop at time t = 5 s is

(a)   I = 0.11 × 10-6 A
(b)   I = 0.67 × 10-6 A
(c)   I = 1.05 × 10-6 A
(d)   I = 5.99 × 10-6 A
(e)   I = 18.6 × 10-6 A

### QUESTION 14**

The direction of the current in the rectangular loop travels

(a)   from A to B.
(b)   from B to A.

### QUESTION 15*

This and the next two questions are about the following situation:

A long wire carrying a 150 A current is suspended to the ceiling. A 0.3 m wide conducting bar is free to move up or down along two vertical conducting rails. With a particular current through the bar, it "floats" 0.005 m below the wire. Its mass per unit length is 0.025 kg/m.

What is the magnetic field due to the 150 A current at the location of the floating bar, 0.005 m below the wire?

(a)   B = 6 × 10-3 T
(b)   B = 12 × 10-3 T
(c)   B = 18 × 10-3 T

### QUESTION 16*

How much current must flow in the bar to maintain equilibrium at this position? Assume that g = 9.8 m/s2 is the gravitational acceleration down the page.

(a)   I = 1.02 A
(b)   I = 8.86 A
(c)   I = 22.2 A
(d)   I = 40.8 A
(e)   I = 62.1 A

### QUESTION 17**

In which direction must the current flow in the bar?

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

### QUESTION 18*

This and the next two questions are about the following situation:

A mechanical crank is used to turn a rectangular loop with area 0.33 m2 at angular frequency w in the presence of a magnetic field B = 0.4 T as shown. The 200 V peak EMF produced in the loop results in a peak voltage across the primary of the transformer of Vp = 200 V. The secondary windings are connected to an oscilloscope.

What is the peak voltage in the secondary, Vs?

(a)   Vs = 66.6 V
(b)   Vs = 200 V
(c)   Vs = 600 V

### QUESTION 19*

What is the maximum flux through the rectangular loop?

(a)   F = 0.13 T m2
(b)   F = 0.36 T m2
(c)   F = 0.40 T m2

### QUESTION 20**

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

(a)   T = 2.3 ms
(b)   T = 3.6 ms
(c)   T = 4.1 ms
(d)   T = 7.2 ms
(e)   T = 9.8 ms

### QUESTION 21**

This and the next question are about the following situation:

Two identical conducting loops are moving in a region of uniform magnetic field (indicated by the X’s). They are moving with equal speed, but loop number 1 is traveling down and loop number 2 is traveling to the right.

Compare the magnitude of the induced current in the two loops.

(a)   I1 < I2
(b)   I1 = I2
(c)   I1 > I2

### QUESTION 22**

The current in the two loops flows in the same direction (e.g. both clockwise or both counter clockwise.)

(T)   True
(F)   False

### QUESTION 23*

This and the next question are about the following situation:

The phasor diagram to the right yields 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 line in the drawing.

At the instant shown in the diagram, the voltage across the inductor is

(a)   positive
(b)   negative
(c)   zero

### QUESTION 24*

This circuit is being driven __________ the resonant frequency

(a)   faster than
(b)   slower than
(c)   at

### QUESTION 25*

This and the next two questions are about this series RLC circuit:

Calculate XC.

(a)   XC = 45 W
(b)   XC = 60 W
(c)   XC = 125 W

### QUESTION 26*

If the overall impedance of the circuit is Z = 82 W, what is R?

(a)   R = 25 W
(b)   R = 50 W
(c)   R = 75 W
(d)   R = 100 W
(e)   R = 125 W

### QUESTION 27*

What is the maximum voltage across the capacitor?

(a)   Vc = 3.32 volts
(b)   Vc = 4.13 volts
(c)   Vc = 9.97 volts
(d)   Vc = 24.8 volts
(e)   Vc = 38.1 volts

### QUESTION 28***

If the value of the inductor is increased slightly, the maximum voltage across the resistor will

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

### QUESTION 29*

This and the next question are about the following situation:

A vertically polarized electromagnetic wave is incident on a series of two linear polarizers as shown below.

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

(a)   I2 = 0
(b)   I2 = 0.35 I0
(c)   I2 = 0.67 I0
(d)   I2 = 0.84 I0
(e)   I2 = 0.91 I0

### QUESTION 30*

You must now add a third polarizer to the stack with its transmission axis rotated 10 degrees from vertical. Where should it be placed to maximize the intensity of light transmitted through all three polarizers?

(a)   in front of polarizer 1
(b)   between polarizers 1 and 2
(c)   after polarizer 2

### QUESTION 31*

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

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

### QUESTION 32**

A distant galaxy emits light that has a wavelength of 500.7 nm. On earth, the light is measured to have a wavelength of 503.7 nm. What direction is this galaxy moving?

(a)   towards earth
(b)   away from earth

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.