### Summer 2011 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 97. The exam period was 75 minutes; the mean score was 60.6; the median was 61. 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 four pertain to the following situation.

In this diagram, a power supply causes current to flow through the solenoid in the direction shown (down on the visible side, upward in the back). The solenoid has radius R = 10 cm and length L = 40 cm along its axis. The single loop surrounding the solenoid has a resistance of 40 Ω and a radius of 24 cm.

How many turns of wire must there be if a current of 4 A will produce a magnetic field B = 0.02 T within the solenoid?

(a)   640
(b)   1000
(c)   1600

### QUESTION 2**

The current in the solenoid is adjusted to produce a magnetic field in the solenoid B = 0.0003 T. An electron (mass and charge obey m = 9.11 × 10- 31 kg and |q| = 1.6 × 10-19 C) inside the solenoid traverses a circular path because of that field (there are no other forces acting on it). How fast can the electron move and still stay inside the solenoid? You may assume the path of the electron lies in a plane perpendicular to the axis of the solenoid.

(a)   5.3 × 106 m/s
(b)   8.8 × 106 m/s
(c)   1.3 × 107 m/s

### QUESTION 3***

In the previous question, would the electron move in the same direction as the current in the solenoid, or in the opposite direction? Assume, again, that the electron stays inside the solenoid.

(a)   same direction as current in the solenoid
(b)   direction opposite to the current in the solenoid

### QUESTION 4***

The current in the solenoid is reduced steadily so that the magnetic field inside it diminishes from 0.3 T to 0 through 8 seconds. What current is induced in the loop?

(a)   0.011 mA
(b)   0.029 mA
(c)   0.085 mA
(d)   0.17 mA
(e)   0.23 mA

### QUESTION 5**

True or false: the induced current in the loop flows in the same direction as the current in the solenoid.

(T)   True
(F)   False

### QUESTION 6**

As in lecture, an aluminum plate is connected as a pendulum and allowed to swing between the poles of a magnet located at the bottom of the path of the plate. In this problem, you may assume that the plate never touches the magnet. True or false: as you pull the magnet away, the plate will be pulled and start to move in the same direction.

(T)   True
(F)   False

### QUESTION 7*

A positively-charged particle, initially moving at constant velocity in a field-free region, enters a region where a magnetic field is present. You may assume the charge's initial velocity is perpendicular to the direction of the magnetic field. Which one of the following is true?

(a)   As the charge enters the field, the field does positive work on it.
(b)   As the charge enters the field, the field does negative work on it.
(c)   As the charge enters the field, the field does no work on it.

### QUESTION 8**

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

The diagram shows a uniform magnetic field directed out of the page. It is confined to the region within the dashed line. At t = 0, a rectangular wire loop moving upward in the plane of the page just begins to enter the magnetic field. It takes 15 ms for the loop to enter the field completely, and an additional 90 ms for the loop to begin to exit the field at the top. If needed, external forces will be applied to maintain constant velocity.

At time t = 10 ms, in which direction does the current in the loop flow?

(a)   There is no current flow at that time.
(b)   The current flows clockwise.
(c)   The current flows counter-clockwise.

### QUESTION 9***

At time t = 45 ms, in which direction does the magnetic force on the top edge of the loop (the edge which entered the field first) act?

(a)   There is no magnetic force on that edge at that time.
(b)   The magnetic force acts downward along the page.
(c)   The magnetic force acts upward along the page.
(d)   The magnetic force acts rightward along the page.
(e)   The magnetic force acts leftward along the page.

### QUESTION 10***

In this experiment, the magnetic field points out of the page, but we could reverse that field direction in a different trial. As the loop leaves the magnetic field, what can we say about forces acting on that loop?

(a)   There will be no forces acting on the loop as it leaves the field.

(b)   There will be a magnetic force acting on the loop as it leaves the field, but its direction does not depend on the direction of the magnetic field.

(c)   The direction of magnetic force acting on the loop as it leaves the field will depend on the direction of the magnetic field.

### QUESTION 11*

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

In the RLC circuit shown, R = 220 Ω, C = 4.7 μF, and L = 33mH. A function generator applies an alternating voltage of frequency f = 640 Hz. Measurements reveal that the peak voltage across the resistor is 6 V.

What rms voltage is measured across the inductor?

(a)   0.12 V
(b)   0.8 V
(c)   2.6 V

### QUESTION 12*

In this circuit, does the function generator voltage lead (peak before) or lag (peak after) the current?

(a)   The function generator voltage leads the current.
(b)   The function generator peaks after the current.
(c)   The function generator voltage and the current peak at the same time.

### QUESTION 13*

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

(a)   0.04 W
(b)   0.08 W
(c)   0.14 W
(d)   0.28 W
(e)   0.56 W

### QUESTION 14**

While this circuit is operating, what maximum amount of charge would be measured on the capacitor?

(a)   2.3 × 10-6 C
(b)   4.8 × 10-6 C
(c)   6.8 × 10-6 C

### QUESTION 15***

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

A generator consists of 19 rectangular loops of wire wound on a rectangular frame of width 5 cm and height 20 cm. This frame -- and the loops on it -- are rotated at a constant rate in a 0.09 T magnetic field, making 18 rotations per second. This generator delivers current to a 22 Ω resistor.

At a certain instant in time, you measure the electromotive force to be zero. How much later is the electromotive force equal to its peak value?

(a)   0.014 sec
(b)   0.028 sec
(c)   0.042 sec

### QUESTION 16*

What is the maximum magnetic flux for a single loop of wire in this problem?

(a)   9 × 10-4 T m2
(b)   4.5 × 10-3 T m2
(c)   2 × 10-2 T m2
(d)   9 × 10-2 T m2
(e)   0.45 T m2

### QUESTION 17***

What is the maximum external torque required to keep the generator rotating 18 times per second?

(a)   9.2 × 10-5 Nm
(b)   3.5 × 10-4 Nm
(c)   1.5 × 10-3 Nm
(d)   0.019 Nm
(e)   0.033 Nm

### QUESTION 18***

Consider how easy or difficult it is to keep the coils of this generator rotating. What effect would it have to increase the resistance attached to it?

(a)   Increasing the resistance makes it easier to rotate the generator.
(b)   Increasing the resistance has no effect on how easy it is to keep it rotating.
(c)   Increasing the resistance makes it harder to keep the generator rotating.

### QUESTION 19*

A transformer consists of 1540 turns of wire in its primary and 990 turns of wire in its secondary. A voltage Vrms = 120 volts is applied to the primary. A current Irms = 0.26 A flows into the primary. A resistor is attached to the secondary. What is its resistance?

(a)   1120 Ω
(b)   720 Ω
(c)   460 Ω
(d)   300 Ω
(e)   190 Ω

### QUESTION 20**

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

The phasor diagram pertains to an RLC circuit connected to a function generator. What can we say about the function generator voltage at the instant shown?

(a)   The function generator voltage is decreasing.
(b)   The function generator voltage is at its peak.
(c)   The function generator voltage is increasing.

### QUESTION 21**

Regarding the RLC circuit in the previous problem: what happens to the average power delivered by the function generator if we slightly increase the frequency?

(a)   The average power would increase.
(b)   The average power would decrease.

### QUESTION 22**

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

A metal bar slides leftward across two parallel metal tracks (separated by = 14 cm) at speed v = 0.44 m/s. A magnetic field B = 0.1 T is perpendicular to the page. A current I = 0.6 mA flows downward through the resistor.

Does the field point into the page or out of the page?

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

### QUESTION 23*

What is the resistance R?

(a)   0.01 Ω
(b)   0.1 Ω
(c)   1 Ω
(d)   10 Ω
(e)   100 Ω

### QUESTION 24***

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

The top and middle wires carry current I = 50 mA to the right; the bottom wire carries I = 50 mA to the left. The separation L = 11 cm.

An electron moves rightward, just above the top wire and parallel to it. What is the direction of the magnetic force acting on that electron?

(a)   out of page
(b)   into page
(c)   upward along page
(d)   downward along page
(e)   There is no magnetic force on it.

### QUESTION 25***

What net (combined) force do the bottom and middle wires exert on each meter of the top wire?

(a)   6.8 × 10-9 N upward
(b)   2.3 × 10-9 N upward
(c)   0
(d)   2.3 × 10-9 N downward
(e)   6.8 × 10-9 N downward

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.