### Spring 2009 Physics 102 Hour Exam 2(28 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 143. The exam period was 90 minutes; the mean score was 109.9; the median was 114. 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 two pertain to the 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 2*

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

(a)   particle a
(b)   particle b

### QUESTION 3*

Let B = 1.5 T, mb = 3.0 × 10 -27 kg, vb = 2 × 107 m/s and qb = 1.6 × 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 4*

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 5*

A square loop of wire with area 2 m2 is rotated around an axis at a constant frequency, in a magnetic field of 2.7 mT. A peak EMF of 0.3 V is produced. At what frequency f is the loop rotated?

(a)   2.75 Hz
(b)   4.42 Hz
(c)   8.84 Hz
(d)   30.31 Hz
(e)   55.56 Hz

### QUESTION 6*

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

A circular loop (radius r = 0.25 m) is sitting (not moving) in a spatially-uniform magnetic field (as shown below). The loop has a resistance per unit length of 5 Ω/m. The strength of the field is given by

B(t)= (1.5 +(0.2)t)

where B is in tesla and t is in seconds.

What is the flux Φ through the loop at time t = 0 s?

(a)   0 T-m2
(b)   0.03 T-m2
(c)   0.26 T-m2
(d)   0.75 T-m2
(e)   1.31 T-m2

### QUESTION 7*

What is the flux Φ through the loop at time t = 6 s?

(a)   0 T-m2
(b)   0.46 T-m2
(c)   1.20 T-m2
(d)   2.73 T-m2
(e)   5.10 T-m2

### QUESTION 8**

What is the magnitude of the induced current at t = 6 s?

(a)   4.3 mA
(b)   2.5 mA
(c)   7.0 mA
(d)   10.3 mA
(e)   25.0 mA

### QUESTION 9*

Two particles are traveling in the same direction at constant speed, v. Particle 1 has mass M and charge Q. Particle 2 has mass 2M and charge 2Q. If the particles are suddenly subject to a magnetic field perpendicular to their direction of motion, what is the ratio of the acceleration experienced by particle 1 (a1) to the acceleration experienced by particle 2 (a2)?

(a)   a1 / a2 = ¼
(b)   a1 / a2 = ½
(c)   a1 / a2 = 1
(d)   a1 / a2 = 2
(e)   a1 / a2 = 4

### QUESTION 10*

In the drawing below, a battery is connected to a resistor in series with a coil of wire wrapped around a tube. The circuit has been connected for a long time.

Which of the following would decrease the magnetic flux through the coil? (Allow the circuit to reach new steady conditions again.)

(a)   increasing battery voltage
(b)   increasing the resistance
(c)   increasing the number of loops around the tube

### QUESTION 11**

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

A conducting bar slides along metal rails 6 meters apart at a constant speed of v = 3 m/s. A resistor with resistance R = 100 Ω is connected across the rails. The bar and rails are in a magnetic field of magnitude B = 2.5 × 10-4 T, pointing out of the page.

In which direction does the induced current flow through the resistor R in the above picture?

(a)   up
(b)   down

### QUESTION 12*

How much power is dissipated in the resistor R at the instant depicted above?

(a)   2.0 × 10-7 W
(b)   4.0 × 10-7 W
(c)   4.5 × 10-5 W
(d)   2.0 × 10-3 W
(e)   4.0 × 10-3 W

### QUESTION 13*

If R is now doubled, and everything else remains the same as above, by what factor does the power dissipated change?

(a)   increases by a factor of 4
(b)   increases by a factor of 2
(c)   remains the same
(d)   decreases by a factor of ½
(e)   decreases by a factor of ¼

### QUESTION 14*

This and the next question are conceptually related.

A charged particle is traveling at speed v. It encounters a static magnetic field oriented perpendicular to its path. The particle changes its

(a)   speed ( = magnitude of the velocity).
(b)   direction.
(c)   both its speed and direction.
(d)   neither its speed or direction.
(e)   All of the above scenarios are possible.

### QUESTION 15*

A charged particle is traveling at speed v. It encounters a static magnetic field oriented parallel to its path. The particle changes its:

(a)   speed ( = magnitude of the velocity).
(b)   direction.
(c)   both its speed and direction.
(d)   neither its speed or direction.
(e)   All of the above scenarios are possible.

### QUESTION 16*

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

The diagram below shows a generator consisting of a square loop of 5 coils of area 0.1 m2 in a magnetic field of 0.05 T. The loop is rotating at an angular frequency ω = 20 rad/s.

What is the maximum induced emf, Vmax?

(a)   0.1 V
(b)   5 V
(c)   0.5 V
(d)   1 V
(e)   1.5 V

### QUESTION 17**

The generator is attached to a resistor of resistance R (in Ω). What is the average power dissipated by the resistor?

(a)   Vmax2 / R
(b)   Vmax2 / (√2R)
(c)   Vmax2 / (2R)
(d)   Vmax R
(e)   Vmax R2

### QUESTION 18**

A solenoid of length 0.5m, with 100 coils, has an inductance L = 15 H. It is now stretched to a length of 1 m. The diameter and number of coils are not changed. The inductance is now:

(a)   7.5 H
(b)   15 H
(c)   30 H
(d)   100 H
(e)   225 H

### QUESTION 19*

This and the next question are conceptually related.

A particle is placed in a uniform magnetic field of strength 1 T. The particle has a velocity of 400 m/s, perpendicular to the magnetic field. If the particle moves in a circle of radius 4 m, what is its ratio of charge to mass (q/m), in Coulombs per kg?

(a)   q/m = 0 C/kg
(b)   q/m = 25 C/kg
(c)   q/m = 50 C/kg
(d)   q/m = 100 C/kg
(e)   q/m = 200 C/kg

### QUESTION 20***

A particle with a charge-to-mass ratio of 50 C/kg is moving at a speed of 200 m/s. What electric potential difference is required to stop the particle?

(a)   200 V
(b)   400 V
(c)   800 V
(d)   1200 V
(e)   1600 V

### QUESTION 21*

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

Given are two long wires carrying currents. The first is located at (x = 0, y = 1 m) and carries a current of 2 A directed out of the page. The second is located at (x = 0, y = -1 m) and carries a current of 1 A directed into the page. (See the figure.)

What is the magnitude of the magnetic field at the origin?

(a)   1 × 10-7 T
(b)   3 × 10-7 T
(c)   6 × 10-7 T
(d)   12 × 10-7 T
(e)   20 × 10-7 T

### QUESTION 22**

What is the approximate direction and magnitude of the magnetic field at (x = 100 m, y = 0)?

(a)   zero
(b)   1 × 10-9 T in the +y direction
(c)   2 × 10-9 T in the +y direction
(d)   1 × 10-9 T in the +x direction
(e)   2 × 10-9 T in the -y direction

### QUESTION 23***

Suppose we double the current I1 (such that this wire now carried a current of 2 A). What direction (exact) will the magnetic field at (x = 100 m, y = 0) now be?

(a)   +x
(b)   +y
(c)   -x
(d)   -y
(e)   There is no magnetic field there.

### QUESTION 24**

A particle of mass m and positive charge q is moving with velocity v along the +x direction. The particle is located near the center of a circular loop of radius R. The current in the loop is I, directed counterclockwise. What is the magnitude and direction of the acceleration of the moving particle?

(a)   0
(b)   0Iv / 2mR in the -y direction
(c)   0Iv / 2mR out of the page
(d)   0Iv / 2mR in the +y direction
(e)   0Iv / 2mR into the page

### QUESTION 25*

An electric current I passes through a straight wire in the direction pointing out of the page. Magnetic compasses are placed at points A and B as shown in the figure.

Which one of the following is true?

(a)   The needles will not deflect.
(b)   The needles will deflect in opposite directions.
(c)   Both the needles will deflect in the same direction.

### QUESTION 26*

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

A mass spectrometer analyzes the elemental composition of a sample by sending it through a magnetic field. In the figure below, an unknown chemical compound traveling at velocity v through a magnetic field B = 1 T pointing into the page is deflected to the right, tracing a circular path of radius R = 10 cm. An analyzer collects the molecules.

What is the sign of the charge?

(a)   positive
(b)   negative

### QUESTION 27*

Calculate the velocity v required such that a molecule with a charge of magnitude |Q = 1.6 × 10-19 C and mass m = 1.6 × 10-24 kg is collected by the analyzer.

(a)   102 m/s
(b)   103 m/s
(c)   104 m/s
(d)   106 m/s
(e)   108 m/s

### QUESTION 28*

If the charge of the molecule is doubled, what is the new radius R'?

(a)   0.05 m
(b)   0.25 m
(c)   0.5 m
(d)   1 m
(e)   2 m

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.