### Fall 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 126. The exam period was 90 minutes; the mean score was 94.7; the median was 95. 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*

The primary coil of a transformer is attached to a power supply of 10 V. The secondary coil is attached to a resistor of 20 Ω. The maximum current through the resistor is 15 A. Np, the number of turns on the primary side is 20 turns. What is Ns, the number of turns on the secondary side?

(a)   20
(b)   100
(c)   200
(d)   600
(e)   800

### QUESTION 2*

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

Two long straight parallel wires are separated by a distance of 3 m. One wire carries a current of 3 A. The other carries a current of 2 A. Both currents run in the same direction.

What is the magnitude of the force on 1 meter of the wire carrying 3 A, due to current in the other wire?

(a)   7 × 10-8 N
(b)   1 × 10-7 N
(c)   4 × 10-7 N
(d)   7 × 10-7 N
(e)   2 × 10-6 N

### QUESTION 3*

What is the magnitude of the magnetic field at a point exactly halfway between the 2 wires?

(a)   0
(b)   1.3 × 10-7 T
(c)   4.0 × 10-7 T
(d)   5.7 × 10-7 T
(e)   6.2 × 10-7 T

### QUESTION 4**

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

A long section of straight wire carrying a current runs parallel to the surface of the earth on the east-west axis. It has a linear mass density of 10 g/m. The earth's magnetic field is approximately 50 μT and points south to north.

What current must the wire carry to balance the force of gravity (i.e. the wire will remain suspended above the ground) ?

(a)   2 kA
(b)   32 kA
(c)   90 kA
(d)   120 kA
(e)   2 MA

### QUESTION 5**

What has to be the direction of the current in the wire?

(a)   east to west
(b)   west to east
(c)   It doesn't matter.

### QUESTION 6*

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

A hemoglobin protein, of mass 10-19 g and charge +3e (three times the charge of the electron), is shot at a velocity v = 60 m/s into a region with a uniform magnetic field 0.5 T (here pointing out of the page).

What is the displacement L?

(a)   1.3 cm
(b)   2.5 cm
(c)   5.0 cm
(d)   10 cm
(e)   12 cm

### QUESTION 7**

How long does the protein spend in the magnetic field region?

(a)   0.66 ms
(b)   1.3 ms
(c)   2.6 ms
(d)   5.2 ms
(e)   7.2 ms

### QUESTION 8*

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

A circular loop of radius R = 0.25 m is rotated at a frequency f = 10 Hz in a uniform magnetic field B = 0.2 T, pointing to the right.

What is the magnitude of the induced EMF when the loop is at the state shown here?

(a)   0 V
(b)   1.38 V
(c)   2.14 V
(d)   3.74 V
(e)   4.89 V

### QUESTION 9***

Based on the direction of the induced current, I, shown in the diagram, what direction is the loop rotating?

(a)   clockwise
(b)   counter-clockwise

### QUESTION 10**

When will the EMF be maximal?

(a)   when the angle between the surface normal (to the loop) and the field is 0°
(b)   when the angle between the surface normal (to the loop) and the field is 90°
(c)   The EMF is constant.

### QUESTION 11**

A charge (q = 2.5 μC, m = 3.1 g) is moving in a uniform magnetic field (B = 2.7 T, pointing in the y direction). The charge has a velocity v = 2.5 cm/s at an angle &theta ;= 25˚ relative to the magnetic field (see illustration below). As the charge moves, it traces out a circle in the x-z plane. What is the radius of the circle?

(a)   0.21 m
(b)   0.42 m
(c)   1.77 m
(d)   2.95 m
(e)   4.8 5 m

### QUESTION 12*

Two parallel wires have current flowing through them in the same direction. The two wires will

(a)   attract each other.
(b)   repel each other.
(c)   not exert force on each other.

### QUESTION 13*

This question and the next two are conceptually related.

A rectangular loop with side lengths a = 4 m and b = 5 m is in a constant magnetic field, pointing into the page. The loop has a resistivity per unit length of 15 Ω/m.

The magnetic field strength begins to decrease at a rate of 0.2 T/sec. What is the magnitude of the induced current in the loop?

(a)   0 A
(b)   1.3 × 10-4 A
(c)   1.5 × 10-2 A
(d)   0.1 A
(e)   3.4 A

### QUESTION 14**

Which direction does the current flow?

(a)   clockwise
(b)   counter-clockwise

### QUESTION 15**

If the lengths a and b are both doubled, at what rate will the field strength need to be changing at in order to maintain the same current?

(a)   decreasing at 0.2 T/sec
(b)   decreasing at 0.1 T/sec
(c)   unchanging
(d)   increasing by 0.1 T/sec
(e)   increasing by 0.2 T/sec

### QUESTION 16**

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

Neglect the force due to gravity in these four questions.

A charged particle is undergoing circular motion in a magnetic field. Its velocity, its acceleration and the magnetic field are best described by which one of the following statements?

(a)   v and a are parallel to each other, B perpendicular to both.
(b)   v and B are parallel to each other, a is perpendicular to both.
(c)   a and B are parallel to each other, v is perpendicular to both.
(d)   All three are parallel to each other.
(e)   All three are perpendicular to each other.

### QUESTION 17**

Next, the particle is suddenly "pushed" (given some velocity) in the direction of the magnetic field. What does the trajectory of the particle look like?

(a)   a cube
(b)   a cone
(c)   a helix
(d)   a straight line
(e)   a circle

### QUESTION 18*

We now turn off the magnetic field. What is the new trajectory of the particle?

(a)   a cube
(b)   a cone
(c)   a helix
(d)   a straight line
(e)   a circle

### QUESTION 19*

We now reset the particle's motion such that it is undergoing circular motion in a magnetic field. If we suddenly double the mass of the particle (while leaving its speed unchanged), the radius of the particle's trajectory will

(a)   stay the same.
(b)   double.
(d)   halve.
(e)   become one quarter.

### QUESTION 20*

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

A mad scientist causes a giant magnetic field to be erected all over the quad, directly upward. You are playing Frisbee using a resistive circular loop of wire. You stand just outside the quad while your friend is inside.

After playing for a while, you notice the Frisbee is getting warm. Why?

(a)   The magnetic field makes the air around the Frisbee hotter, heating the Frisbee.

(b)   The radiation caused by the magnetic field cooks the Frisbee.

(c)   Current is induced by the magnetic field, heating the Frisbee.

### QUESTION 21*

Now you step inside the quad and continue your game. Does the Frisbee continue getting hotter?

(a)   yes
(b)   no

### QUESTION 22**

A flat square is 3 meters on each side. The square is oriented as seen in the picture. What is the flux going through the square?

(a)   12.1 Tm2
(b)   30 Tm2
(c)   45 Tm2
(d)   78 Tm2
(e)   90 Tm2

### QUESTION 23*

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

A particle of mass 3 × 10-8 kg, negative charge -20 μC, and velocity v = 2000 m/s enters a region with magnetic field B1 perpendicular to the plane of the paper. The direction and magnitude of B1 are not given.

The particle performs a quarter circle trajectory and enters another region with a different magnetic field B2 perpendicular to the plane of the paper. In this region, it performs a semi-circular trajectory. The direction and magnitude of B2 are not given.

What is the direction of the magnetic field B1?

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

### QUESTION 24*

What is the direction of the magnetic field B2?

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

### QUESTION 25*

What is the magnitude of the magnetic field B1?

(a)   B1 = 1 T
(b)   B1 = 1.2 T
(c)   B1 = 3 T
(d)   B1 = 3.6 T
(e)   B1 = 5 T

### QUESTION 26*

Compare the relative magnitudes of B1 and B2.

(a)   B1 < B2
(b)   B1 = B2
(c)   B1 > B2

### QUESTION 27**

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

A conducting rod of length L and mass m slides down frictionless vertical wires at a constant velocity v. The wires are connected across a light bulb. A uniform magnetic field B is present everywhere, pointing into the page as indicated. Assume the rod and wires are perfect conductors, and that gravity is present.

Which way does the current flow?

(a)   clockwise
(b)   counterclockwise

### QUESTION 28***

If v = 1 m/s, m = 2 g, and the induced current I = 2 A, what is the resistance of the light bulb?

(a)   4.9 mΩ
(b)   9.8 mΩ
(c)   There is not enough information to answer this question.

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.