### Summer 2001 Physics 102 Hour Exam 2(23 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 23 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 122. When the exam was given, the mean was 73.3; the median was 74. 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:

In the circuit shown, the capacitor is initially uncharged. At t = 0, the switch is closed. The time constant t for the charging of the capacitor is

(a)   54 µs
(b)   18 µs
(c)   36 µs
(d)   0.019 µs
(e)   15 µs

### QUESTION 2**

What is the approximate current through the 3 W resistor when t = t / 100 ?

(a)   3/8 A
(b)   1/2 A
(c)   3/4 A
(d)   1 A
(e)   3/2 A

### QUESTION 3***

The figure shows the motion of electrons in a wire, which is near the N pole of a permanent magnet. The wire will be pushed:

(a)   towards the magnet
(b)   away from the magnet
(c)   downward
(d)   upwards
(e)   along its length

### QUESTION 4**

Four long straight wires carry equal currents into the page as shown. The magnetic force exerted on wire F is:

(a)   north
(b)   east
(c)   south
(d)   west
(e)   zero

### QUESTION 5***

Two straight wires pierce the plane of the paper at vertices of an equilateral triangle as shown below. They each carry 2 A, out of the paper. The magnetic field at the third vertex (P) has a magnitude (in Tesla) of

(a)   1.0 × 10-5
(b)   1.7 × 10-5
(c)   2.0 × 10-5
(d)   5.0 × 10-6
(e)   8.7 × 10-6

### QUESTION 6***

The circuit shown below is in a uniform magnetic field that is into the paper and is decreasing in magnitude at the rate of 150 T/s. The current in the circuit (in amperes) is:

(a)   0.18
(b)   0.22
(c)   0.40
(d)   0.62
(e)   none of these

### QUESTION 7*

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

Two fixed conductors are connected by a resistor of resistance 30 ohms. The two fixed conductors are separated by 3 m. A moving conductor slides on them at velocity 14 m/s. Out of the page is a uniform magnetic field (shown by the black dots in the figure) with a magnitude of 2 × 10-3 T. Points a and b are two points on the circuit just above and below the resistor.

What is the magnitude of the rate of change of the magnetic flux through the loop defined by the bar and the wire?

(a)   0 V
(b)   1.7 × 10-3 V
(c)   5.3 × 10-4 V
(d)   42 × 10-3 V
(e)   84 × 10-3 V

### QUESTION 8*

What is the potential at point a relative to that at point b?

(a)   42 × 10-3 V
(b)   -84 × 10-3 V
(c)   -1.7 × 10-3 V

### QUESTION 9*

What is the power dissipated in the resistor?

(a)   28 × 10-4 J
(b)   0 J
(c)   2.35 × 10-4 J

### QUESTION 10*

What force is acting on the bar due to the magnetic field? (Assume 10-3 A for the current flowing.)

(a)   0.39 × 10-5 N
(b)   1.7 × 10-5 N
(c)   6 × 10-6 N
(d)   12 × 10-5 N
(e)   53 x 10-5 N

### QUESTION 11**

What external force must be applied to the bar to keep it moving at constant velocity?

(a)   0.39 × 10-5 N
(b)   1.7 × 10-5 N
(c)   5.3 × 10-5 N
(d)   12. × 10-5 N
(e)   6 × 10-6 N

### QUESTION 12**

If the velocity of the moving bar is doubled, what happens to the power dissipated by the resistor?

(a)   The power increases by a factor of 2.
(b)   The power decreases by a factor of 2.
(c)   The power increases by a factor of 4.
(d)   The power decreases by a factor of 4.
(e)   The power is unchanged.

### QUESTION 13*

This and the next question are about the following situation:

In the circuit shown, the capacitance C is 5µ F, and the inductance L is 50 mH, and V0 = 10 Volts. Switch S is first pushed up to charge the capacitor.

When S is then pushed down, the current in the circuit will oscillate at a frequency of

(a)   f = 318 Hz
(b)   f = 0.01 Hz
(c)   f = 12.5 Hz
(d)   f = 2000 Hz
(e)   f = 300 Hz

### QUESTION 14**

The total energy stored in the oscillating circuit (with switch S pushed down) is:

(a)   500 × 10-6 J
(b)   250 × 10-6 J
(c)   0 J
(d)   123 × 10-6 J
(e)   200 × 10-6 J

### QUESTION 15***

The sum of the reactances of the capacitor and inductance at resonance is

(a)   100 W
(b)   200 W
(c)   0 W
(d)   50 W
(e)   300 W

### QUESTION 16*

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

A square loop is in a uniform B field of .01 T. The loop carries a current of 5 A in the direction shown. The loop and the field lie in the plane of the page The B-field makes an angle of 45° with the sides of the loop. The length of a loop side is .03 m.

What is the magnitude of the magnetic force on side ab ?

(a)   0 N
(b)   3.6 × 10-4 N
(c)   10.6 × 10-4 N
(d)   15.0 × 10-4 N
(e)   |18.4 × 10-4 N

### QUESTION 17***

The magnitude of the force on the loop

(a)   0 N
(b)   10.6 × 10-4 N
(c)   21.2 × 10-4 N
(d)   5.3 × 10-4 N
(e)   42.4 × 10-4 N

### QUESTION 18***

The magnitude of the torque on the loop is

(a)   0 Nm
(b)   3.18 × 10-5 Nm
(c)   4.50 × 10-5 Nm

### QUESTION 19*

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

A circular conducting loop with resistance of 5 ohms and diameter d=0.4 m surrounds a long solenoid with radius R = 0.15 m and 2 × 104 turns/meter. The current in the solenoid is flowing in the counterclockwise direction producing a uniform magnetic field inside the solenoid of B = 0.75 T.

From the end view, the magnetic field at the center of the solenoid is pointing

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

### QUESTION 20*

Calculate the magnitude of the current Is flowing in the solenoid.

(a)   Is = 9.5 A
(b)   Is = 16.4 A
(c)   Is = 29.8 A
(d)   Is = 58.7 A
(e)   Is = 68.2 A

### QUESTION 21***

The current in the solenoid is steadily decreased to zero over a period of 5 seconds. Calculate IL the magnitude of the average induced current in the outside loop while the current in the solenoid is being decreased.

(a)   IL = 2.12 × 10-3 A
(b)   IL = 2.46 × 10-3 A
(c)   IL = 3.18 × 10-3 A
(d)   IL = 3.76 × 10-3 A
(e)   IL = 4.30 × 10-3 A

### QUESTION 22**

When the number of loops in the solenoid is doubled, its self-inductance

(a)   remains the same.
(b)   is reduced by a factor of 2.
(c)   doubles.
(e)   is reduced by a factor of 4

### QUESTION 23*

Which procedure maximizes the magnitude of the magnetic field at the center of the solenoid, where L is the length of the solenoid?

(a)   L, N and I are doubled.
(b)   L and I are doubled and N is intact.
(c)   N and I are doubled and L is intact.

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.