### Summer 2007 Physics 102 Hour Exam 2(22 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 81. The exam period was 75 minutes; the mean score was 49.2; the median was 49. Click here to see page1 page2 page3 of the formula sheet that came with the exam.

### QUESTION 1**

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

As in lecture, a cylindrical magnet ('cow magnet') is released to fall through a narrow copper pipe. Assume, as shown, that it is dropped with the north pole closer to the ground. The tube is at rest; arrow is drawn only for the sake of reference.

An induced current flows in the direction indicated by the arrow (which shows a current flowing left to right in the part of the copper pipe closest to you).

(T)   True
(F)   False

### QUESTION 2***

As in lecture, imagine instead trying this with an insulating (PVC) pipe rather than the copper pipe. Will there be an electromotive force in the PVC pipe?

(a)   There will be no emf in the PVC pipe.
(b)   There will be an emf in the PVC pipe.

### QUESTION 3**

Using the copper pipe once again, suppose the South Pole had been closer to the ground. How does this compare to the original case, where the North Pole had been closer to the ground?

(a)   The induced current and the force on the cow magnet will be the same in both cases.
(b)   The induced current will reverse direction but the force will be the same in both cases.
(c)   Both induced current and force on the magnet will switch directions when the poles are reversed.

### QUESTION 4*

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

A charge (|q| = 1.6 × 10-19C, m = 1.673 × 10-27 kg) enters a circular region in which a constant field (B = 0.08 T) points out of the page. It enters moving vertically at the exact bottom of the field and emerges moving horizontally at the rightmost side. Ignore all forces other than that exerted by the magnetic field. Call the radius of this circular region r.

What is the sign of the charge?

(a)   positive
(b)   negative

### QUESTION 5*

If the radius of this circular region is r = 15 cm, what is speed of this charge?

(a)   1200 m/s
(b)   1700 m/s
(c)   2 × 105 m/s
(d)   1 × 106 m/s
(e)   8 × 106 m/s

### QUESTION 6**

Suppose instead this same charge traversed a small circle entirely within the field. How long would it take this charge to travel a complete circle once?

(a)   8 × 10-7 sec
(b)   5 × 10-6 sec
(c)   7 × 10-6 sec

### QUESTION 7**

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

A square loop of side 1 cm moves at constant velocity to the right of 1 cm/sec. It moves from a field-free region on the left into a uniform magnetic field and then back into the field-free region on the right. The field points into the page. The region in which the field is contained is 5 cm wide. The loop has a total resistance of 500 Ω.

At various times a current may be detected in the square loop. For how many seconds, in total, will this current flow?

(a)   1 sec
(b)   2 sec
(c)   3 sec
(d)   5 sec
(e)   6 sec

### QUESTION 8**

The magnetic field B = 0.5 T. When a current does flow in the loop, what is its magnitude?

(a)   I = 1 × 10-3 A
(b)   I = 1 × 10-5 A
(c)   I = 1 × 10-7 A

### QUESTION 9***

Parts of this loop may experience a magnetic force. During the time before the loop reaches the center of the field, in what direction does the magnetic force act on the top segment of wire in the loop?

(a)   The top segment is pulled to the left.
(b)   The top segment is pulled upward along the page.
(c)   The top segment is pulled downward along the page.

### QUESTION 10*

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

Consider the AC circuit shown here. The resistor R = 1000 Ω, the capacitive reactance XC = 1800 Ω, and the inductive reactance XL = 900 Ω. The function generator produces a voltage given by the formula Vgen(t) = 40 sin(800t).

Calculate the capacitance.

(a)   0.7 μF
(b)   2.2 μF
(c)   4.4 μF

### QUESTION 11**

Calculate the rms voltage across the inductor.

(a)   19 V
(b)   27 V
(c)   34 V
(d)   37 V
(e)   40 V

### QUESTION 12***

Calculate the peak power delivered by the function generator.

(a)   0.31 W
(b)   0.44 W
(c)   0.63 W
(d)   0.88 W
(e)   0.99 W

### QUESTION 13**

You now increase the frequency applied by the function generator. Does the peak current in the circuit increase or decrease?

(a)   It increases.
(b)   It decreases.

### QUESTION 14***

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

The diagram shows a wire loop of height 8 cm and width 4 cm. The loop has a total resistance R = 15 Ω. A magnetic field of strength B = 0.9 T points into the page. In the first question, a battery is connected, causing a current to flow. Points P and Q are labeled for the sake of reference.

If an 18 volt battery is attached, what is the magnitude of the torque acting on this loop?

(a)   0
(b)   0.0035 Nm
(c)   35 Nm

### QUESTION 15**

Now the battery is removed. The loop is closed to form a rectangle and made to rotate at constant angular velocity. The picture shows this loop as viewed from above. From this point of view, the magnetic field points upward along the page and the loop rotates clockwise. At the instant shown, the angle θ = 38°. Assume the total resistance is still R = 15 Ω and the applied field is still 0.9 T.

In which direction does the induced current flow?

(a)   from P towards Q
(b)   from Q towards P

### QUESTION 16**

If the induced current is 0.12 mA at this instant, what is the angular velocity ω?

### QUESTION 17*

If the induced current 0.12 mA, what is the torque τ at that instant?

(a)   0.6 × 10-7 Nm
(b)   1.3 × 10-7 Nm
(c)   2.1 × 10-7 Nm
(d)   3.5 × 10-7 Nm
(e)   4.5 × 10-7 Nm

### QUESTION 18*

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

This diagram shows a pair of very long wires carrying currents I1 = 0.4 A and I2 = 1.6 A, both flowing to the right. The wires are separated by L = 15 cm.

Which one of these is true?

(a)   These two wires repel each other.
(b)   These two wires attract each other.

### QUESTION 19**

Where between these two wires is the magnetic field B = 0?

(a)   nowhere between them
(b)   10 cm above the bottom wire
(c)   12 cm above the bottom wire

### QUESTION 20**

Calculate the magnitude of the force exerted on a 4 cm long segment of the bottom wire.

(a)   3.4 × 10-8 N
(b)   5.1 × 10-8 N
(c)   8.5 × 10-8 N
(d)   1.7 × 10-7 N
(e)   2.1 × 10-7 N

### QUESTION 21**

The phasor diagram shown here refers to a particular series RLC circuit connected to a function generator. You are told nothing about the circuit other than the lengths shown represent voltages associated with the three components. What peak voltage is measured across the function generator?

(a)   9 V
(b)   11 V
(c)   12 V

### QUESTION 22***

The diagram depicts an ideal transformer. Assume there are four times as many turns of wire in the primary as in the secondary. A peak voltage 70 V is applied by the function generator; the attached resistor R = 18 Ω. Find the average power dissipated in the resistor.

(a)   6 W
(b)   8.5 W
(c)   12 W
(d)   17 W
(e)   24 W

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.