PHY3054 - Exam 3 - Fall 1995 - Selman Hershfield
1. A loop of wire sits in the plane of the page. You are given that at a particular instant in time the magnetic field is 3 Tesla into the page. You are not given the value of the magnetic field before or after this time. Which of the following is true?

1. The induced current flows clockwise.
2. The induced current flows counterclockwise.
3. There is no induced current.
4. There is an induced current which can either flow clockwise or counterclockwise.
5. Not enough information is given.

2. As shown in Fig. 1, a square loop of wire travels at a constant velocity (RIGHT to LEFT) from a region where there is a magnetic field (into page) to a region where there is no magnetic field and finally back to the region with a magnetic field. Use the convention that the force is positive when it is in the positive x direction and the induced current is positive when it is flowing clockwise. Which of the graphs in Fig. 1 is the current and which is force?

1. Fig. 1.2 is the current and Fig. 1.3 is the force.
2. Fig. 1.3 is the current and Fig. 1.1 is the force.
3. Fig. 1.3 is the current and Fig. 1.2 is the force.
4. Fig. 1.4 is the current and Fig. 1.1 is the force.
5. Fig. 1.4 is the current and Fig. 1.2 is the force.

3. Suppose that the square loop in Fig. 1 is 20cm on a side and that the magnetic field is 3 Tesla. Assume that the resistance of the loop is 1 kilo-Ohm. If the speed of the loop is 1 m/s, what is the magnitude of the induced current when the loop is at x = 1m?

1. 0.6 mA
2. 1.5 mA
3. 6 mA
4. 0.15A
5. 0.6A

4. In Fig. 2.1 a rod moves to the right. In Fig. 2.2 the current in the wire is increasing. What is the direction of the induced current in wire in the closed circuit in Fig. 2.1 and in the loop in Fig. 2.2?

1. Clockwise in Fig. 2.1 and clockwise Fig. 2.2.
2. Clockwise in Fig. 2.1 and counterclockwise in Fig. 2.2.
3. Counterclockwise in Fig. 2.1 and counterclockwise in Fig. 2.2.
4. Counterclockwise in Fig. 2.1 and clockwise in Fig. 2.2.
5. None of the above.

5. A magnetic field of 1 Tesla is perpendicular to the plane of a Square loop of wire with side 10 cm and resistance 1 Ohm. If in one second the Square loop is stretched so that its final sides are 17 cm, keeping the resistance the same, what is the magnitude of the induced current in the loop?

1. 7 mA
2. 13 mA
3. 19 mA
4. 25 mA
5. 31 mA

6. Which of the following is true of light?

1. The higher the frequency the greater the speed of the light.
2. The shorter the wavelength the greater the speed of the light.
3. The speed of light is the same in all media.
4. The speed of light is at most the speed of light in a vacuum.
5. None of the above.

7. The net power hitting a sphere of radius R centered about the sun is 1 x 1026 Watts. What is the net power hitting a sphere of radius R/2?

1. 0.25 x 1026 Watts
2. 0.5 x 1026 Watts
3. 1 x 1026 Watts
4. 2 x 1026 Watts
5. 4 x 1026 Watts

8. A satellite in outer space experiences light of intensity 1.5 kW m-2. If the satellite has a perfectly absorbing sail normal to the incident light and with area 104 m2 , what is the net force on the satellite?

1. 5 x 10-5 N
2. 5 x 10-4 N
3. 10-4 N
4. 0.05 N
5. 0.1 N

9. Put your name, social security number, and test number correctly on the bubble sheet and the exam printout.

1. Done correctly.
2. Done incorrectly.
3. Done incorrectly.
4. Done incorrectly.
5. Done incorrectly.

10. Three polarizers are arranged as shown in Fig. 3. What are All the distinct values of the angle A so that no light emerges from the final filter? The incoming light is assumed to be unpolarized.

1. 22.5 degrees
2. 90 degrees
3. 135 degrees
4. 90 degrees and 135 degrees
5. 22.5 degrees, 90 degrees, and 135 degrees

11. Six polarizers are arranged in series with each polarizer making an angle of 30 degrees with the preceding one. If unpolarized light of intensity I is incident upon the first polarizer, what is the intensity of the light emerging from the final polarizer?

1. 0.1 I
2. 0.015 I
3. 0.2 I
4. 0.25 I
5. 0.4 I

12. A light ray is incident upon a wedge of glass in Fig. 4. What is the angle marked X in this figure?

1. 21 degrees
2. 34 degrees
3. 40 degrees
4. 58 degrees
5. 67 degrees

13. Light is incident upon an air (n = 1) - glass (n = 1.4) interface. Which of the rays in Fig. 5 will be totally internally reflected?

1. Fig. 5.1
2. Fig. 5.2
3. Fig. 5.3
4. Fig. 5.4
5. Figs. 5.2 and 5.4

14. An object is placed 3 meters in front of a concave mirror with radius 1 meter. Which of the following is true of the image formed?

1. The image is real and erect.
2. The image is real and inverted.
3. The image is virtual and erect.
4. The image is virtual and inverted.
5. Not enough information given.

15. In Figure 6 which of the principal rays is drawn incorrectly?

1. Fig. 6.1
2. Fig. 6.2
3. Fig. 6.3
4. Figs. 6.1 and 6.3
5. All the rays are drawn correctly.

16. In Figure 7 which of the principal rays is drawn incorrectly?

1. Fig. 7.1
2. Fig. 7.2
3. Fig. 7.3
4. Figs. 7.1 and 7.3
5. All the rays are drawn correctly.

17. Two converging lenses each with focal length 30cm are placed 1m apart. If an object is placed 1m to the left of the left lens, how far is the final image (through both lenses) to the right of the right lens?

1. 0.5 m
2. 0.6 m
3. 0.7 m
4. 0.8 m
5. 1.0 m

18. A converging lens has a focal length of 2 m. If the image is virtual and 0.5 meters from the lens, how far is the object from the lens?

1. 0.4 m
2. 0.5 m
3. 0.6 m
4. 0.66 m
5. 1 m

19. An image in a concave mirror with radius 1 m has a magnification of -1/3. What is the distance of the object from the mirror?

1. 0.5 m
2. 0.66 m
3. 1 m
4. 2 m
5. 3 m

20. A fish rests at the center of a completely spherical fish bowl of radius 2m. If the index of refraction of water is 1.33 and the index of refraction of air is 1.0, how far from the surface of the fish bowl does the fish appear to an observer (outside the fish bowl!)?

1. 0.8 m
2. 1.2 m
3. 1.6 m
4. 2.0 m
5. 2.4 m

1. Answer = (5) Not enough information is given.
2. Answer = (4) Fig. 1.4 is the current and Fig. 1.1 is the force.
3. Answer = (1) 0.6 mA
4. Answer = (3) Counterclockwise in Fig. 2.1 and counterclockwise in Fig. 2.2.
5. Answer = (3) 19 mA
6. Answer = (4) The speed of light is at most the speed of light in a vacuum.
7. Answer = (3) 1 x 1026 Watts
8. Answer = (4) 0.05 N
9. Answer = (1) Done correctly.
10. Answer = (4) 90 degrees and 135 degrees
11. Answer = (1) 0.1 I
12. Answer = (5) 67 degrees
13. Answer = (3) Fig. 5.3
14. Answer = (2) The image is real and inverted.
15. Answer = (3) Fig. 6.3
16. Answer = (4) Figs. 7.1 and 7.3
17. Answer = (2) 0.6 m
18. Answer = (1) 0.4 m
19. Answer = (4) 2 m
20. Answer = (4) 2.0 m