Physics 72: Chapter 32 Problem Set
Summer 2010
Name: Lecturer: Mr Carlos Baldo III
Score: Section: X7-7
Encircle the letter of the correct answer. Solve each problem clearly and completely for full credit. 1.
EM Romantic Quotes. Which of the following are electromagnetically valid love notes of the magnetic field to the electric field? I. “If you are tired and weary, I can do all the oscillations while you are asleep and steady.” II. “My oscillation oscillates you, your oscillation oscillates me – we will never be weary.” III. “Through water, through oil, through prism, my love is not changing, like my frequency.” IV. “We may pass through sweet honey or bitter coffee but our union and our wavelength will not be disturbed.” A. B. C. D. E.
I and III I and IV II and III II and IV I, II, III and IV
2.
Maxwell. Which of the following statements violates Maxwell’s equations? A. The absence of a magnetic monopole B. Induction of a conservative electric field can be induced by a time-varying magnetic flux. C. Mutually perpendicular time-varying electric and magnetic field D. A and B E. B and C
3.
Big Bang. Sheldon is studying the EM wave that is transmitted to his Walkman along the x-axis. Assuming that that the speed of EM wave in a medium is v=2 x 108m/s, he determined its frequency (f=100 Hz), and E-field’s amplitude (4 N/C) and direction (y-axis). Accordingly, he writes in his notes this result: E= 4 cos(____x – ____t) N/C j. A. 2∏ x 10-6m-1; 200∏ rad/s B. ∏ x 10-6m-1; 200∏ rad/s C. 2∏ x 106m-1; 100∏ rad/s D. 2∏ x 106m-1; 200∏ rad/s E. ∏ x 106m-1; 200∏ rad/s
4.
Kamehame wave. A linearly-polarized electromagnetic kamehame wave has its electric field given by
E( x,t ) = 2c[cos( kx + ωt ) varying along the +y direction in vacuum. What is the corresponding magnitude and direction its magnetic field? Note: c = speed of light. A. B. C. D. E. 5.
B( x,t ) = 2c[cos( kx − ωt )] , - z B( x,t ) = 2c[cos( kx + ωt )] , +z B( x,t ) = 2c[cos( kx + ωt )] , - z B( x,t ) = 2[cos( kx + ωt )] , +z B( x,t ) = 2[cos( kx + ωt )] , - z
Transverse Wave. Juliet transformed her belongings into an EM wave (gamma rays). She transmitted it to her lover, Naruto. Romeo, through his magic camera, discovered that at point x=xo and time t=to, the electric field of the gamma ray is as shown. Heartbroken, Romeo sketched the corresponding magnetic field and the Poynting vector. Which of the following is his sketch? Egamma ray
+y
+x Naruto +z
Juliet
Physics 72: Chapter 32 Problem Set
Summer 2010
6.
Magnetic Field. An EM wave travels in vacuum along the x-axis. At some point and at some instant, the Efield has its maximum value of 200 N/C and is along the z-axis. What is the corresponding the magnitude and direction of the magnetic field? A. 6 × 1010 T; -z-axis B. 6 × 1010 T; -y-axis C. (3/2) × 10-6 T; -z-axis D. (2/3) × 10-6 T; -z-axis E. (2/3) × 10-6 T; -y-axis
7.
Poynting. Electric field vector and magnetic field vector at time t and position r in a vacuum are given by E = (4 V/m)i and B=(5 T)j, what is the magnitude and direction of the Poynting vector? A. j(20/µ0) W/m2 B. i(10/µ0) W/m2 C. Zero D. k(20/µ0) W/m2 E. k(10/µ0) W/m2
8.
Polarized. If the Electric field and magnetic field of a linearly polarized EM wave in +y-axis is as shown in the figure, what is the direction of the Poynting vector? A. –x B. +x C. +y D. –y E. +z
9.
Easy kuno. For plane waves, which of the following statements is TRUE? A. The E and B fields are longitudinal. B. Waves striking a small area travel in various directions. C. Waves striking a small area all travel in the same direction. D. Plane waves in vacuum move at a speed much lower than c. E. The fields are uniform over the planes that are parallel to the Poynting vector.
10. EM Sores. Which of the following cannot be a source of EM waves? A. A lovely deer that swallowed an electron, going around in circles B. A closed DC circuit consisting of an ideal battery and a resistor C. A proton riding on a ball thrown by Kobe Bryant D. A cellphone E. A lightning 11. Source A. B. C. D. E.
of EM Wave. Which of the following cannot be a source of EM waves? A closed circuit consisting of a fully charged capacitor and an inductor A point charge with initial velocity vO that is under a uniform magnetic field An electron riding on a ball thrown by Dwayne Wade A pendulum with a proton suspended on the mass-less string A charged point marble sliding on a frictionless table
For the next two questions, consider the figure at the right. Five gummy bears with laser guns emitting light similar to their colors are arranged equally spaced in a semi-circle. A blue gummy bear is found lying at the center of the semi-circle, after being hit by the lasers coming from the five other bears. 12. Gummy bear 1. If the five bears fire lasers at the same time, which of them could have hit the blue gummy bear first if the gummy bears are assumed to be in vacuum? A. Green gummy bear B. Red gummy bear C. Orange gummy bear D. Yellow gummy bear E. None of the above, the lasers reached the gummy bear at the same time.
Physics 72: Chapter 32 Problem Set
Summer 2010
13. Gummy bear 2. If the shooting incident actually happened underwater, which of them could have hit the blue gummy bear first? A. Green gummy bear B. Red gummy bear C. Orange gummy bear D. Yellow gummy bear E. None of the above, the lasers reached the gummy bear at the same time. 14. So fishy. A robot janitor fish cleans a fish tank once it detects that the index of refraction of the tank reaches 1.50 or greater. At what value of the speed of light will the robot janitor fish stop cleaning? (Note speed of light is assumed to be within the range:[0, 3x108m/s]) A. Speed of light inside the medium is equal to 2x108 m/s. B. Speed of light inside the medium is less than 2x108 m/s. C. Speed of light inside the medium is greater than 2x108 m/s. D. Speed of light inside the medium is less than or equal to 2x108 m/s. E. Speed of light inside the medium is greater than or equal to 2x108 m/s. 15. Ibong Pepito. Pepito, studying, wondered loudly: “What is the refractive index of wine?” His pet goldfish replied: “I do not know. But the amplitudes of E-field and B-field in my house are already 2π N/C and π x 10-8T, respectively. So it is time you clean my bowl!” What is the index of refraction of the liquid in the goldfish’s bowl? A. π B. 3 C. 5 D. 2/3 E. 3/2 16. Gummy wave. A gummy wave has a wavelength in vacuum of 1200 nm. What are its frequency f and wavelength λ in a medium where its speed is c/4? A. f = 5.00 X 1014 Hz; λ= 300 nm B. f = 2.50 X 1014 Hz; λ= 300 nm C. f = 2.50 X 1014 Hz; λ= 1200 nm D. f = 1.00 X 1015 Hz; λ= 300 nm E. f = 1.00 X 1015 Hz; λ= 1200 nm 17. Womb Raider. Lara Crocs is choosing among 5 different light bulbs for her study lamp. The bulbs are of the same radius. She wants a bulb with the greatest intensity. Which bulb will Lara choose? A. A bulb emitting 25-W power on its surface. B. A bulb emitting 20-W power on its surface. C. A bulb emitting 15-W power on its surface. D. A bulb emitting 10-W power on its surface. E. A bulb emitting 5-W power on its surface. 18. Pressured KNB? A 10-W source produces light incident over an area of 5m2 reflecting surface. If the surface reflects 50% of the incident light, what is the average force exerted on the surface? (Note c = speed of light) A. 5/10c N B. 5/2c N C. 5/c N D. 10/c N E. 2/c N 19. Yucky gummy. Light is incident on a platform with yummy gummy bears on top of it. Assume a normal axis at the midpoint of the platform. If the left side with respect to the axis is 50% reflecting while the right side is 50% absorbing, which of the following is TRUE? A. The platform will move upward. B. The platform will move downward. C. The platform will rotate clockwise. D. The platform will rotate counterclockwise. E. The gummy bears will bounce from the reflecting surface.
Physics 72: Chapter 32 Problem Set
Summer 2010
20. UFO. An unidentified flying object was seen at the UP Sunken Garden. Although its origin is not yet known, Physics 72 students discovered that its purpose is to absorb energy from the sun. Moreover, they found that it is 50% reflecting, 30% transmitting and 20% absorbing. What is the total force associated with radiation pressure felt by the UFO if it has a total area of 2.0 m2 and the sun has an intensity of approximately 1 kW/m2? A. 4 x 10-6 N B. 8 x 10-6 N C. 4 x 10-5 N D. 6 x 10-5 N E. 8 x 10-5 N 21. Radiation Pressure. In a space simulator facility, a series of overhead arc lamps can produce light of intensity 1 500 W/m2 at the floor of the facility. What is the average radiation pressure on a totally reflecting section of the floor? A. 0.5 x 10-5 N/m2 B. 1.0 x 10-5 N/m2 C. 1.5 x 10-5 N/m2 D. 2.0 x 10-5 N/m2 E. 2.5 x 10-5 N/m2 22. Electric field. At a certain moment in time, an EM wave is found to propagate towards
+ kˆ and the
ˆ ˆ magnetic field vector is pointing in the +i + j direction. What is the corresponding direction of the electric field vector? +ˆi + ˆj ˆ ˆ B. +i − j ˆ C. +i ˆ D. + j + kˆ A.
E.
For the next 2 questions, consider our gummy planet being invaded by gummy worms. To save the planet, Super Gummy Bear has waged a war against the gummy worms using his super uber lethal ultraelectromagneticalicious green laser gun (with an attack rating of 3, critical +15 and additional racial damage to gummy worms (150%), undead (43%) and humans (3%)). 23. Puro na lang Gummy. If the speed of the green laser inside the gummy worm is 2.5 x 108 m/s, what is the refractive index of the gummy worm? A. 0.83 B. 1.2 C. 1.4 D. 1.6 E. 2.0 24. Gummy na Naman. Which of the following statements is/are TRUE regarding the green laser light inside the gummy worm? A. Its wavelength shifts towards the red end of the visible light spectrum. B. Its wavelength shifts towards the blue end of the visible light spectrum. C. Its frequency inside the gummy worm is less than its frequency in air. D. The ratio of the electric and magnetic field (E/B) decreases. E. B and D 25. Waves in Vacuum. Two light waves are travelling in vacuum. Light wave A has thrice the wavelength of light wave B. What can be said about the frequencies of A and B? A.
fA = fB
B.
f A = 3 fB
C.
fA = 1 fB 3
D. E.
f A = 3 fB fA = 1
3
fB
The SAND Prince