Selected Topics from Modern Physics 1. According to the special theory of relativity, if a 30-year old astronaut sent on a space mission is accelerated to speeds close to that of light, and then returns to earth after 20 years as measured on earth, what would be his biological age upon returning? a. less than 50 years b. 50 years c. more than 50 years d. exactly 100 years 2. A spaceship, which has length 50 m and width 10 m when at rest, is moving in a direction along its length at a speed where its length contraction results in a 40-m length measured by a stationary observer. What would the measured width be in this case? a. between 12 and 13 m b. 10 m c. 8 m d. less than 8 m 3. According to Wien s displacement law, if the absolute temperature of a radiating blackbody is tripled, then the peak wavelength emitted will change by what factor? a. 1/3 b. 1 c. 3 d. 9 4. If a monochromatic light beam with quantum energy value of 3.0 ev incident upon a photocell where the work function of the target metal is 1.60 ev, what is the maximum kinetic energy of ejected electrons? a. 4.6 ev b. 4.8 ev c. 1.4 ev d. 2.4 ev 5. A monochromatic light beam is incident on a barium target, which has a work function of 2.50 ev. If a stopping potential of 1.0 V is required, what is the light beam photon energy? a. 1.0 ev b. 1.5 ev c. 2.5 ev d. 3.5 ev 6. Of the following energies for photons, which is the least energy that could result in photoelectron production if the work function is 3.00 ev? a. 1.50 ev b. 2.90 ev c. 3.50 ev d. 6.01 ev 1
7. Sources of red, blue, and yellow light each emit light with a power of 50 mw. Which source emits more photons per second? a. the red source b. the blue source c. the yellow source d. They all emit the same number per second. 8. According to de Broglie, as the momentum of a moving particle is tripled, the corresponding wavelength changes by what factor? a. 1/9 b. 1/3 c. 3 d. 9 9. According to Heisenberg, as the uncertainty in the measurement of a particle s momentum is reduced by a factor of 2, by what factor is the uncertainty in that same particle s position changed? a. 1/2 b. 1 c. 2 d. 4 10. When a wire carries high current causing it to glow, it will emit which type of spectrum? 11. When a high voltage is applied to a low-pressure gas causing it to glow, it will emit which type of spectrum? 12. When a cool gas is placed between a glowing wire filament source and a diffraction grating, the resultant spectrum from the grating is which one of the following? 13. An alpha particle is: a. a neutral helium atom. b. any positively charged nucleus. c. an x-ray. d. None of the above. 14. The atomic mass number of a nucleus is equivalent to which of the following numbers? a. number of neutrons present b. number of protons present c. difference in neutron and proton numbers d. sum of neutron and proton numbers 2
15. The ratio of the numbers of neutrons to protons in the nucleus of naturally occurring isotopes tends to vary with atomic number in what manner? a. increases with greater atomic number b. decreases with greater atomic number c. is maximum for atomic number = 60 d. remains constant for entire range of atomic numbers 16. An element is emitting alpha, beta, and gamma radiation. Rank order them in terms of the thickness of protective shielding they will need for safety, from least to most. a. alpha, beta and gamma b. gamma, beta and alpha c. beta, gamma and alpha d. alpha, gamma and beta 17. A radioactive material initially is observed to have an activity of 1 000 decays/sec. If three hours later it is observed to have an activity of 125 decays/sec, what is its half-life? a. 1/2 hour b. 1 hour c. 3 hours d. 8 hours 18. A radioactive isotope that emits an alpha particle will change in what respect? a. Atomic number decreases by four. b. Mass number decreases by four. c. Both choices a and b are valid. d. None of the above choices are valid. 19. What particle is emitted when 240 Pu decays to 236 U? (atomic numbers of Pu and U are, respectively, 94 and 92) a. alpha b. beta (electron) c. beta (positron) d. gamma quantum 20. The isotope 14 C cannot be used in dating old samples of which of the following? a. charcoal from a fire b. a bronze implement from a cave c. a bone buried in mud d. All of the above can be dated using 14 C. 21. The water surrounding the fuel rods in a nuclear fission reactor serves what purpose(s)? a. coolant b. moderator c. Both choices above are valid. d. None of the choices above are valid. 22. The advantage of a fusion reactor when compared to a fission reactor is which of the following? a. The fuel is cheaper. b. There is less radioactive waste material. c. Both choices above are valid. d. None of the above choices are valid. 3
23. What is meant by a particle being elementary? a. It is too small to see. b. It is subatomic. c. It is not composed of other particles. d. It has no charge. 24. Which of the following particles is (are) considered to be elementary? a. the neutron b. the meson c. the electron d. All of the above. 25. Which of the following forces is the weakest? a. strong nuclear b. weak nuclear c. electromagnetic d. gravitational 26. Which force can act over distances comparable to the distance between planets? a. only gravitational b. only electrical c. only magnetic d. All of the above. 27. A proton and antiproton each with total energy 400 GeV collide head-on. What is the total energy (particles + energy) released? a. 800 GeV b. 400 GeV c. zero d. 1 600 GeV 28. The particle thought responsible for particle masses is: a. the graviton. b. the gluon. c. the _ particle. d. the Higgs boson. 29. The cosmic background radiation appears compatible with a blackbody source at what temperature? a. 0.000 3 K b. 0.03 K c. 3 K d. 300 K 30. The Big Bang occurred: a. about 15 to 20 billion years ago. b. about 5 to 7 thousand years ago. c. about 4 to 6 billion years ago. d. infinitely far in the past. 4
ID: A Selected Topics from Modern Physics Answer Section MULTIPLE CHOICE 1. A 2. B 3. A 4. C 5. D 6. C 7. A 8. B 9. C 10. C 11. A 12. B 13. D 14. D 15. A 16. A 17. B 18. B 19. A 20. B 21. C 22. C 23. C 24. C 25. D 26. D 27. A 28. D 29. C 30. A 1