Physics 102.001

Fall 2011

Exam 3

1) A subatomic particle in flight is observed to have a lifetime of 2 µs (2 × 10-6 s). It’s moving at a speed corresponding to a relativistic γ factor of 2. If you’d observe the same particle at rest you’d measure its lifetime to be A) B) C) D) E)

2 µs. 1 µs. 4 µs. 8 µs. 0.5 µs.

2) Consider a wave in a given medium (which then determines/fixes the wavespeed), say a water wave. You double its frequency. What happens to its wavelength? A) B) C) D)

It also doubles. It drops to one half. It remains the same. Can’t answer without more info, for instance on amplitude.

3) The mass of the water molecule is A) B) C) D) E)

less than the sum of m(H2) + m(O). greater than the sum in answer A. the same as the sum in answer A. Depends on the temperature of the water. None of the above.

4) Consider light traveling in vacuum. An ultraviolet photon has more energy than an infrared photon. Therefore the ultraviolet photon also travels faster. A) B) C) D)

False. True. Depends on the light source. No, it varies from photon to photon.

5) A refrigerator uses 1 kWh of electric energy and outputs 3 kWh of thermal energy into your kitchen. In the process it extracts the following amount of thermal energy from the contents of your refrigerator. A) B) C) D)

1 kWh 2 kWh 3 kWh 4 kWh

E) 5 kWh 6)  What is not predictable in the double-slit experiment with electrons?   A) The overall pattern of hits on the screen, as formed by a large number of electrons. B) The individual impact point of each electron on the screen. C) Both of the above. D) None of the above. 7) One similarity between electrons and photons is A) B) C) D) E)

both are particles of matter. both are the quanta of a field. Both of the above. both are forms of electromagnetic radiation. None of the above.

8) Which one has the shortest wavelength, assuming that they all have the same speed? A) B) C) D) E)

Helium atom Water molecule Electron Baseball None of these objects have wavelengths of any kind.

9) In the double-slit experiment with light, A) the impact point of each photon can be predicted, and so can the overall pattern of impact points of a large number of photons. B) the impact point of each photon can be predicted, but the overall pattern of impact points cannot be predicted. C) the impact point of each photon cannot be predicted, but the overall pattern can be predicted. D) the impact point of each photon cannot be predicted, and neither can the overall pattern of impact points. 10) You are a passenger in a moving car, and you drop a coin from your right hand, holding your left hand directly below the point of release. If the car is moving at unchanging velocity, the coin will come down A) in front of your left hand. B) behind your left hand.

C) in your left hand. D) to the side of your left hand. 11) Velma is in a train headed north past Mort at 30 m/s. Velma has a slingshot that can shoot pebbles at 40 m/s. Velma shoots a pebble toward the rear of the train. Mort observes the pebble's velocity to be A) B) C) D) E)

70 m/s north. 10 m/s north. 10 m/s south. 70 m/s south. 40 m/s south.

12) Einstein proposed two basic principles or laws as the foundation of his Special Theory of Relativity. One of these principles is intuitively plausible, while the other seems quite implausible or odd to our intuitions. The principle that is intuitively plausible is A) B) C) D) E)

E = mc2. the principle of time dilation. the principle of the constancy of the speed of light. the principle of inertia. the principle of relativity.

13) If you drop a coin inside a car that is turning a corner to the right, the coin will land A) B) C) D) E)

to the left of the drop point. to the right of the drop point. right under the drop point. to the rear of the drop point. in front of the drop point.

14) Mort stands on Earth. Velma moves away from him in a rocket, at 299,000 km/s (1000 km/s slower than lightspeed). Mort sends a laser beam toward Velma. As the tip of the beam passes her rocket, Mort observes A) B) C) D) E)

the beam moving away at 300,000 km/s. the beam moving just 1000 km/s faster than Velma. Both of the above. the beam moving 300,000 km/s faster than Velma. All of the above.

15) Suppose that a certain atom has just the 4 different energy levels shown in the

figure below. Which quantum jump produces radiation with the shortest wavelength?

A) B) C) D) E)

Nonsense – quantum jumps don’t produce radiation. 4 to 3. 2 to 1. 4 to 1. 3 to 2.

16) Suppose that a certain atom has just 4 different energy levels. How many different frequencies of radiation can generally be emitted by this atom upon deexcitation? A) B) C) D) E)

4 6 10 16 infinitely many

17) When an atom emits a photon, the atom's mass A) B) C) D) E)

remains exactly the same. increases slightly. becomes negative. decreases slightly. Could do any of the above.

18) A certain type of atom has only four energy levels, as shown in the figure in problem 15. The spectral lines produced by this element are all visible, except for one ultra-violet (UV) line. The quantum jump that produces the UV line is A) B) C) D) E)

3 to 1. 2 to 1. 4 to 3. 4 to 2. None of the above.

19) When an atom emits electromagnetic radiation, A) B) C) D) E)

it emits a small portion of its electric charge. a proton strikes a neutron to create a photon. a photon makes a quantum jump from one quantum state to another. an electron moves in precisely one complete orbit around the nucleus. an electron makes a quantum jump from one quantum state to another.

20) When an atom undergoes a transition (quantum jump) from its ground state to an excited state, A) B) C) D) E)

it absorbs an electron. it emits a photon. it emits an electron. it absorbs a photon. None of the above.

21) The wavelength of electromagnetic radiation with a frequency of 300 MHz is A) B) C) D) E)

3 m. 10 m. 100 m. 0.1 m. 1 m.

22) How long does it take a radio signal to travel from New York to San Francisco, about 5000 km? (1 µs = 10-6 s) A) B) C) D) E)

About 5 µs. About 17 µs. About 10 µs. About 2 µs. About 1 µs.

23) Alphonse moves eastward in a car at 30 m/s. Velma, moving eastward in a train at 70 m/s, passes him. Alphonse's velocity relative to Velma is A) B) C) D) E)

40 m/s westward. 40 m/s eastward. 100 m/s westward. 100 m/s eastward. 30 m/s westward.

24) In the gamma ( = high energy photon) detection process inside a detector,

gamma  e- + e+, are energy / mass / rest mass conserved? A) B) C) D) E)

Yes / Yes / Yes Yes / No / No Yes / Yes / No No / No / Yes No / Yes / Yes

25) The wavelength of visible light is A) B) C) D)

similar to the typical size of atoms. much smaller than the typical size of atoms. much larger than the typical size of atoms. Makes no sense to compare since they carry different units.

26) An object with a rest mass of 10 kg moves past you at half the speed of light. You’d measure its mass to be about A) B) C) D) E)

1.5 kg. 15 kg. 0.5 kg. 11.5 kg. 8.7 kg.

27) How is a proton similar to a photon? A) B) C) D) E)

Neither carries electric charge. Both are field quanta. Both travel with the speed of light. In a double-slit experiment both impact the screen at a tiny spot. Both B and D are correct.

28) Suppose in an atom the energy difference between E3 and E2 is 1/3 the energy difference between E3 and E1. In that case a quantum jump from 2 to 1 will emit a photon with A) B) C) D) E)

twice the energy of a jump from 3 to 2. half the energy of a jump from 3 to 2. the same energy of a jump from 3 to 2. 3 times the energy of a jump from 3 to 2. 1/3 the energy of a jump from 3 to 2.

29) A star is heading away from Earth at 10% of the speed of light when it explodes. With what speed does the light from the explosion leave the star? With what speed

(as measured on Earth) does the light approach Earth? A) B) C) D) E)

c and 1.1 c c and 0.9 c 0.9 c and c c and c None of the above.

30) Across the US on average 200 W of solar radiation power strike every m2. Let’s assume this is all in the visible range, and, for simplicity, take an average wavelength of 5 × 10-7 m for visible light. How many photons does this correspond to in one second? A) B) C) D) E)

About 5 × 1020. About 1019. About 4 × 1021. About 2 × 1022. About 4 × 1019.

31) Assume a 100 W old fashioned, incandescent light bulb is only 10% efficient, i.e. only 10% of the power consumption ends up in visible light. Yes, they are that inefficient, 90% goes into heat/thermal energy, it’s disgusting! Using the same average wavelength as in the previous problem, how many photons does such a light bulb emit per second? A) B) C) D) E)

About 1018. About 2.5 × 1019. About 5 × 1019. About 1020. About 2 × 1018

32) A photon with an energy exactly equal to the difference between the ground state and the first excited state of hydrogen is incident on a hydrogen atom at rest and in its ground state. Will this photon be able to excite the hydrogen atom into its first excited state? A) Yes, of course, since energy is conserved. B) Maybe. C) No, there is a problem with the combined conservation of energy & momentum. D) Yes, since momentum can be conserved. E) Such photons don’t exist. 33) You pump up a bike tire, which, as you know, warms up as a result. From a Special Relativity perspective, have you increased rest mass and mass, respectively,

of the air content of the tire?   A) No (rest mass) and yes (mass). B) No and no. C) Yes and no. D) Yes and yes. E) Air has no rest mass. 34) A meter stick with a rest mass of 1 kg moves past you. You measure it to have a mass of 2 kg and a length of 1.2 m. How fast is it moving and with what orientation? A) B) C) D)

Impossible! You made a measurement mistake. It’s moving at a speed such that γ = 2, and perpendicular to its motion. Same as B), but oriented parallel to its motion. None of the above.

35) How does the matter wavelength of typical macroscopic objects in motion (baseballs, cars, plains, etc.) compare with typical atomic dimensions?   A) It is larger than atomic dimensions. B) It is comparable, say within one order of magnitude. C) Such objects have no wavelength, they behave like “particles” after all. D) It is much, much smaller than atomic dimensions. E) None of the above. 36) In the context of the double-slit experiment, quantum uncertainty refers to A) the fact that each interaction with the screen occurs over a broad area of the screen. B) the fact that the electron beam or light beam goes through both slits rather than only one. C) the fact that the particles of matter or radiation are instantaneously replaced by a spread-out matter field or EM field when the particles impact the screen. D) the fact that the entire matter field or EM field instantaneously collapses to the single interaction point when the field interacts with the screen. E) our inability to precisely predict the point at which a spread-out matter field or EM field will interact with the screen. 37) Gamma rays are associated with A) B) C) D)

the thermal vibrations of atoms and molecules. electrons in atoms and molecules. nuclear processes. electrons vibrating back and forth in antennae.

38)

Two brief waves or "pulses" are sent down a rope, in opposite directions, as shown in the figure. Pulse A is 1 cm in height, and pulse B is 2 cm in depth. When they meet, they will create A) B) C) D) E)

a disturbance 3 cm high. a disturbance 3 cm deep. a disturbance 1 cm high. a disturbance 1 cm deep. no disturbance, because they cancel.

39) Consider two protons that you move closer together. Have you increased the overall mass and overall rest mass, respectively, of this two-proton system as a result of moving them closer together? A) B) C) D) E)

Yes (mass) and yes (rest mass). No and yes. Depends on their separation before and after. Yes and no. No and no.

40) Mort observes Velma's light-clock, as Velma passes. He observes that, during a 1 second tick of her clock, her light beam travels A) B) C) D) E)

a distance of one light year. further than 300,000 km. less than 300,000 km. no distance at all. 300,000 km.

Answers: 1. B 2. B 3. A 4. A 5. B 6. B 7. B 8. D 9. C 10. C

11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

C E A C D B D E E D E B A C C D E A D A B C D A D E C D D B