Quantum Physics
Which of the following statements is true about quantum physics?
A) Atoms are the smallest indivisible particles of matter.
B) Quantum physics is not necessary for our modern understanding of semiconductor devices.
C) The emission of electrons from a substance when light strikes its surface is explained by the theory of relativity.
D) In the photoelectric effect, electrons are emitted only if the light frequency exceeds a threshold.
D - In the photoelectric effect, electrons are emitted only if the light frequency exceeds a threshold.
(Lau Final Exam Practice, Q2)
In the beta-minus decay of an atomic nucleus:
A. The radioactive nucleus emits a 4He nucleus, changing its chemical identity to an element with two fewer protons.
B. The radioactive nucleus emits a 4He nucleus, changing its chemical identity to an element with four fewer protons.
C. The radioactive nucleus emits an electron and an antineutrino, changing its chemical identity to an element with one fewer proton.
D. The radioactive nucleus emits an electron and an antineutrino, changing its chemical identity to an element with one additional proton.
D; Remember that Beta minus emits an antineutrino, while Beta plus emits a neutrino
(Lau Final Exam Practice, Q11)
Which of the following statements is NOT true about matter waves?
A) A smoke particle has de Broglie wavelength comparable to an atom’s size, hence exhibits wave-like behavior.
B) Electrons that create a double-slit interference pattern are behaving like waves.
C) The de Broglie wavelength of an electron depends on the kinetic energy it carries.
D) Diffraction patterns can be produced by both X-rays and electrons passing through an aluminum foil.
A; Smoke particles do NOT exhibit wave-like behavior (deBroglie wavelength is extremely small and negligible)
(Lau Final Exam Practice, Q6)
Identify the unknown atom/isotope X given the following alpha decay:
X -> 224Ra + [alpha particle]
X is 228Th
(Lau Week 10 HW, 30.31 Part A)
The 1.0 mW light beam from a laser pointer (λ = 670 nm) shines on a screen. How many photons strike the screen each second?
3.4*1015 photons/s
R = P/hf
(Lau Mar3slides, Slide 4)
One of the main radioactive waste products from a nuclear fission power plant is the iodine isotope 131I, which has a half-life of 8 days. Suppose a sample of 131I registers 160 decays per second. After 50 days, how many decays per second will it register? Note the unit of decays per second is the becquerel, Bq.
2.1 Bq
(Lau Final Exam Practice, Q21)
A 100 W incandescent lightbulb emits about 5 W of visible light. (The other 95 W are emitted as infrared radiation or lost as heat to the surroundings.) The average wavelength of the visible light is about 600 nm, so make the simplifying assumption that all the light has this wavelength. How many visible-light photons does the bulb emit per second?
1.5 x 1019 photons/sec
R = P / hf
(Lau Week 9 HW, 28.21)
Calculate the binding energy per nucleon for 240Pu
7.58 MeV
HW 30A, Q4
The work function of copper is 4.8 eV and that of lithium 2.9 eV. Can blue light of wavelength 480 nm eject photoelectrons from either of these materials?
No; neither copper nor lithium.
[wavelength] = hc / E
(Lau Final Exam Practice, Q17)
A sample of wood from an archeological excavation is dated by measuring the relative numbers of 14C and 12C atoms in the sample. Suppose 35 atoms of 14C are found for every 1.2×1015 atoms of 12C. When the wood was alive, the ratio of 14C atoms to 12C atoms was 1.3×10−12, the same as is found naturally in the atmosphere. The half life of 14C is 5730 years. What is the age of the wood?
31,400 years
(Lau Final Exam Practice, Q22; Modified numbers)