PHY100/Exam2 Page 2 of 5 Name: (12 pts )

Transcription

1 PHY100 November 12, 2008 Exam 2 Name: Aran Garcia-Bellido Please read the problems carefully and answer them in the space provided. Write on the back of the page, if necessary. Show your work where requested in order to be considered for partial credit. In problems where you are requested to show your work, no credit will be given unless your work is shown. Potentially useful formulas: F = G m 1m 2 r 2 F = k q 1q 2 r 2 F = ma Distance = Speed Time v = x t a = v t Momentum = p = mv Length contraction (space is longer in proper frame) : L = γl Space dilation (time is shorter in proper frame) : T = γt 1 γ = 1 ( v c )2 Wave properties : v = λν ; ν = 1/T (T = period) Energy of a particle/wave : E = hν De Broglie matter waves : λ = h p = h mv Uncertainty principle : x p > h ; E t > h Constants : c = 300, Nm2 m/s ; G = kg 2 k = Do not forget to put units in your results! Nm2 C 2 ; h = Js 1. According to quantum theory, a macroscopic object such as a baseball (a) has a very long wavelength (b) has a very short wavelength (c) has no wavelength at all (d) has an infinite wavelength (e) is really made out of microscopic baked beans 2. When we say that an electromagnetic field is quantized, we mean that (a) it fills up a region of space (b) it is really made of particles known as electrons (c) it is really made of particles known as photons (d) it is allowed only certain particular values of total energy (e) it vibrates at one particular frequency

2 PHY100/Exam2 Page 2 of 5 Name: ( ) 3. Here is the energy level diagram for the hydrogen atom. The lower line represents the lowest energy level, the upper line (at 13.6 ev) represents the highest energy level. Several possible transitions of electrons between energy levels are indicated by numbered arrows (1-5). No need to show your work. (a) (3 pts) Which transition, as shown, is not possible? (a) 1 (b) 2 (c) 3 (d) 4 (e) 5 (b) (3 pts) Which transition represents an electron that absorbs a photon with 10.2 ev of energy? (a) 1 (b) 2 (c) 3 (d) 4 (e) 5 (c) (3 pts) True or False: transition number 4 represents the absorption of a photon with lower frequency than transition number 5. (d) (3 pts) This atom is illuminated with blue light (at a frequency ν = Hz). Knowing that the value of Planck s constant is h = ev s, which transition will occur (the energy needed for each transition is given in parenthesis): (a) 1 (10.2 ev) (b) 2 (-10.2 ev) (c) 3 (11.5 ev) (d) 4 (1.9 ev) (e) 5 (2.6 ev)

3 PHY100/Exam2 Page 3 of 5 Name: ( ) 4. As a particle s uncertainty in position gets smaller, its uncertainty in momentum (a) must remain unchanged (b) must increase enough to satisfy the uncertainty principle (c) must decrease enough to satisfy the uncertainty principle 5. In quantum mechanics, if two photons are entangled it means: (a) they always move in the same direction (b) the two photons share the same wavefunction and observations of one are reflected in the state of the other (c) they are moving directly apart from each other (d) the two photons share the same wavefunction and, consequently, observations of one do not yield information about the other (e) they had a bit much to drink the night before 6. Five radioactive decays are shown below. Label each as to whether it is an example of alpha decay, beta decay or gamma decay. The X represents whatever other stuff comes out in the reaction, be it alpha, beta or gamma. No need to show your work. (a) Po Pb + X (b) Bi Po + X (c) Bi Bi + X (d) 19 10Ne 19 9 F + X (e) 14 6 C 14 7 N + X (f) U Th + X (10 pts ) 7. Why is it that the spectrum of light emitted by an atom is often said to be a fingerprint for that atom? 10 pts 34 pts

4 PHY100/Exam2 Page 4 of 5 Name: 8. The subatomic particles that make up the atom are: (a) neutrons, protons and electrons all evenly distributed (b) protons and antiprotons together in the nucleus, electrons and photons in orbitals (c) alpha particles (d) protons and neutrons in the nucleus, electrons in orbitals (e) electrons in the nucleus, neutrons and protons scattered around 9. Consider the three following forces: gravitational, electromagnetic, and strong nuclear. Of these three, which one or ones have significant effects (i.e., experimentally detectable effects) within the nucleus? (a) all three (b) electromagnetic and strong nuclear only (c) gravitational and strong nuclear only (d) electromagnetic only (e) strong nuclear only 10. One of the radioactive isotopes that fell on the land and got into the food chain after the Chernobyl accident was 131 I. Its half-life is 8 days. How long was it, following its deposit on the land, before its radioactivity had fallen to 1/32 (about 3%) of its initial value? Show your reasoning. (a) 40 days (b) 48 days (c) 16 days (d) several years (e) it is impossible to say, even approximately ( ) 11. One of your friends knows you are closet science geek, so they ask you: How does the Sun work? Briefly write below what you would tell your friend. 30 pts

5 PHY100/Exam2 Page 5 of 5 Name: ( ) 12. How were the elements up to iron in the periodic table created? What about the elements beyond iron? Where did the chemical elements that form your body come from? Explain.