Name Score. Physics Dr. E. F. Redish

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Physics 131 1 Dr. E. F. Redish I. (25 points) The circuit diagram shown at the right contains two 1.5 Volt batteries (labeled A

1 Physics Dr. E. F. Redish I. (25 points) The circuit diagram shown at the right contains two 1.5 Volt batteries (labeled A and B) and two identical 30 Ω resistors. A. (10 pts) Can you determine the magnitude of the potential drops across resistors 1 and 2? If you can, find them and explain how you found them and if any of Kirchhoff s laws helped. If you can t, explain why not. ΔV 1 = ΔV 2 = B. (10 pts) Can you determine the currents through resistors 1 and 2? If you can, find them and explain how you found them and if any of Kirchhoff s laws helped. If you can t, explain why not. I 1 = I 2 = C. (5 points) Can you determine the currents through batteries A and B? If you can, find them and explain how you found them and if any of Kirchhoff s laws helped. If you can t, explain why not. I A = I B =

2 Physics Dr. E. F. Redish II. (25 points) Assume we have one mole each of two different weakly-interacting gases in identical thermally-insulated glass balls on either side of a valve as shown in the top figure at the right. The gas on the left is at a temperature of 300 K while the gas on the right is at a temperature of 400 K. A. (5 points) Before the valve is opened, can you tell which side has the higher pressure, or are they the same? If you can, indicate which and explain how you know. If you can t, explain which you can t. L R Same Can t tell B. (5 points) Before the valve is opened, can you tell which side has the higher internal energy, or are they the same? If you can, indicate which and explain how you know. If you can t, explain why you can t. L R Same Can t tell C. (5 points) Before the valve is opened, can you tell which side has the higher entropy, or are they the same? If you can, indicate which and explain how you know. If you can t, explain why you can t. L R Same Can t tell D. (5 points) When the valve is opened, what happens to the entropy of the system? If you can, indicate how it changes and explain how you know. If you can t, explain why you can t. Increases Decreases No change Can t tell E. (5 points) When the valve is opened, what happens to the Gibbs free energy of the system? If you can, indicate how it changes and explain how you know. If you can t, explain why you can t. Increases Decreases No change Can t tell

3 Physics Dr. E. F. Redish III. (25 points) A laser of the color marked on the spectrum (wavelength = 600 nm) is put through two slits and produces an intensity pattern on the screen that is represented by the graph at the right (solid line). A. (10 pts) If the screen is 2 m away from the slits, can you find the separation of the slits? If you can, find it. If you can t, explain why you can t. Separation of slits = B. (10 pts) Assuming that the two slits have the same width, can you find the width of one of the slits? If you can, find it. If you can t, explain why you can t. Width of slits = C. (5 pts) In part B we assumed that the slits had the same width. Is that assumption reasonable? Check an answer box and explain why you think so. Yes No

average behavior. However, when the number is very small, random fluctuations in the actual number present become important.

4 Physics Dr. E. F. Redish IV. (15 points) When the number of molecules, atoms, or ions of a particular type (here we consider H+ ions) is in the hundreds of millions, we can confidently speak of concentrations and analyze an average behavior. However, when the number is very small, random fluctuations in the actual number present become important. As a rule of thumb we need a different type of description (stochastic equations!) when describing 10 or fewer atoms, molecules, or ions. Consider two compartments of comparable size but very different function inside eukaryotic cells: mitochondria and lysosomes (#9 and #12 in the schematic at right). These have very different ph from the rest of the cell. Mitochondria have a ph of ~8. This means that there are about 10-8 moles/liter of H + ions. Lysosomes on the other hand have a ph of ~5. Estimate whether we need to consider stochastic equations in either compartment. Be sure to clearly state your assumptions and how you came to the numbers you estimated, since grading on this problem will be mostly based on your reasoning, not on your answer. Figure source: Wikimedia commons

5 Physics Dr. E. F. Redish V. (10 points) A biology student who took physics many years ago complained, So many of the topics we discussed had no use in biology. For example we spent all that time studying harmonic oscillation. The only thing that seemed good for was to describe a mass hanging on a spring, and who cares about that? Do you agree with her? Do you see any potential applications for what you have learned about harmonic oscillation in biology or chemistry? If not, do you see any general skills or knowledge that might have been useful from this example? If neither, discuss why you think we might have included this topic in a physics class for biologists. Note: This is an essay question. Your answer will be judged not solely on its correctness, but for its depth, coherence, and clarity.

6 Physics Dr. E. F. Redish VI. (5 points extra credit) Have you learned anything in this class that you think will be of any use to you in five years? If so, what? If not, why not? Any thoughtful answer will receive full extra credit points. (Don t do this until you have completely finished the rest of the exam.)

University of Maryland Department of Physics. Spring 2009 Final Exam 20. May (175 points) Post grades on web? (Initial, please) Yes No

University of Maryland Department of Physics Physics 122 20. May 2009 (175 points) Post grades on web? (Initial, please) Yes No (If you agree, I will post your grades and your detailed scores for each