PHY 7B-Final Exam with Solutions

# PHY 7B-Final Exam with Solutions - | Last name First name...

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1. Consider the present effort to cap the gushing oil well in the Gulf of Mexico. We can imagine the situation as shown in the diagram with a cap over the well (open to the sea floor pressure) sucking up oil by means of a pump at the surface, 1500 m above. Today BP is claiming to be pumping 30 liters per second from the bottom. (That is, 0.03 m 3 /s.) Let’s assume the oil density is 900 kg/m 3 . (Recall that the density of water is 1000 kg/m 3 .) Assume there is no fluid resistance in the tubes. a. (1.0 pts) Determine the flow velocity through the pipe, which has 20 cm (0.2 m) radius. I = Av, so v = I/A = (0.03 m 3 /s)/( ! (0.2 m) 2 ) = 0.239 m/s b. (2.5.pts) Which terms in the extended Bernoulli equation are relevant in comparing the fluid energy density at the entrance to the 20 cm radius tube at the sea floor to the outflow into the boat? Indicate which terms are zero, and on the next lines write out as much as you can about the remaining terms you know, including numbers. (Hint: be careful of the different densities!) " P + # g " h + \$ # " (v 2 ) = " E pump /vol - IR (P boat - P sea floor ) + # oil g (1500 m) = " E pump /vol (P atm - # water g (1500 m)) + # oil g (1500 m) = " E pump /vol 10 5 N/m 2 - (100 kg/m3)(10 m/s2)(1500 m) = -1.4x10 6 J/m 3 = " E pump /vol c. (1.0 pts) If there is no resistance does the pump energy come out to be positive or negative? Explain briefly, and if the pump energy is negative (which it cannot be), explain which assumption is likely false. The pump energy per unit volume comes out negative; if there is no resistance the lower density of the oil makes it tend to squirt to the top on its own. Of course, the assumption of no resistance is most likely not realistic. | | | | p 2 Last name First name DL Sec First three initials of last name grade (for office use only)

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2. Each of the resistors in the circuit to the right represents a light bulb. Each light bulb is identical. a) (2.5 pts) Rank the brightness of the light bulbs while the circuit is active. Put your answer in this box: D > C > A = B b) You wish to make bulb B brighter by short-circuiting one of the other light bulbs. Decide which bulbs you could short-circuit to increase the brightness of bulb B and check the appropriate boxes below. Explain briefly.
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## This note was uploaded on 08/21/2011 for the course PHYS 7B taught by Professor Mcraven during the Spring '10 term at UC Davis.

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PHY 7B-Final Exam with Solutions - | Last name First name...

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