Physics 7b Midterm Exam #1 Fall 2004 Name:__________________________Disc. Section_______________ SID__________ #1______ #2______ #3______ #4______ #5______ Total________ Mw of O2= 32 g/mol, kb=1.38x10-23J/K, NA=6x1023 The weight of a 1kg mass on the Earth’s surface is 2.2pounds. The relative weight of each problem is indicated next to the problem number. Most credit will be given for algebraic work. Please do not insert numerical values until you have a final algebraic answer inside a box. If you don’t know a particular constant, use a symbol to get partial credit. It is almost impossible to award partial credit when a student has inserted numbers too early. If you get stuck on one problem, go on to the next and come back to the difficulties later in the exam period. Do not leave early until you have completed everything. Do not quit! Never, never, never quit!!
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1. (20 pts) PG&E charges for natural gas in a unit called a therm. 1 therm = 105BTU. 1BTU= heat to raise the temp of one pound of water by 1oF. The gas price is $0.67/therm. The price of electricity is $0.14/kW hr When buying energy from PG&E, what is the ratio of cost per Joule for gas compared to electricity?