Physics 121 Hour Exam #1 60 Minutes, Take Home, Closed Book

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1 Physics 121 Hour Exam #1 60 Minutes, Take Home, Closed Book This exam is given under the Bryn Mawr honor code. Take it in a single sitting lasting no more than one hour. This is a closed-book exam: do not use any references (text, notes, etc.). You will need a calculator capable of basic arithmetic, but you are not allowed to store formulas, etc., in the calculator memory, and if the calculator has built-in unit conversion functions, you are not allowed to use them. You may use a computer if it acts only as a calculator to do basic arithmetic. This exam covers the same topics which were on homework assignments 1 and 2, that is, material covered in class through Wednesday, September 16. It includes questions on metric prefixes and abbreviations as outlined on homework 1. The values of g and σ are given in the assignment, as are the formulas for conversion between Celsius and Kelvin and for thermal radiation emission. All other formulas and concepts should be memorized. Return your exam in class on Monday, September 21. Do not discuss this exam with anyone until all the exams have been turned in. When you begin the exam, remove all the staples except the one in the upper left corner, and remove this first sheet from the exam booklet. Write your answers directly on the exam booklet. Show your work and box your final answers. Partial credit is important! Always show your work! I can t give you credit if I don t know what you re thinking. In multi-part questions, if you get stuck on an early part, try to answer the later parts anyway either make up some answer for the early parts to use in the later parts, or describe what approach you would use to solve the later parts. In any exam or assignment in this class, if you are given data, constants, or equations, please do not assume that you have to use every last piece of information given to solve a problem. Sometimes life gives you more information than you need. Good luck!!

2 Physics 121 Hour Exam #1 Name Return this exam in class on Monday, September 21. There are four problems on five pages. Point values are given for each problem. Within a problem, the points are not necessarily evenly divided between the parts. The total is 100 points. g = 9.8 m s 2 T (Kelvins) = T ( C) σ = W m 2 K 4 P = eσat 4 1. [15 points] Which would appear brighter to an observer on Earth: a single star 1 parsec away, or a cluster of 1000 stars 100 parsecs away? Justify your answer; a brief explanation or calculation is sufficient. Assume that all the stars have exactly the same properties. 1

3 2. [30 points] (a) The Large Hadron Collider, a particle accelerator now coming on line in Europe, will accelerate protons to that they are given energies of 7 tera electron volts (7 TeV). i. How many electron volts (ev) is this? ii. If 1 ev = J, what is 7 TeV in µj? (b) The intensity of sunlight at the location of Jupiter is 51 W m 2. The radius of Jupiter is 71,000 km. How much sunlight power is intercepted by Jupiter? 2

4 3. [15 points] A light bulb consumes 25 W of electricity. It is completely surrounded by a cube-shaped hollow metal box 0.2 m on a side. The box heats up until it reaches some uniform steady temperature, T. What is T? Assume that there are no other sources of energy which impact this experiment. (The box is not near a star; it is not in a warm, air-filled room, etc.) 25W electrical power 3

5 4. [40 points] The Hoover Dam is located on the Colorado River. The head of the dam is about 170 m high. This means that if water spilled over the dam, it would fall 170 m. The flow rate of water at the location of the dam is about m 3 /year. There are seconds in one year. (a) How many cubic meters of water flow through the dam in one second? (b) What is the mass of water which flows through the dam in one second? (The density of water is 1000 kg/m 3.) (c) If that amount of water fell from a height of 170 m down to 0 m, what would its kinetic energy be at the end of the fall? 4 Continued on the next page...

6 (d) Assume all the kinetic energy is converted to electric power by turbines at the base of the dam. If water flows continuously, how much power can be produced, in Watts? (e) Average electricity consumption in the U.S. is about 1200 Watts per person. (This is split fairly evenly between residential, commercial, and industrial uses.) What population could be supported by the Hoover Dam? 5