Refrigerators Physics 202 Professor Lee Carkner Lecture 19.
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Refrigerators Physics 202 Professor Lee Carkner Lecture 19
PAL # 18 Engines Engine #1 W = 10, Q H = 45 =W/Q H = 0.22 Engine #2 Q C = 25, Q H = 30 = 1 – Q C /Q H = 0.17 Engine #3 T H = 450 K, T C = 350 K C = 1 – T C /T H = 0.22 Engine #4 W = 20, Q H = 30, T H = 500, T C = 400 = 0.66 > C = 0.2 Engine #5 W = 20, Q H = 15 = 1.33 > 1
Engines and Refrigerators Heat from the hot reservoir is transformed into work (+ heat to cold reservoir) By an application of work, heat is moved from the cold to the hot reservoir
A Refrigerator The boiling point of a liquid depends on its pressure Heat can be moved from a cold region to a hot region by adjusting the pressure so that the circulating fluid boils in the cold region and condenses in the hot We can make heat flow “uphill” from cold to hot, but we must add work
Refrigerator Cycle Liquid Gas Compressor (work =W) Expansion Valve Heat removed from inside fridge by evaporation Heat added to room by condensation High Pressure Low Pressure QLQL QHQH
Refrigerator as a Thermodynamic System K = Q L /W K is called the coefficient of performance Q H = Q L + W W = Q H - Q L This is the work needed to move Q L out of the cold area
Refrigerators and Entropy We can rewrite K as: From the 2nd law (for a reversible, isothermal process): So K becomes: K C = T L /(T H -T L ) Refrigerators are most efficient if they are not kept very cold and if the difference in temperature between the room and the refrigerator is small
Perfect Systems A perfect refrigerator converts Q L directly into Q H with no work But each violates the second law: Q L /T L = Q H /T H Perfect systems are impossible
Gas Motions Why don’t gasses diffuse more rapidly? The molecules collide with each other constantly and are scattered The constant collisions means the gas becomes thermalized Energy and information is quickly transmitted through the gas
Mean Free Path The average distance between collisions: = 1 /[√2 d 2 (N/V)] Where: N is the number of molecules N/V is the number density Millions of collisions per second!
Speed Distribution Maxwell’s distribution is not symmetrical This means there are several ways to characterize a “average” speed Most probable speed, v p v p = (2RT/M) ½ Average speed, v avg v avg = (8RT/ M) ½ root-mean-squared speed, v rms v rms = (3RT/M) ½ rms speed reflects the way the molecules produce pressure and carry energy
Planetary Atmospheres Why do some planets have atmospheres and others do not? Gas molecules are moving and may escape So equating escape velocity to thermal velocity should define conditions for atmosphere retention Escape velocity needs to be about 10 times large than rms velocity in order to keep an atmosphere for a long time: v escape > 10v rms
Next Time Test #2 Same format as test #1 Sample equation sheet and practice problems posted For Monday Read: 33.1-33.7 Homework: Ch 33, P: 2, 10, 20, 21