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Refrigerators

Refrigerators. Physics 313 Professor Lee Carkner Lecture 13. Exercise #12 Engines. V 1 = 6.25X10 -4 m 3 , P 1 = 12X10 6 Pa, n = 3 moles P 1 V 1 g = P 2 V 2 g P 2 = P 1 V 1 g /V 2 g = 385 MPa h = 1 – T 1 /T 2 = 0.75 W = h Q H , Q H = nc V D T 23

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Refrigerators

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  1. Refrigerators Physics 313 Professor Lee Carkner Lecture 13

  2. Exercise #12 Engines • V1 = 6.25X10-4 m3, P1 = 12X106 Pa, n = 3 moles • P1V1g = P2V2g • P2 = P1V1g/V2g = 385 MPa • h = 1 – T1/T2 = 0.75 • W = hQH, QH = ncVDT23 • P3 = 500X106 Pa, V3 = 7.8X10-5 m3 • QH = (3)(3/2)(8.31)(1564-1205) = 13424 J

  3. Limits on Engines • Engines convert heat into work and waste heat • Second Law of Thermodynamics • An engine cannot have 100% efficiency

  4. 1st and 2nd Laws • Converting heat completely into work does not violate the 1st law • The second law is an independent statement

  5. Refrigerators • A refrigerator is a device that uses work to move heat from low to high temperature • A heat pump does this to heat a room (want large QH)

  6. How a Refrigerator Works • Fluid flows through the cold chamber and evaporates, adding heat QL to the fluid from the chamber • The fluid is pumped into the hot chamber and compressed, adding work W • The fluid condenses releasing heat QH

  7. Refrigerator Cycle Compressor (work =W) Gas QL QH Low Pressure High Pressure Heat removed from fridge by evaporation Heat added to room by condensation Liquid Expansion Valve

  8. Refrigerator Performance • The equivalent of efficiency for a refrigerator is the coefficient of performance K K = QL/(QH-QL) • Unlike efficiency, K can be greater than 1

  9. Air Conditioner • Air conditioners also have condensers that dry out the air

  10. Heat Pump • The heat removed from the inside of a refrigerator is ejected into the kitchen • A refrigerator that has the cold chamber as the outdoors and the hot chamber as the house is called a heat pump • Many heat pumps can be reversed in summer to function as air conditioners

  11. Refrigerators and the Second Law • You cannot move heat from low to high temperature without the addition of work

  12. Statements of the Second Law • Kelvin-Planck Statement: • Clausius Statement:

  13. Equivalence • One implies the other • For example: • A 100% efficient engine connected to a high T reservoir powering a refrigerator cooling a low T reservoir to the same high T reservoir • The refrigerator by itself is “legal” but the net effect to is move QL from low to high T with no other effect

  14. Engines and Refrigerators • Efficiency: h = W/QH = (QH-QL)/QH = 1 - (QL/QH) • Can rewrite using: • Coefficient of performance: K = QL/W = QL/(QH-QL) (refrigerator)

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