Refrigerators Physics 313 Professor Lee Carkner Lecture 13

Refrigerators Physics 313 Professor Lee Carkner Lecture 13

Exercise #12 Engines á V 1 = 6. 25 X 10 -4 m 3, P 1 = 12 X 106 Pa, n = 3 moles á á P 1 V 1 g = P 2 V 2 g á á P 2 = P 1 V 1 g/V 2 g = 385 MPa á á h = 1 – T 1/T 2 = 0. 75 áW = h. QH, QH = nc. VDT 23 á P 3 = 500 X 106 Pa, V 3 = 7. 8 X 10 -5 m 3 á á QH = (3)(3/2)(8. 31)(1564 -1205) = 13424 J á

Limits on Engines áEngines convert heat into work and waste heat á áSecond Law of Thermodynamics á áAn engine cannot have 100% efficiency

1 st and 2 nd Laws áConverting heat completely into work does not violate the 1 st law á áThe second law is an independent statement á

Refrigerators á áA refrigerator is a device that uses work to move heat from low to high temperature á áA heat pump does this to heat a room (want large Q H)

How a Refrigerator Works áFluid flows through the cold chamber and evaporates, adding heat QL to the fluid from the chamber áThe fluid is pumped into the hot chamber and compressed, adding work W á áThe fluid condenses releasing heat QH

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

Refrigerator Performance áThe equivalent of efficiency for a refrigerator is the coefficient of performance K K = QL/(QH-QL) áUnlike efficiency, K can be greater than 1 á á

Air Conditioner á áAir conditioners also have condensers that dry out the air á

Heat Pump áThe heat removed from the inside of a refrigerator is ejected into the kitchen á áA refrigerator that has the cold chamber as the outdoors and the hot chamber as the house is called a heat pump á áMany heat pumps can be reversed in summer to function as air conditioners

Refrigerators and the Second Law áYou cannot move heat from low to high temperature without the addition of work á á

Statements of the Second Law áKelvin-Planck Statement: á áClausius Statement: á

Equivalence á áOne implies the other áFor example: áA 100% efficient engine connected to a high T reservoir powering a refrigerator cooling a low T reservoir to the same high T reservoir á áThe refrigerator by itself is “legal” but the net effect to is move QL from low to high T with no other effect

Engines and Refrigerators áEfficiency: h = W/QH = (QH-QL)/QH = 1 - (QL/QH) áCan rewrite using: · · áCoefficient of performance: K = QL/W = QL/(QH-QL) (refrigerator) á