Refrigerators

Physics 313Professor Lee

CarknerLecture 13

Exercise #12 Engines

V1 = 6.25X10-4 m3, P1 = 12X106 Pa, n = 3 moles

P1V1 = P2V2

P2 = P1V1

/V2 = 385 MPa

= 1 – T1/T2 = 0.75

W = QH, QH = ncVT23

P3 = 500X106 Pa, V3 = 7.8X10-5 m3

QH = (3)(3/2)(8.31)(1564-1205) = 13424 J

Limits on Engines

Engines convert heat into work and waste heat

Second Law of Thermodynamics

An engine cannot have 100% efficiency

1st and 2nd Laws

Converting heat completely into work does not violate the 1st law

The second law is an independent statement

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)

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

Refrigerator Cycle

Liquid

Gas

Compressor (work =W)

Expansion Valve

Heatremovedfrom fridgeby evaporation

Heat added to room bycondensation

HighPressure

Low Pressure

QL QH

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

Air Conditioner

Air conditioners also have condensers that dry out the air

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

Refrigerators and the Second Law

You cannot move heat from low to high temperature without the addition of work

Statements of the Second Law

Kelvin-Planck Statement:

Clausius Statement:

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

Engines and Refrigerators Efficiency:

= W/QH = (QH-QL)/QH = 1 - (QL/QH) Can rewrite using:

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Coefficient of performance:K = QL/W = QL/(QH-QL) (refrigerator)