CH 5 The Second Law of Thermodynamics Introduction

CH. 5 The Second Law of Thermodynamics

Introduction to the Second Law of Thermodynamics l 0 th law of thermodynamics If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. l 1 st law of thermodynamics Conservation of energy principle

Introduction to the Second Law of Thermodynamics l A cup of hot coffee does not get hotter in a cooler room. l Transferring heat to a wire will not generate electricity. l Transferring heat to a paddle wheel will not cause it to rotate.

Heat Engines l Net work output and thermal efficiency relations for any heat engine Wnet, out = QH – QL l Thermal efficiency of a heat engine is always less then unity.

Refrigerators and Heat pumps l 냉공기 : 냉각실에서 QL 을 제거 l QH: Magnitude of the heat rejected to the worm environment at TH. l Wnet, out: Net work input to the refrigerator

Refrigerators and Heat pumps l l 냉동기의 효율 : 성적계수 Coefficient of Performance (COP) Objective of a refrigerator: Remove heat QL Conservation of energy principle for a cyclic device requires Wnet, in = QH - QL

Refrigerators and Heat pumps 열펌프 (Heat pumps) 저온부에서 고온부로 열전달 장치 (따뜻한 방에 열 QH를 공급) Heat pumps operates on the same cycle with refrigerators but differs in their objectives. l

역 카르노 사이클 (The Reversed Carnot Cycle) 카르노 열기관은 내외적 가역사이클 따라 서 모든 과정 역을 될 수 있다. -> Carnot 냉동 사이클 (Carnot Refrigeration Cycle. ) Heat in the amount of QL is absorbed from the low-temperature reservoir. Heat in the amount of QH is rejected to a high-temperature reservoir. Work input of Wnet, in is required to accomplish all this.

The Thermodynamic Temperature Scale l l l Heat Q 1 is supplied to HE. A and HE. C Engine C rejects Q 3 to the low-temperature reservoir. Engine B receives the heat Q 2 rejected by engine A and rejects heat in the amount of Q 3.

The Thermodynamic Temperature Scale l Thermodynamic Temperature l Kelvin scale, Absolute temperatures l On the Kelvin scale, the temperature ratios depend on the ratios of heat transfer b/w a reversible heat engine and the reservoirs and are independent of the physical properties of any substance. On this scale, temperatures vary b/w zero and infinity

The Thermodynamic Temperature Scale l It gives us only a ratio of absolute temperatures. l What is the magnitude of a kelvin degree? l The triple point of water (the state at which all three phases of water exist in equilibrium) was assigned the value 273. 16 K. l The magnitude of a kelvin is defined as 1/273. 16 of the temperature interval between absolute zero and the triplepoint temperature of water. l 1 K = 1 C and T( C) = T(K) -273. 15

The Carnot Heat Engine l Thermal efficiency: th = 1 - QL/QH l Carnot engine or any reversible heat engine Carnot efficiency: th, rev = 1 - TL/TH l This is the highest efficiency a heat engine operating b/w the two thermal energy reservoirs at temperatures TL and TH. l Note that TL and TH are absolute temperature

The Carnot Heat Engine The thermal efficiencies of actual and reversible heat engines operating b/w the same temperature limits. < th, rev Irreversible heat engine th = th, rev Reversible heat engine > th, rev Impossible heat engine l l l The maximum efficiency of a steam power plant is 60%. The thermal efficiency of actual heat engines can be maximized by supplying heat to the engine at the highest possible temperature and rejecting heat from the engine at the lowest possible temperature.

The Carnot Heat Engine The quality of energy l Efficiency values show that energy has quality as well as quantity. l More of the high temperature thermal energy can be converted to work. Therefore, the higher the temperature, the higher the quality of the energy.

The Carnot Refrigerator and Heat Pump l Coefficient of Performance (COP) QL: Amount of heat absorbed from the low-temperature medium QH: Amount of heat rejected to the high-temparature medium l COPs of all reversible refrigerator or heat pump can be expressed by the ratios of the absolute temperatures of highand low-temperature media.

The Carnot Refrigerator and Heat Pump l These are the highest coefficients of performance that a refrigerator or a heat pump operating between the temperature limits of TL and TH can have.