6-1 introduction to the second lawE.g: we use electric resistor to heat the room. Is itpossible to transfer some heat to an equivalent amountof electric energy to be generated in the resistor ?Heat=0W·Transferring heat to a paddlewheel will not cause itto rotate.Heat
6-1 introduction to the second law • E.g: we use electric resistor to heat the room. Is it possible to transfer some heat to an equivalent amount of electric energy to be generated in the resistor ? 6 •Transferring heat to a paddle wheel will not cause it to rotate
6-1 introduction to the second lawSatisfying thefirstlawdoes not ensurethattheprocesscan actually occur. We need to introduce the secondlawofthermodynamicsAprocessmustsatisfybothIstlaw2nd lawPROCESSthefirstandsecondlawsofthermodynamicstoproceed.A process cannot occur unless it satisfies both laws
6-1 introduction to the second law • Satisfying the first law does not ensure that the process can actually occur. We need to introduce the second law of thermodynamics. • A process cannot occur unless it satisfies both laws. 7 A process must satisfy both the first and second laws of thermodynamics to proceed
6-1 introduction to the second law The second law of thermodynamics:- identify the direction of a process.assertthatenergyhasnotonlyquantity(数量）butalsoquality(品质）- provide the necessary means to determine the quality as well asthe degradation of energy during a process-be usedto determinethetheoretical limitsforperformanceofcommonlyusedengineeringsystems(heatenginesandrefrigerators).-Twostatementswill be discussedlater
6-1 introduction to the second law • The second law of thermodynamics: – identify the direction of a process. – assert that energy has not only quantity(数量) but also quality(品质). – provide the necessary means to determine the quality as well as the degradation of energy during a process – be used to determine the theoretical limits for performance of commonly used engineering systems (heat engines and refrigerators) . – Two statements will be discussed later. 8
6-2 thermal energy reservoirsThermal energy reservoir: supply or absorb finiteamounts of heat without undergoing any changein temperature: has big thermal energy capacity(mass*specific heat, mCp)Heat source(热源): a reservoir that suppliesenergy in the form of heat. Heat sink(热沉): a reservoir that absorbs energyin the form of heat
6-2 thermal energy reservoirs • Thermal energy reservoir: supply or absorb finite amounts of heat without undergoing any change in temperature: has big thermal energy capacity (mass*specific heat, mCP) • Heat source(热源): a reservoir that supplies energy in the form of heat. • Heat sink(热沉): a reservoir that absorbs energy in the form of heat. 9
6-3 heat engines (热机) Work can be converted to heat directly and completely,but converting heat to work requires the use of somespecial devices. These devices are called heat engines (热机)Receive heat (Qμ) from a high temperature source (solar energyfurnace)Convert part of these heat to work (W) (usually rotating shaft)- Reject the remaining waste heat (Q) to a low-temperature sink (theatmosphere,rivers,etc.)Operateonacycle Usually involves a fluid, called working fluid(工质)Broadly heat engine is often include work producing devices that do notoperate in a thermodynamic cycle. (like gas turbines, car engines, whichoperate in a mechanical cycle not in a thermodynamics (working fluiddoesnotundergoacompetecycle))10
6-3 heat engines (热机) • Work can be converted to heat directly and completely, but converting heat to work requires the use of some special devices. These devices are called heat engines ( 热机). – Receive heat (QH) from a high temperature source (solar energy, furnace) – Convert part of these heat to work (W) (usually rotating shaft) – Reject the remaining waste heat (QL ) to a low-temperature sink (the atmosphere, rivers, etc.) – Operate on a cycle. – Usually involves a fluid, called working fluid(工质). – Broadly heat engine is often include work producing devices that do not operate in a thermodynamic cycle. (like gas turbines, car engines, which operate in a mechanical cycle not in a thermodynamics (working fluid does not undergo a compete cycle)) 10