2013-3-6 Kelvin-Planck Statement of the Second Law It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir. Entropy Statement of the Second Law Mass and energy are familiar examples of extensive properties used in thermodynamics Entropy is another important extensive property. How entropy is evaluated and applied is detailed in Chapter 6. Unlike mass and energy,which are conserved, entropy is produced within systems whenever non-idealities such as friction are present. The Entropy Statement is: It is impossible for any system to operate in a way that entropy is destroyed. 6
2013-3-6 6 Kelvin-Planck Statement of the Second Law It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir. Entropy Statement of the Second Law ►Mass and energy are familiar examples of extensive properties used in thermodynamics. ►Entropy is another important extensive property. How entropy is evaluated and applied is detailed in Chapter 6. ►Unlike mass and energy, which are conserved, entropy is produced within systems whenever non-idealities such as friction are present. ►The Entropy Statement is: It is impossible for any system to operate in a way that entropy is destroyed
2013-3-6 Irreversibilities One of the important uses of the second law of thermodynamics in engineering is to determine the best theoretical performance of systems. By comparing actual performance with best theoretical performance,insights often can be had about the potential for improved performance. Best theoretical performance is evaluated in terms of idealized processes. Actual processes are distinguishable from such idealized processes by the presence of non- idealities-called irreversibilities. Irreversibilities Commonly Encountered in Engineering Practice Heat transfer through a finite temperature difference Unrestrained expansion of a gas or liquid to a lower pressure Spontaneous chemical reaction Spontaneous mixing of matter at different compositions or states Friction-sliding friction as well as friction in the flow of fluids
2013-3-6 7 Irreversibilities ►One of the important uses of the second law of thermodynamics in engineering is to determine the best theoretical performance of systems. ►By comparing actual performance with best theoretical performance, insights often can be had about the potential for improved performance. ►Best theoretical performance is evaluated in terms of idealized processes. ►Actual processes are distinguishable from such idealized processes by the presence of nonidealities – called irreversibilities. Irreversibilities Commonly Encountered in Engineering Practice ►Heat transfer through a finite temperature difference ►Unrestrained expansion of a gas or liquid to a lower pressure ►Spontaneous chemical reaction ►Spontaneous mixing of matter at different compositions or states ►Friction – sliding friction as well as friction in the flow of fluids