Data Sheet No. 155 - First and Second Laws of Thermodynamics—Thermal Cycles

Abstract

This chapter describes the first and second laws of thermodynamics. According to the first law of thermodynamics, energy can be neither created nor destroyed but only changed in form. This law can be expressed as a general energy equation, representing a thermodynamic system. Second law of thermodynamics enables the proportion of heat that cannot be converted into work to be measured. The latter is best defined by entropy, which is a measure of unavailable energy. Entropy allows the heat change in a thermal cycle to be expressed as an area, which measures the extent of the conversion of heat into work. The coordinates are temperature (ordinate) and entropy, measured from some standard condition (abscissa). Carnot cycle is the ideal cycle, in which heat is generated, or added, at a constant temperature and rejected at a lower temperature. The chapter presents a temperature entropy diagram representing this change.