Thermodynamics

A process during which there is no heat transfer is called an adiabaticprocess (Fig. 2–14). The word adiabatic comes from the Greek wordadiabatos, which means not to be passed. There are two ways a processcan be adiabatic: Either the system is well insulated so that only a negligibleamount of heat can pass through the boundary, or both the system andthe surroundings are at the same temperature and therefore there is nodriving force (temperature difference) for heat transfer. An adiabatic processshould not be confused with an isothermal process. Even though there isno heat transfer during an adiabatic process, the energy content and thusthe temperature of a system can still be changed by other means suchas work.As a form of energy, heat has energy units, kJ (or Btu) being the mostcommon one. The amount of heat transferred during the process betweentwo states (states 1 and 2) is denoted by Q 12 , or just Q. Heat transfer perunit mass of a system is denoted q and is determined fromq Q m 1kJ>kg2 (2–14)only as it crosses the system boundary.Chapter 2 | 612 kJthermalenergySURROUNDINGHEATAIR2 kJheatBAKED POTATOSystem 2 kJboundary thermalenergyFIGURE 2–13Energy is recognized as heat transferSometimes it is desirable to know the rate of heat transfer (the amount ofheat transferred per unit time) instead of the total heat transferred over sometime interval (Fig. 2–15). The heat transfer rate is denoted Q . , where theoverdot stands for the time derivative, or “per unit time.” The heat transferrate Q . has the unit kJ/s, which is equivalent to kW. When Q . varies withtime, the amount of heat transfer during a process is determined by integratingQ . over the time interval of the process:InsulationADIABATICSYSTEMQ = 0t 2Q Q # dt1kJ2When Q . remains constant during a process, this relation reduces tot 1Q Q # ¢t1kJ2(2–15)(2–16)where t t 2 t 1 is the time interval during which the process takes place.Historical Background on HeatHeat has always been perceived to be something that produces in us a sensationof warmth, and one would think that the nature of heat is one of the firstthings understood by mankind. However, it was only in the middle of thenineteenth century that we had a true physical understanding of the nature ofheat, thanks to the development at that time of the kinetic theory, whichtreats molecules as tiny balls that are in motion and thus possess kineticenergy. Heat is then defined as the energy associated with the random motionof atoms and molecules. Although it was suggested in the eighteenth andearly nineteenth centuries that heat is the manifestation of motion at themolecular level (called the live force), the prevailing view of heat until themiddle of the nineteenth century was based on the caloric theory proposedby the French chemist Antoine Lavoisier (1744–1794) in 1789. The calorictheory asserts that heat is a fluidlike substance called the caloric that is amassless, colorless, odorless, and tasteless substance that can be poured fromone body into another (Fig. 2–16). When caloric was added to a body, itsFIGURE 2–14During an adiabatic process, a systemexchanges no heat with its surroundings.Q = 30 kJm = 2 kg∆t = 5 sQ = 6 kWq = 15 kJ/J/kg30 kJheatFIGURE 2–15The relationships among q, Q, and Q . .