2 In discussing thermodynamics, we often refer to particular systems. A systemis any object or set of objects that we wish to consider (see Section 14-4). Every-thing else in the universe is referred to as its "environment" or the "surroundings."In this Chapter, we examine the two great laws of thermodynamics. The firstlaw of thermodynamics relates work and heat transfers to the change in internalenergy of a system, and is a general statement of the conservation of energy. Thesecond law of thermodynamics expresses limits on the ability to do useful work,and is often stated in terms of entropy, which is a measure of disorder. Besidesthese two great laws, we also discuss some important related practical devices:heat engines, refrigerators, heat pumps, and air conditioners.15-1 The First Law of ThermodynamicsIn Section 14-2, we defined the internal energy of a system as the sum total of allthe energy of the molecules within the system. Then the internal energy of asystem should increase if work is done on the system, or if heat is added to it.Similarly the internal energy should decrease if heat flows out of the system or ifwork is done by the system on something in the surroundings.Thus it is reasonable to extend conservation of energy and propose an impor-tant law: the change in internal energy of a closed system, AU, will be equal tothe energy added to the system by heating minus the work done by the system onthe surroundings. In equation form we writelean basAU = Q - W(15-1)where is the net heat added to the system and W is the net work done by the system.We must be careful and consistent in following the sign conventions for Q and W.Because Win Eq. 15-1 is the work done by the system, then if work is done on thesystem, W will be negative and U will increase. Similarly, Qis positive for heatadded to the system, so if heat leaves the system, Q is negative. [Caution: Elsewhereyou may sometimes encounter the opposite convention for W where W is definedas the work done on the system; in that case Eq. 15-1 is written as AU = Q + W.]Equation 15-1 is known as the first law of thermodynamics. It is one of thegreat laws of physics, and its validity rests on experiments (such as Joule's) towhich no exceptions have been seen. Since Q and W represent energy transferredinto or out of the system, the internal energy changes accordingly. Thus, the firstlaw of thermodynamics is a general statement of the law of conservation of energy.Note that the conservation of energy law was not able to be formulated untilthe 1800s, because it depended on the interpretation of heat as a transfer of energy.A given system does not "have" a certain amount of heat or work. Rather, workand heat are involved in thermodynamic processes that can change the systemfrom one state to another; they are not characteristic of the state itself. Quantitieswhich describe the state of a system, such as internal energy U, pressure P, volume V,temperature T, and mass m or number of moles n, are called state variables.Q and W are not state variables.FIGURE 15-3 PV diagra forEXAMPLE 15-1 Using the first law. 2500 J of heat is added to a system,and 1800 J of work is done on the system. What is the change in internal energyof the system?APPROACH We apply the first law of thermodynamics, Eq. 15-1, to our system.SOLUTION The heat added to the system is Q = 2500 J. The work W doneby the system is -1800 J. Why the minus sign? Because 1800 J done on thesystem (as given) equals - 1800 J done by the system, and it is the latter we needfor the sign conventions we used for Eq. 15-1. HenceAU = 2500 J (-1800J) = 2500 J + 1800 J = 4300 J.FIRST LAW OFTHERMODYNAMICSNOTE We did this calculation in detail to emphasize the importance of keepingcareful track of signs. Both the heat and the work are inputs to the system, so weexpect AU to be increased by both.CAUTIONHeat added is +Heat lost is -Work on system is-Work by system is +sidsvoM.CAUTIONP, V, T, U, m, n are statevariables. W and Q are not:a system does not have an RUDHamount of heat or work buites 101 mei 19 s-ar BAUDAIsaiso gator obnu ang inobiTangihSECT 15-2 abimanybormed Hd awon SECTION 15-1 413 2. An ideal gas undergoes an isothermal expansion from one state to another. In this processdetermine whether Q, W and AU are positive, negative or zero (using the sign conventions onpage 413):Q = 0, Q> 0 or Q<0W = 0, W> 0 or W<0AU = 0, AU >0 or AU <0OOO

An ideal gas undergoes an isothermal expansion from one state to another. In this process determine whether Q, W and AU are positive, negative or zero (using the sign conventions on page 413): Q = 0, Q> 0 or Q<0 W = 0, W>0 or W<0 AU = 0, AU >0 or AU <0 O O O