lIbTHEFIRSTLAW OFTHERMODYNAMICS3·5116FIGURE3-46Energy cannot be created or destroyed;it can only change forms.\,So far we have considered various forms of energy such as heatQ,workW,and total energyEindividually,andno attempthasbeen made torelate them to each other during a process.Thefirst lawofthermodynamics,also known asthe conservationofenergy principle,provides a sound basis for studying the relationshipsamongthe variousforms of energy and energy interactions. Based on experimental observa-tions, the first law of thermodynamics states thatenergy can be neithercreatednordestroyed; it can only change forms.Therefore,every bit ofenergy should be accounted for during a process. The first law cannot beproved mathematically, but no process in nature is known to haveviolated the first law, and this should be taken as sufficient proof.We all know that a rock at some elevation possesses some potentialenergy, andpartof this potential energy is converted to kinetic energy asthe rock falls (Fig. 3-46). Experimental data show that the decrease inpotential energy(mgilz)exactly equals the increase in kinetic energy[m(V~VD/2] when the air resistance is negligible, thus confirming theconservation of energy principle.Consider a system undergoing a series ofadiabaticprocesses from aspecified state 1 toanotherspecified state 2. Being adiabatic, theseprocesses obviously cannot involve any heat transfer but they mayinvolve several kinds of work interactions. Careful measurements duringthese experiments indicate the following:For all adiabatic processesbetween two specified statesofa closed system, the networkdoneisthesame regardlessofthe natureofthe closed systemandthe detailsoftheprocess.Considering that there are an infinitenumberof ways to performwork interactions under adiabatic conditions, the statement aboveappears to be very powerful with a potential for far-reaching implications.This statement which is largely based on the experiments of Joule in thefirst half of the nineteenth century cannot be drawn from anyotherknown physical principle, and is recognized as a fundamental principle.This principle is called the first lawofthermodynamics or just the firstlaw.A major consequence of the first law is the existence and thedefinition of the propertytotal energyE.Considering that the net work isthe same for all adiabatic processes of a closed system between twospecified states, the value of the net work must depend on the end statesof the system only, and thus it must correspond to a change in a propertyof the system. This property is thetotal energy.Note that the first lawmakes no reference to the value of the total energy of a closed system ata state.Itsimply states that thechangein the total energy during anadiabatic process must be equal to the net work done. Therefore, anyconvenient arbitrary value can be assigned to total energy at a specifiedstate to serve as a reference point.Implicit in the first law statement is the conservation of energy.Although the essence of the first law is the existence of the propertytotalenergy,the first law is often viewed as a statement of theconservationofPEl=10 kJKE,=0mCHAPTER 3TheFirstLawofThermodynamics:Closed SystemsI'iI,,l,~-/",IIIiI117The First Law orThennodynamicsQ2=-3kJQ=5 kJ!:J.E= Qnet=I2kJFIGURE3-47Theincrease in theenergyof apotatoin an oven is equal to theamountofheat transferred to it.Q,=15kJFIGURE3-48In the absence of any workinteractions, energy change of a systemis equal to the net heat transfer.+-5kJr-----------r---~------~I,I1I1111!:J.E5 kJ, II 1'--------1Now consider a well-insulated (i.e., adiabatic)roomheatedby anelectricheaterasoursystem (Fig. 3-49). As a result of electrical workdone, the energy of the system will increase. Since the system is adiabaticand cannot have any heat interactions with the surroundings(Q0), theconservation of energy principle dictatesthatthe electrical workdoneonthe system must equal the increase in energy ofthesystem.Thatis,We=AE.Thenegative sign isdueto the sign conventionthatwork done on asystem is negative. This ensures that workdoneon a system increases theenergy of the systemandwork done by a system decreases it.Now let us replace the electricheaterwith a paddle wheel (Fig. 3-50).As a result of the stirring process, theenergyof the system will increase.Again since there is no heat interaction betweenthesystem and itssurroundings(Q=0), the paddle-wheel workd()n~u?nthesystem mustshow up as an increase in theenergyofthesystem.Thatis, -Wpw=AE.(Adiabatic)energyprinciple. Below we developthefirst law ortheconservation ofenergy relation for closed systems with the help of some familiarexamples using intuitive arguments.Letus consider first some processes that involveheattransferbutnowork interactions.Thepotatointheoven that we have discussedpreviously is a good example for this case (Fig. 3-47). As a result of heattransfer to the potato, the energy of thepotatowill increase.Ifwedisregard any mass transfer (moisture loss from thepotato),the increasein the total energy of thepotatobecomesequal totheamountofheattransfer.Thatis, if 5 kJ ofheatis transferred to the potato, theenergyincrease ofthepotatowill also be 5 kJ. Therefore, the conservation ofenergy principle for this case can be expressed asQ=6.E.Asanotherexample, consider the heatingofwaterin a pan on top ofa range (Fig. 3-48).If15 kJofheatis transferred tothewater from theheatingelementand 3 kJ of it is lost from the water to the surroundingair, the increase in energy of thewaterwill be equal to the netheattransfer to the water, which is 12 kJ.Thatis,Q=Qncl=6.E.Theabove conclusions can be summarized as follows:In the absenceofany work interactions between a systemandits surroundings, theamountofnet heat transfer is equal to the change in the total energyofa closed system.Thatis,Q=6.EwhenW =0118(Adiabatic).(Adiabatic)(3-34) .(kJ)owhenQW11£ = I1U + I1KE + I1PEwhere!J.E=8 kJW =-8kJpw.(\•liVQ netheattransferacrosssystemboundaries( L Q;n - LQOUl)Wnetworkdonein all forms ( LWOul- L'Win)I1E = netchangein totalenergyofsystem(E2EJAs discussed inChap.I,thetotalenergyE of a system isconsideredto consist ofthreeparts:internalenergyU, kineticenergyKE,andpotentialenergyPE.Thenthechangein totalenergyofa systemduringaprocesscan beexpressedas thesumofthechangesin its internal, kinetic,andpotentialenergies:Now wearein apositiontoconsidersimultaneousheatandworkinteractions.Asyoumayhavealreadyguessed,whenasysteminvolvesbothheatandworkinteractionsduringa