Thermodynamics

200 | ThermodynamicsSolution A person switches from regular potato chips to fat-free ones. Theweight the person loses in one year is to be determined.Assumptions Exercising and other eating habits remain the same.Analysis The person who switches to the fat-free chips consumes 75 fewerCalories a day. Then the annual reduction in calories consumed becomesThe metabolizable energy content of 1 kg of body fat is 33,100 kJ. Therefore,assuming the deficit in the calorie intake is made up by burning bodyfat, the person who switches to fat-free chips will losem fat E reduced 175 Cal>day2 1365 day>year2 27,375 Cal>yearE reducedEnergy content of fat (about 7.6 pounds) of body fat that year.27,375 Cal33,100 kJ>kg4.1868 kJa b 3.46 kg1 CalSUMMARYWork is the energy transferred as a force acts on a systemthrough a distance. The most common form of mechanicalwork is the boundary work, which is the work associatedwith the expansion and compression of substances. On a P-Vdiagram, the area under the process curve represents theboundary work for a quasi-equilibrium process. Variousforms of boundary work are expressed as follows:2(1) GeneralW b P dV(2) Isobaric processW b P 0 1V 2 V 1 2(3) Polytropic process(P 1 P 2 P 0 constant)W (PV n b P 2V 2 P 1 V 11n 12 constant)1 n(4) Isothernal process of an ideal gasV 2V 2W b P 1 V 1 ln mRT (PV mRT 0 constant)V 0 ln1 V 1The first law of thermodynamics is essentially an expressionof the conservation of energy principle, also called the energybalance. The general energy balances for any system undergoingany process can be expressed as1Net energy transferby heat, work, and massChange in internal, kinetic,potential, etc., energiesIt can also be expressed in the rate form asRate of net energy transferby heat, work, and massRate of change in internal,kinetic, potential, etc., energiesTaking heat transfer to the system and work done by thesystem to be positive quantities, the energy balance for aclosed system can also be expressed aswhereE in E out ⎫ ⎪⎬⎪⎭ ⎫⎪⎪⎬⎪⎪⎭E . in E . out Q W ¢U ¢KE ¢PE1kJ2W W other W b¢U m 1u 2 u 1 2¢KE 1 2m 1V 2 2 V 2 12¢PE mg 1z 2 z 1 2¢E system 1kJ2dE system >dt1kW2For a constant-pressure process, W b U H. Thus,Q W other ¢H ¢KE ¢PE1kJ2⎫⎪⎬⎪⎭⎫⎪⎪⎬⎪⎪⎭