Both the two statements of the 2nd law are negative statements and a negative statement cannot be proved The two statements are equivalent in their consequences, and either statement can be used as the expression of the 2 nd Any device that ates the Kelvin-Planck statement also violates the Clausius statement nd vice versa Any device that violates either Ist law or 2nd law of thermodynamics is called a perpetual-motion machine Example 3 A car engine produces 136 hp on the output shaft with a thermal efficiency of 30%. The fuel it burns gives 35 000 kJ/kg as energy release. Find the total rate of energy rejected to the ambient and the rate of fuel consumption in kg/s Solution From the definition of a heat engine efficiency and the conversion of hp we have The energy equation for the overall engine gives From the energy release in the burning we have Notes An actual engine rejects energy to the ambient through the radiator cooled by atmospheric air, as heat transfer from the exhaust system and the exhaust flow of hot gases Entropy(熵) The second law is expressed mathematically in terms of the concept of entropy When a body absorbs an amount of heat O from a reservoir at temperature T, the body gains and the reservoir loses an amount of entropy S=Q/T If an amount of heat Q flows from a hot to a cold body, the total entropy increases because S-o/T is larger for smaller values of T, the cold body gains more entropy than the hot body loses The statement that heat never flows from a cold to a hot body can be generalized by saying that in no spontaneous process does the total entropy decrease 4.3 Reversible reversible Process a process is reversible if the system and its surroundings can be returned to their initial states The system and its surroundings cannot be restored to their initial states if the process is irreversible Notes All real processes are irreversible but many of our calculations ignore that fact because the assumption of reversibility makes the calculations easier We go ahead assuming most processes are reversible and then correct our calculations to get approximate answers. The Our goal as engineers is to minimize the degree of irreversibility 4.4 The 3rd Law of Thermodynamics a postulate related to but independent of the 2nd law is that it is impossible to cool a body to absolute zero by any finite process. Although one can approach absolute zero as closely as one desires, one cannot actually reach this The 3rd law of thermodynamics, formulated by Walter Nernst and also known as the Nernst heat theorem states The limiting value of the entropy of a system can be taken as zero as the absolute value of temperature approaches zer 5. Thermal Deformation Thermal deformation means that as the thermal energy (and temperature)of a material increases, so does the vibration of itsBoth the two statements of the 2nd law are negative statements, and a negative statement cannot be proved. . The two statements are equivalent in their consequences, and either statement can be used as the expression of the 2nd law. . Any device that violates the Kelvin-Planck statement also violates the Clausius statement, and vice versa. . Any device that violates either 1st law or 2nd law of thermodynamics is called a perpetual-motion machine. . Example 3 A car engine produces 136 hp on the output shaft with a thermal efficiency of 30%. The fuel it burns gives 35 000 kJ/kg as energy release. Find the total rate of energy rejected to the ambient and the rate of fuel consumption in kg/s. Solution From the definition of a heat engine efficiency and the conversion of hp we have: The energy equation for the overall engine gives: From the energy release in the burning we have: Notes An actual engine rejects energy to the ambient through the radiator cooled by atmospheric air, as heat transfer from the exhaust system and the exhaust flow of hot gases. Entropy （熵） The second law is expressed mathematically in terms of the concept of entropy. When a body absorbs an amount of heat Q from a reservoir at temperature T, the body gains and the reservoir loses an amount of entropy S = Q / T . If an amount of heat Q flows from a hot to a cold body, the total entropy increases; because S=Q/T is larger for smaller values of T, the cold body gains more entropy than the hot body loses. . The statement that heat never flows from a cold to a hot body can be generalized by saying that in no spontaneous process does the total entropy decrease. 4.3 Reversible & Irreversible Process A process is reversible if the system and its surroundings can be returned to their initial states. . The system and its surroundings cannot be restored to their initial states if the process is irreversible. . Notes All real processes are irreversible but many of our calculations ignore that fact because the assumption of reversibility makes the calculations easier. . We go ahead assuming most processes are reversible and then correct our calculations to get approximate answers. The corrections are based on experience. . Our goal as engineers is to minimize the degree of irreversibility. 4.4 The 3rd Law of Thermodynamics A postulate related to but independent of the 2nd law is that it is impossible to cool a body to absolute zero by any finite process. Although one can approach absolute zero as closely as one desires, one cannot actually reach this limit. . The 3rd law of thermodynamics, formulated by Walter Nernst and also known as the Nernst heat theorem, states : . The limiting value of the entropy of a system can be taken as zero as the absolute value of temperature approaches zero. 5. Thermal Deformation Thermal deformation means that as the thermal energy (and temperature) of a material increases, so does the vibration of its atoms/molecules ;