(2)+(分)-(-7) The second law says that the entropy change must be equal to or greater than zero. This corresponds to the statement that heat must flow from the higher temperature source to the lower temperature source. This is one of the statements of the second law given in Section 1. B. 1 Muddy points In the single reservoir example, why can the entropy decrease? (MP 1B.6 19 Why does the entropy of a heat reservoir change if the temperature stays the same?(MP9 1BZ7)9 How can the heat transfer from or to a heat reservoir be reversible?(MP 1B. 8)9 How can AS be less than zero in any process? Doesn't entropy always increase? (MP 1B. 99 If <= As for a reservoir, could you add o to any size reservoir and still get the same 4s? MP 1B. 10) c) Possibility of obtaining work from a single heat reservoir We can regard the proposed process as the bsorption of heat, Q, by a device operating in a cycle, rejecting no heat, and producing work. The total entropy change is the sum of the change in the reservoir, the system or device, and the surroundings. The entropy change of the reservoir is AS=-O/Tu. The entropy evIe change of the device is zero because we ar considering a complete cycle(return to initial state) and entropy is a function of state. The urroundings receive work only so the entropy change of the surroundings is zero Work from a single heat reservoir The total entropy change is +△S -Q/Tm+0+0 The total entropy change in the proposed process is thus less than zero As<0 which is not possible. The second law thus tells us that we cannot get work from a single reservoir only. The"only"is important; it means without any other changes occurring. This is the other statement of the second law we saw in Section lB.1 Muddy points What is the difference between the isothermal expansion of a piston and the(forbidden) production of work using a single reservoir?(MP 1B.11) For the"work from a single heat reservoir"example. how do we know there is no ASu.2 (MP 1B. 12) 1B-8H ∆S =   −Q  +   Q   = Q (TH − TL )  TH   TL  T TL The second law says that the entropy change must be equal to or greater than zero. This corresponds to the statement that heat must flow from the higher temperature source to the lower temperature source. This is one of the statements of the second law given in Section 1.B.1. Muddy points In the single reservoir example, why can the entropy decrease? (MP 1B.6)9 Why does the entropy of a heat reservoir change if the temperature stays the same? (MP9 1B.7)9 How can the heat transfer from or to a heat reservoir be reversible? (MP 1B.8)9 How can ∆S be less than zero in any process? Doesn't entropy always increase? (MP 1B.9)9 If Q = ∆S for a reservoir, could you add Q to any size reservoir and still get the same ∆S? T (MP 1B.10) c) Possibility of obtaining work from a single heat reservoir We can regard the proposed process as the absorption of heat, Q, by a device or system, operating in a cycle, rejecting no heat, and producing work. The total entropy change is the sum of the change in the reservoir, the system or device, and the surroundings. The entropy change of the reservoir is ∆S = −Q/TH . The entropy change of the device is zero, because we are considering a complete cycle (return to initial state) and entropy is a function of state. The surroundings receive work only so the entropy change of the surroundings is zero. The total entropy change is Work from a single heat reservoir ∆Stotal = ∆Sreservoir + ∆Sdevice + ∆Ssurroundings = −Q T/ H + 00 + The total entropy change in the proposed process is thus less than zero, ∆Stotal < 0 which is not possible. The second law thus tells us that we cannot get work from a single reservoir only. The “only” is important; it means without any other changes occurring. This is the other statement of the second law we saw in Section 1.B.1. Muddy points What is the difference between the isothermal expansion of a piston and the (forbidden) production of work using a single reservoir? (MP 1B.11) For the "work from a single heat reservoir" example, how do we know there is no ∆Ssurr? (MP 1B.12) 1B-8