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Notes_17_Consequences_of_the_Second_Law

Notes_17_Consequences_of_the_Second_Law - Meeting 17...

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1 Meeting 17, Sections 6-8–6-12
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Announcements Use this week to practice and get ahead. Read chapters 5, 6, and 7. Work more examples. Make major progress on the Group Project. 2
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What we covered Friday The Second Law Various Statements of the Second Law. Thermal reservoirs. Perpetual Motion Machines. Heat Engine efficiencies. 3
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Topics for today Consequences of the Second Law Carnot Corollaries. Absolute temperature scale. Maximum efficiency. 4
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5 Analytical form of KP statement. Let us limit ourselves to the special case of one TER (thermal energy reservoir): TER HE W Q
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6 Analytical form of the KP statement. However, it would not violate the KP statement if work were done on the system during the cycle, or if work were zero. These are __________________ forms of the KP statement. Wcycle ≤ 0 (single reservoir) Qcycle ≤ 0 (single reservoir)
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7 Analytical forms of the KP statement. Both the equations may be regarded as analytical forms of the KP statement. It can be shown that the equality applies to reversible processes and that the inequality applies to irreversible processes. Get used to the inequalities. They are not going away.
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8 Analytical form of KP Statement: Conservation of Energy for a cycle says E = 0 = Qcycle - Wcycle, or Qcycle = Wcycle We have not limited the number of heat reservoirs (or work interactions, for that matter). Qcycle could be QH - QC, for example.
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9 R I QH QH WI WR QC=QH-WR I H C W Q Q - = Hot reservoir Cold reservoir
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10 Carnot’s first principle Each engine receives identical amounts of heat QH and produces WR or WI. Each discharges an amount of heat Q to the cold reservoir equal to the difference between the heat it receives and the work it produces.
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11 R I QH QH QH WI WR WR QC QC=QH-WR I H C W Q Q - = Hot reservoir Cold reservoir
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12 Carnot’s first principle. Taken together, Now reverse the reversible engine. C H C H I R Q Q Q Q W W - + - = + C H C H I R Q Q Q Q W W - + + - = + -
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13 R I QH QH QH WI WR QC QC=QH-WR I H C W Q Q - = Cold reservoir
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14 Carnot’s first principle If WI  WR, the system puts out net work and exchanges heat with one reservoir. This violates KP. So, WI cannot be  WR.
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