复旦大学：《大学物理——热学》课程教学资源（PPT课件）第16次课

大学物理(热学) 1) Review 2) Carnot's cycle eNtropy 2006-12-21 2021/9/5

2021/9/5 1 大学物理（热学） 1) Review 2) Carnot’s cycle 3) Entropy 2006-12-21

High temperature High-pressure ste from boiler Steam Water Intake valve Boiler ng expansion) Exhaust valve Piston (closed during expansion) Pump pressure steam, exhausted to condenser Wat temperature FIGURE 20-2 Steam engines (a)Reciprocating type (b)Turbine(boiler and condenser not shown)

FIGURE 20-3 Four-cycle internal combustion engine: (a)the gasoline-air mixture flows into the cylinder as the piston moves down:; (6) the piston moves upward and compresses the gas; c) firing of the spark plt plug ignites the gasoline-a mixture, raising it to a high temperature; (d) the gases, now at high temperature and pressure, expand against the piston in he power stroke; (e) the burned gases are pushed out to the exhaust pipe; the intake valve then opens, and the whole cycle repeats. Intake Exhaust Both valves Both valy alve Both valves Exha closed To exhaust Ga . cylinder ixture from carburetor Rings Connectin rod Crankshaft Intake (b)Compression (c)Ignition(d)Expansion (e)Exhaust (power stroke)

用热做功—存在限制否?

用热做功——存在限制否？

Carnot's ideas

Carnot’s ideas Q W

Carnot's ideas The necessary condition of the maximum is, then, that in the bodies employed by realize the motive power of heat there should not occur any change of temperature which may not due to a change of volume

“The necessary condition of the maximum is, then, that in the bodies employed by realize the motive power of heat there should not occur any change of temperature which may not due to a change of volume. ” Carnot’s ideas

Carnot's Cvcle FIGURE 20-5 The Carnot cycle Heat In a cyo the cycle for the Carnot engine begins at point a on this Pl diagram (1) The gas is first panded isothermally, with addition of heat @), along path ab at temperature TH. (2)Next d→ he gas expands adia Adiabatic Adiabatic from b to c-no heat is exchanged compression lexpansion but the temperature drops to TL (3)The gas is then compressed at int temperature Ti, path c to d 0=0 and heat e. flows out. (4)Finally he gas is compressed adiabatically path da, back to its original state. No Carnot engine actually exists, but as a theoretical engine it played Isothermal Important role in the develop of thermodynamics compression

Carnot’s Cycle

High temperature, TH Engine Ow temperature, FIGURE 20-1 Schematic diagram of energy transfers for a heat engine

Carnot's theorem cAll reversible engines operating between the same two constant temperatures TH and tl have the same efficienc y Any irreversible engine operating between the same two fixed temperatures will have an efficiency less than this

“All reversible engines operating between the same two constant temperatures TH and TL have the same efficiency. Any irreversible engine operating between the same two fixed temperatures will have an efficiency less than this.” Carnot’s theorem

Clausus statement Not possible Q

Clausuis statement Not possible