21st Law of Thermodynamics Conservation of energyWhen heat flows to or from a system, the system gains or loses an amount of energy equal to the amount of heat transferredHeat added to system = increase in internal energy + external work done by the system
3Adiabatic ProcessesCompression or expansion of a gas where no heat enters or leaves the systemIf we do work on a system by compression, we heat it up.When we expand a system we cool it offRemember blowing on your hands?Warm air rises, expands, cools and forms clouds
52nd Law of Thermodynamics Thermal energy never flows spontaneously from cold object to a hot object.A machine cannot be 100% efficientEntropy of closed systems always increases.One of the greatest shortcoming of would-be inventors is lack of understanding of the 1st and 2nd law of Thermodynamics.
6Maximum Efficiency of Heat Engine High TemperatureInput HeatWork doneHeat ExhaustLow Temperature
8Efficiency T hot - Tcold Ideal Efficiency = ------------------------ If a steam turbine runs with an input of 400 K and an output of 300 K it has an efficiency of:Eturbine = ( )/400 = ¼ or 25%
9Entropy 2nd Law of Thermodynamics The entropy of a thermally isolated system never decreases, it can only increase or stay the same. Entropy is a measure of the disorder of a system.Drop an egg and the process can never reverse and become an egg again.In a natural process, high quality energy tends to transform into lower quality energy – order tends to disorder
10Class ProblemThe air temperature at an altitude of 10 kilometers is a chilling –35°C. Cabin temperatures in airplanes flying at this altitude are comfortable because of air conditioners rather than heaters. Why?
11Class ProblemThe air temperature at an altitude of 10 kilometers is a chilling –35°C. Cabin temperatures in airplanes flying at this altitude are comfortable because of air conditioners rather than heaters. Why?Airliners have pressurized cabins. The process of stopping and compressing outside air to near-sea-level pressures would normally heat the air to a roasting 55°C (130°F). So air conditioners must be used to extract heat from pressurized air.
12Class ProblemA piece of iron has a temperature 10°C. A second identical piece of iron is twice as hot. What is the temperature of the second piece of iron?200 C2730 C2930 C3130 CNone of the above
13Class ProblemThe twice-as-hot iron is 293°C: Consider a stick that is units long. This is like a thermometer that extends from absolute zero (–273°C) to 10°C. Can you see that a stick twice as long is 2 x units long? (Or temperature-wise, 566 K?)
14Class Problems1. To wholly convert a given amount of heat energy into mechanical energy isA) possible using a simple machine.B) possible using an atomic reactor.C) possible using a steam engine.D) impossible regardless of the technique used.2. The first law of thermodynamics is a restatement of theA) law of heat addition.B) Carnot cycle.C) principle of entropy.D) conservation of energy.E) none of these3. Systems that are left alone, tend to move toward a state ofA) more entropy.B) less entropy.C) no entropy.
15Class Problems 4. Entropy measures A) temperature at constant pressure.B) temperature as volume increases.C) temperature at constant volume.D) temperature as pressure increases.E) messiness.5. During an adiabatic compression of an ideal gas,A) the temperature of the gas does not change.B) the internal energy of the gas remains constant.C) no heat is supplied to or removed from the gas.D) no work is done on the gas.E) None of the above choices are true.6. Entropy can beA) neither created nor destroyed.B) created but not destroyed.C) sometimes destroyed but never created.
16Class Problems7. One hundred joules of heat is added to a system that performs 60 joules of work. The internal energy change of the system isA) 60 J.B) 40 J.C) 100 J.D) 0 J.E) None of the above choices are correct.8. A heat engine would have 100 percent efficiency if its input reservoir wereA) 1000 times hotter than the exhaust sink.B) 100 times hotter than the exhaust sink.C) 100 times cooler than the exhaust sink.D) any finite temperature if the exhaust sink were at absolute zero.E) at any finite temperature regardless of the heat sink temperature.
17Class Problems9. An adiabatic process is characterized by the absence of1) temperature change.2) pressure change.3) entropy.4) heat exchange.5) None of the above choices are true.10. Two identical blocks of iron, one at 10 degrees C and the other at 20 degrees C, are put in contact. Suppose the cooler block cools to 5 degrees C and the warmer block warms to 25 degrees C. This would violate the1) 1st law of thermodynamics.2) 2nd law of thermodynamics.3) both of these4) neither of these11. Suppose you put a closed, sealed can of air on a hot stove burner. The contained air will undergo an increase in1) pressure.2) temperature.3) temperature and pressure.4) internal energy, temperature and pressure.5) internal energy.
18Class Problems12. Your refrigerator is in your kitchen. If you try to cool your kitchen on a hot day by operating your refrigerator with its door open, the kitchen temperature willincrease.decrease.remain unchanged.13. The ideal efficiency for a heat engine operating between temperatures of 2700 K and 300 K is1) 89%.2) 24%.3) 10%.4) 80%.5) none of these