Physics 15: The Laws of Thermodynamics

1. Physics 15: The Laws of Thermodynamics Christopher Chui The Laws of Thermodynamics - C. Chui

2. Thermodynamics Terminology • Thermodynamics is the study of processes in which energy is transferred as heat and as work • Heat is the transfer of energy due to temperature difference; work is a transfer of energy NOT due to a temperature difference • A closed system is one which no mass enters or leaves, but energy must be exchanged; an open system is one which mass/energy may enter or leave • A closed system is isolated if no energy passes across its boundaries The Laws of Thermodynamics - C. Chui

3. The 1st Law of Thermodynamics • Change in internal energy, DU= H, heat added – W, work done by the system • Heat added is +; heat lost is -; work on system is -; work by the system is + • 1st law of thermodynamics is conservation of energy • Internal energy is a property of a system; work and energy are not • Work and heat are involved in thermodynamic processes that can change the system from one state to another The Laws of Thermodynamics - C. Chui

4. 1st Law applied to simple systems • Isothermal means constant temperature, PV=constant • A heat reservoir is one that temperature does not change during compression or expansionisotherms • Isothermal process (ideal gas): T=const. DU=0, Q=W • Adiabatic process is one in which no heat is allowed to flow into or out of the system: Q=0 or DU = -W • Work is done on the gasU and T increase • Isobaric process is one in which the pressure is kept constant P = constant, W = PDV • Isochoric or isovolumetric process is one in which the volume is kept constant V = constant, W = 0 • Work = area under PV curve The Laws of Thermodynamics - C. Chui

5. 1st Law and Human Metabolism • Heat flows out of the body • Internal energy (chemical potential energy) is stored in food • When we eat food, internal energy increases. This energy goes into work and heat flow from the body • Metabolic rate is the rate at which internal energy is transformed within the body • For a 65-kg adult, metabolic rates in kcal/h: sleeping 60; sitting upright 100; light activity 200; moderate work 400; running 1000; bicycling 1100 The Laws of Thermodynamics - C. Chui

6. The 2nd Law of Thermodynamics • Clausius (1880): heat flows naturally from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object • A heat engine is a device that changes thermal energy into mechanical work • Heat engine: heat input QH at a high temperature TH is partly transformed into work W and partly exhausted as heat QL at a lower temperature TL • Efficiency, e = W/QH = (QH-QL)/QH = 1 – QL/QH • Carnot (ideal) engine is reversible, eff = 1 – TL/TH • Kelvin-Planck: no device is possible whose sole effect is to transform a given amount of heat completely into work The Laws of Thermodynamics - C. Chui

7. Refrigerators, and Air Conditioners • Coefficient of performance, CP = QL / W = TL/(TH-TL) • For both refrigerators and air conditioners, work is done to remove heat from inside to the outside • For heat pump, work is done to move heat from outside to inside: CP = QH / W • The 2nd law can be stated in terms of entropy, which is a measure of order or disorder of a system • Entropy change, DS = Q / T > 0, never decreases • 2nd Law: the total entropy of any system plus that of its environment increases as a result of any natural process. • 2nd Law: Natural processes tend to move toward a state of greater disorder. In any natural process, some energy becomes unavailable to do useful work • Heat death is the final outcome of our universe The Laws of Thermodynamics - C. Chui

8. Evolution, Growth, & Time’s Arrow • The 2nd law is a downhill process, and evolution is an uphill process. These processes constantly compete • Growth is possible because of goal-directed programs and input of energy. Given enough time, growth will be overpowered by decay • Life is a composite of highly ordered machinery directed by sophisticated programs. Given enough time, the 2nd law wins • Mutation can be considered as a downhill process • The 2nd is the time’s arrow showing which way time goes. Time, once elapsed, will never come back The Laws of Thermodynamics - C. Chui

9. Statistical Interpretation of 2nd Law • Boltzmann (1844-1906) proposed the microstate and macrostate of a system • The microstate refers to the position and velocity of every particle • The macrostate refers to macroscopic properties of the system, such as temperature, pressure, etc • Each microstate is equally probable • Macroscopic events are based on probabilities The Laws of Thermodynamics - C. Chui

10. Energy Resources & Pollution • Conversion of energy from one form to another involves air pollution and thermal pollution • Fossil-fuel steam plants produce 90% of US energy • Nuclear energy uses a steam engine • Geothermal energy gives thermal and chemical pollution • Hydroelectric power plants damage the environment • Tidal energy affects wildlife • Wind power are generally clean • Solar energy is expensive. Solar cell manufacturing produces pollution • Hydrogen fuel cells may be promising The Laws of Thermodynamics - C. Chui

11. Problem Solving in Thermodynamics • Identify the system and the surroundings • Be careful about the signs with work and heat. Work done by the system is +ve; work done on the system is –ve. Heat added is +ve; heat removed is –ve • Watch units used for work and heat; work is J; and heat in cal or kcal. Choose a consistent unit • Temperatures must be in K • Efficiency is a ratio of 2 energies or powers: out / in • Efficiency is always less than 1 or < 100% • Entropy increases when heat is added, and decreases when heat is removed The Laws of Thermodynamics - C. Chui