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Chapter 24 Thermodynamics: 1st Law of Thermodynamics What is thermodynamics? Thermodynamics is the study of heat and its transformations into mechanical energy. What is the focus of Thermodynamics? Focus is off the molecular level and on the macroscopic level State the first law of thermodynamics. Energy is not created or destroyed Also called the law of conservation of energy What happens when energy leaves a system? When energy leaves a system, external work is done Work = F x d Internal energy decreases How can the energy of a system increase? When external work is done on a system, internal energy increases. What is an adiabatic process? Adiabatic Process: changing system energy without heat transfers. Describe how the energy changes during this process. Expansion: decreases temperature (energy) in a system Contraction: increases Temperature (energy) in a system What is the change in temperature that occurs as an air mass rises? The temperature drop for a rising air mass is about 10ºC for every 1 km increase in elevation. How is a Chinook wind created? Chinook winds are created when descending dry, cold air warms. If work is done adiabatically on a system, will the internal energy of the system increase or decrease? If work is done by a system will the internal energy increase or decrease? Summary If work is done by a system will the internal energy decrease Chapter 24 Thermodynamics: 2nd Law of Thermodynamics (parts I-III) Part I State the 2nd Law of Thermodynamics. Heat will never of itself flow from a colder object to a hot object. How can you make heat flow from cold to hot? Heat can be made to flow in the reverse direction only with External Effort What is a heat engine? Heat engine: any device that changes internal energy into mechanical work Work can create heat, but heat can never be completely What does the 2nd Law of converted into work. Part II Thermodynamics say about heat engines? In all heat engines only some of the heat that flows will be converted to mechanical energy. Some will be lost as heat to the surrounding area What is thermal pollution? Thermal Pollution: Unwanted waste heat expelled into the environment. What is a heat reservoir? A heat reservoir is an area where energy is stored in a heat engine Heat Sink: Place where heat travels to. Part II (continued) What does the 2nd Law of thermo say about conversion of work to mechanical energy? How is Ideal Efficiency of a heat engine calculated? When work is done by a heat engine only some of the input heat can be converted into work, the rest is expelled as waste heat. What temperature scale is used in this calculation? Kelvin scale is used to calculate efficiency How can efficiency of a heat engine be increased? Natural systems tend to proceed toward a state of greater disorder. Entropy will increase. Work input decreases the entropy in a system What is entropy? Entropy: A measure of disorder in a system Part III What does the 2nd Law of thermodynamics tell us about entropy? Natural systems tend to proceed toward a state of greater disorder Entropy will increase. Work input decreases the entropy in a system Distinguish between the first and second laws in terms of whether or not exceptions occur. Summary The first law has no exceptions. The second law, because it is dealing with probability, may have exceptions. For example, it is possible, though not probable for entropy to decrease through random chance. Also, energy can be made to flow from cold to hot if external work is done on it.