Second Law of Thermodynamics: The Flow of Thermal Energy and Entropy - Prof. Brian N. Espi, Study notes of Physics

Download Second Law of Thermodynamics: The Flow of Thermal Energy and Entropy - Prof. Brian N. Espi and more Study notes Physics in PDF only on Docsity! Second law of thermodynamics. Looking at energy changing processes in last chapter, quite often the energy is transferred to thermal energy in the end. For example, as a ball is dropped, there is some air resistance which heats up the ball. Think of the space shuttle as an extreme example. Second law of t.d. is the tendency of nonthermal energy to end up as thermal energy. Conservation of energy states that the total energy of all the components is held constant. Unlike other forms of energy, thermal energy is not easily transformed into other forms. “thermal energy can be transformed into other forms with limited efficiency.” Pretend you had some kinetic energy switched to thermal energy. It’s hard to get the kinetic energy back. You won’t be able to get all of it back from the thermal energy. Compare lake to coffee cup Thermal energy also depends on the amount of the material. Take a cool lake and a hot cup of coffee. The coffee has a higher temperature even if the lake has more thermal energy. This is because the lake is so much more massive. However, a single cup of the lake water will have less thermal energy than the cup of coffee. Heat engines Use thermal energy to do work. Convert thermal to kinetic. - Gasoline engine takes TE from burning gas to do work. - Steam engine uses hot steam to do work. heat engine – cyclic device that uses thermal energy to do work. Cyclic devices repeat a set of steps over and over again. Notable feature is that heat engines eject lots of thermal energy. – car gets hot under the hood, also hot tailpipe. This ejected thermal energy is called the exhaust. TE input = work + exhaust energy efficiency = work/thermal energy input Because there is exhaust, efficiency < 100% Energy quality Energy is conserved yet the quality or usefulness is lost. See rock swing from string. Thermal energy is created and that energy cannot be entirely reconverted back to kinetic energy. When we use the Earth’s energy resources, we don’t reduce the total energy of the Earth. Instead we turn the energy from highly useful forms such as the chemical energy of oil to a lesser useful form, usually thermal energy. Entropy • entropy is a quantitative description of the disorganization of a system. • see picture on top of page 141 • 1 kg of water has more entropy than 1kg of ice because in the water the molecules are not organized into a regular crystal lattice. • 2nd Law of TD stated as entropy. The total entropy of a system in any physical process cannot decrease, but it can increase. (also can stay the same, but need to be very careful to do so.) Energy Use Cars are definitely an important technology. Also a huge consumer of energy. See graphs on page 144. internal combustion engine – burn a fuel-air mixture inside the cylinder of an engine external combustion – occurs in fuel that is burned. This provides thermal energy to a second substance that does the work. example: steam engine See making estimates on page 144 See flow chart of energy for a car. Overall energy efficiency of entire vehicle is about 14%. electric vehicles – powered entirely by batteries. no tailpipe, no emissions, no pollution short range , heavy batteries, $$$ gas-electric hybrid – small gas engine powers an electric generator. Generator provides charge to battery to drive car. Fuel cell vehicle – fueled by hydrogen, get energy from chemical energy of hydrogen gas. See transportation efficiency tables trains good commuting cars bad