Second Law of Thermodynamics Prof. Marlon Flores Sacedon Department of Mathematics and Physics College of Arts and Sciences Visayas State University, Visca Baybay City, Leyte, Phiippines

The Second Law of Thermodynamics Introduction Many thermodynamic processes proceed naturally in one direction but not opposite. Example: Heat always flows from hotter body to cooler body, never reverse. . Hot Cool Hot Heat flows from cooler body to hotter body not violates the First Law of Thermodynamics, energy would be conserved. But it doesn’t happen in nature. Why not? It is easy to convert mechanical energy completely into heat.

The Second Law of Thermodynamics Converts from mechanical energy to heat energy Yes! possible to happen. By gravity The answer to both of these questions has to do with the directions of thermodynamic processes and is called… No! impossible to happen by gravity Yes! possible to happen by machine engine Converts from heat energy to mechanical energy Inventors have never succeeded in building a machine that converts heat completely into mechanical energy. Again why or why not?

The Second Law of Thermodynamics This law places fundamental limitations on the efficiency of an engine or a power plant. It also places limitations on the minimum energy input needed to operate a refrigerator. The second law is directly relevant for many important practical problems.

Directions of Thermodynamics Processes Thermodynamic processes that occurs in nature are all Irreversible Process These are processes that proceed spontaneously in one direction but not the other. Example of irreversible process. The flow of heat from a hot body to cooler body is irreversible. Sliding book across a table converts mechanical energy into heat by friction. Reversible Process is an idealization that can never be precisely attained in the real world. Example of reversible process. Heat flow between two bodies whose temperature differ only infinitesimal can be reversed by making only a very small change in one temperature or the other.

Disorder There is a relationship between the direction of a process and the disorder or randomness of the resulting state. For example, imagine a thousand names written on file cards and arranged in alphabetical order. Throw the alphabetized stack of cards into the air, and they will likely come down in a random, disordered state.

Heat Engines Heat Engine undergoes cyclic process. Any device that transforms heat partly into work or mechanical energy is called Heat Engine Heat Engine undergoes cyclic process. so All Heat Engines absorbs heat from a source at a relatively high temperature, perform some mechanical work, and discard or reject some heat at a lower temperature.

Experimental evidence suggests strongly that it is impossible to build a heat engine that converts heat completely to work—that is, an engine with 100% thermal efficiency. This impossibility is the basis of one statement of the second law of thermodynamics, as follows: “It is impossible for any system to undergo a process in which it absorbs heat from a reservoir at a single temperature and converts the heat completely into mechanical work, with the system ending in the same state in which it began.”

Problems: 1. A diesel engine performs 2200 J of mechanical work and discards 4300 J of heat each cycle. (a) How much heat must be supplied to the engine in each cycle? (b) What is the thermal efficiency of the engine? 2. An aircraft engine takes in 9000 J of heat and discards 6400 J each cycle. (a) What is the mechanical work output of the engine during one cycle? (b) What is the thermal efficiency of the engine?

Internal-Combustion Engines

Internal-Combustion Engines The FOUR Strokes Intake stroke Compression stroke Power stroke Exhaust stroke

Gasoline Engine/ Otto Cycle

Gasoline Engine/Otto Cycle (Thermal efficiency in Otto cycle)

Problems: 3. A Gasoline/Otto Engine. A gasoline engine takes in 1.61x104 J of heat and delivers 3700 J of work per cycle. The heat is obtained by burning gasoline with a heat of combustion of 4.60x104J/g. (a) What is the thermal efficiency? (b) How much heat is discarded in each cycle? (c) What mass of fuel is burned in each cycle? (d) If the engine goes through 60.0 cycles per second, what is its power output in kilowatts? In horsepower? 4. A gasoline engine has a power output of 180 kW (about 241 hp). Its thermal efficiency is 28.0%. (a) How much heat must be supplied to the engine per second? (b) How much heat is discarded by the engine per second?

Problems: 5. The pV-diagram in Figure shows a cycle of a heat engine that uses 0.250 mol of an ideal gas with 𝛾=1.40. Process ab is adiabatic. (a) Find the pressure of the gas at point a. (b) How much heat enters this gas per cycle, and where does it happen? (c) How much heat leaves this gas in a cycle, and where does it occur? (d) How much work does this engine do in a cycle? (e) What is the thermal efficiency of the engine?

Problems: 1. (a) Calculate the theoretical efficiency for an Otto-cycle engine with 𝛾=1.40 and r = 9.50. (b) If this engine takes in 10,000 J of heat from burning its fuel, how much heat does it discard to the outside air? 2. The Otto-cycle engine in a Mercedes-Benz SLK230 has a compression ratio of 8.8. (a) What is the ideal efficiency of the engine? Use 𝛾=1.40. (b) The engine in a Dodge Viper GT2 has a slightly higher compression ratio of 9.6. How much increase in the ideal efficiency results from this increase in the compression ratio?

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