Chapter 4The Second Law of Thermodynamics

Back GroundThe first law state about the conservation of energy, but in itself it does not give a complete description of how thing behave.For example; hot coffee will transfer the heat to its surrounding, but cold coffee in surrounding will not be hot spontaneously.Other example If a box is slid across the floor against friction, mechanical energy is converted into heat (first law of thermodynamics), but one never observes the reverse process where a box spontaneously decide to start sliding while the floor cools down.To sump up in the nature the process of conservation energy in first law is done in certain direction and need certain condition. And it is stated obviously by The Second Law of Thermodynamics

Kelvin-Plank Statement and Heat EngineThe second law of thermodynamics can be stated by Kelvin-Plank and Clausius statements. Kelvin-Plank statement of the second law of thermodynamics It is impossible to construct a heat engine that, operating in a cycle, produces no effect other than the absorption of energy from a reservoir and the performance of an equal amount of work.Heat engine is a device that transform heat partly into work. An engine utilizes a working substances (for example, gasoline and air in the car engine and water in steam engine.Most engine use a cyclic process in which the working substance return to the same state at periodic interval.

Heat Engine W= QH- QCQH = QC + WEfficiency is the term that show the comparison ofThe work (W) and the Heat in (Qin)., So

ProblemEach cycle an engine gains 120 J of heat from fuel and does 30 J of work. What is the efficiency of the engine?

Reversible and Irreversible ProcessIn a reversible process, the system undergoing the process can be returned to its initial conditions along the same path shown on a PV diagram, and every point along this path is an equilibrium state.A process that does not satisfy these requirements is irreversibleAll natural processes are known to be irreversible.

Carnot EngineCarnot engine is heat engine that all of process designed in reversible condition.Designed by French physicist Sadi Carnot, it is the most efficient engine.It is an ideal engine and just a theoretical view, In practice it used to measure efficient of engine.

A-B isothermal expansionB-C adiabatic expansionC-D isothermal compressionD-A adiabatic compression Carnot engine efficiency

ExampleA steam engine has a boiler that operates at 500 K. The energy from the burning fuel changes water to steam, and this steam then drives a piston. The cold reservoirs temperature is that of the outside air, approximately 300 K. What is the maximum thermal efficiency of this steam engine?

Example of Heat Engine CycleOtto Cycle for gasoline Engine

Diesel Cycle

Clasius Statement and Heat PumpIt is impossible to construct a cyclical machine whose sole effect is the continuous transfer of energy from one object to another object at a higher temperature without the input of energy by work.In simpler terms, energy does not flow spontaneously from a cold object to a hot object.Heat pump or refrigerator is the system that used to transfer heat from a cold to a hot object.

Heat PumpRefrigerator System

The effectiveness of a heat pump is described in terms of a number called the coefficient of performance (COP). In the heating mode, the COP is defined as the ratio of the energy transferred to the hot reservoir to the work required to transfer that energy:

EntropyEntropy is the common statement of the second law of thermodynamic.entropy is a macroscopic scale as it was first expressed by Clausius in 1865.

EntropyStatement of entropyEntropy is a measure of a system's ability to do useful work. As a system loses the ability to do work, its entropy increases.Entropy is a measure of disorder. A system naturally moves toward a state of greater disorder. The more order, the less entropy.

EntropyEntropy is related to temperature and heatThe change in entropy, when an amount of heat is added to (or removed from) a system by a reversible process at a constant temperature is given by

where is the temperature in kelvins.The units of entropy are joules per kelvin: J/K.Total change entropy for a cycle

For adiabatic S = 0For isothermal process

For isochoric process

In general, the direction of any process is toward an increase in entropy:the entropy of an isolated system never decreases or the entropy of an isolated system increases for every natural process .The entropy of a system is a function of its state: each state of a system has a particular value of entropy, and a change in entropy depends only on the initial and final states for a process (analogous to the change in internal energy for an ideal gas).In this context, an unnatural process is one which has never been observed (water at room temperature turning naturally into ice), and a natural process is one which is observed to occur.

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