Second law of thermodynamics

To explain the irreversible thermodynamic processes, the scientists formulated the second law of thermodynamics. The second law of thermodynamics explains what processes can occur in the universe and what processes cannot happen. One scientist named R. J. E. Clausius (1822-1888) made the following statement:

Naturally heat moves from high-temperature objects to low-temperature objects; naturally, heat does not proceed from low-temperature object to high-temperature object (Second law of thermodynamics – Clausius’s statement).

Clausius’s statement is one of the special statements of the second law of thermodynamics. It is called special statement because it only applies to one process just, related to heat transfer. Since this statement is not related to other processes, we need a more general statement. The development of a general statement of the second law of thermodynamics is based on the study about heat engine. Therefore we discuss heat engine first.

Heat Engine

Much of the energy we use comes from chemical potential energy contained in petroleum, gas, coal. Chemical potential energy applied to be directly utilized must be burned first. Usually burning of fossil fuels (oil, gas, and coal) produces heat. Heat can be used directly to cook food, heating room. To move something (such as moving a vehicle), we must convert heat into kinetic energy or mechanical energy (mechanical energy = potential energy + kinetic energy).

A tool that uses heat to do work was discovered in 1700. It was a steam engine. The steam engine was first used to pump water out of a coal mine.

Use of steam engines is based on the fact that steam can move things. Steam engines include heat engine (a heat engine is a tool for converting heat to mechanical energy). Now the steam engine is used to generate electrical energy. Modern heat engines are internal combustion engines such as car engines, motorcycle engines, etc.

Second law of thermodynamics 1The basic idea behind the use of heat engines is heated can be converted into mechanical energy if heat is allowed to flow from high temperatures to low temperatures. During this process, some of the heat is converted into mechanical energy (some of the heat is used to do the work), some of the heat is discharged in low-temperature places. The process of changing the shape of energy and energy transfer in the heat engine looks like this diagram.

High temperature (TH) and low temperature (TL) are called machine operating temperature. QH is the heat flowing from the high temperature, whereas QL is heat flowing to the low-temperature place. When flowing from high temperatures to low temperatures, some heat is converted into mechanical energy (used to work), some of the heat is disposed of as QL. All heat cannot be transformed into work (W), there is always heat that released (QL). Thus, based on conservation of energy, QH = W + QL.

There are several heat engines, including steam engines and internal combustion engines.

Steam Engine

Steam engines use water vapor as a heat transfer medium. Steam is working fluid. There are two types of steam engines: alternating steam engine and turbine steam engine. Design of this engine is different but these two types of steam engines use steam that is heated by burning oil, gas, coal or using nuclear energy.

Internal Combustion Engines

Motorcycle engines and car engines are examples of internal combustion engines. Called internal combustion engine because combustion process occurs inside closed cylinders. The presence of an internal combustion engine is the result of the engineering concept of adiabatic compression and expansion.

Heat Engine Efficiency

The efficiency of the heat engine (e) is a comparison between Work (W) performed by machine with a Heat input at high temperature (QH).

Second law of thermodynamics 2

W is the gain received, while QH is the cost incurred to buy and burn fuel. As human beings who always want to gain the maximum profit and the smallest expenditure, we hope that the profit increased (W) is proportional to the cost we spend (QH). Could it happen?

Based on conservation of energy, heat (QH) must be equal to Work (W) + Discharged heat (QL).

Second law of thermodynamics 3Substitute W in equation 1 with W in equation 2

Second law of thermodynamics 4
This is equations of heat engine efficiency.

Question 1 :

A heat engine absorbs 3000 Joule (QH) heat, does work (W) and removes 2500 Joule (QL) heat. Calculate heat engine efficiency.


Second law of thermodynamics 5

Heat engine efficiency = 17 %.

Question 2 :

A heat engine absorbs 3000 Joule heat (QH), does work (W) and removes 2000 Joule heat (QL). Calculate the efficiency of the heat engine.


Second law of thermodynamics 7

Heat engine efficiency = 34 %.

Question 3 :

A heat engine absorbs 3000 Joule heat (QH), does work (W) and throws as much as 1500 Joules of heat (QL). Calculate the efficiency of a heat engine?


Second law of thermodynamics 8

Heat engine efficiency = 50 %.


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