Por klarigi la nemaligeblajn termodinamikajn procezojn, la sciencistoj formulis la duan leĝon de termodinamiko. La dua leĝo de termodinamiko klarigas, kiaj procezoj povas okazi en la universo kaj kiaj procezoj ne povas okazi. Unu sciencisto nomita RJE Clausius (1822-1888) faris la jenan deklaron:
Kompreneble, varmo moviĝas de alt-temperaturaj objektoj al malalt-temperaturaj objektoj; nature, varmo ne iras de malalt-temperaturaj objektoj al alt-temperaturaj objektoj (Dua leĝo de termodinamiko - la aserto de Clausius).
La aserto de Clausius estas unu el la specialaj asertoj de la dua leĝo de termodinamiko. Ĝi nomiĝas speciala aserto ĉar ĝi validas nur por unu procezo, rilata al varmotransigo. Ĉar ĉi tiu aserto ne rilatas al aliaj procezoj, ni bezonas pli ĝeneralan aserton. La disvolviĝo de ĝenerala aserto de la dua leĝo de termodinamiko baziĝas sur la studo de varmomotoroj. Tial, ni unue diskutas motorvarmon.
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 since steam can move things. Steam engines include a 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.
The 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 heat is converted into mechanical energy (some heat is used to do the work), some 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 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.
Vapormaŝino
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. The 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.
Internaj Brulaj Motoroj
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 the machine with a Heat input at high temperature (QH).
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).
Substitute W in equation 1 with W in equation 2
This is equations of heat engine efficiency.
Demando 1:
A heat engine absorbs 3000 Joule (QH) heat, does work (W) and removes 2500 Joule (QL) heat. Calculate heat engine efficiency.
solvaĵo
Heat engine efficiency = 17%.
Demando 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.
solvaĵo
Heat engine efficiency = 34%.
Demando 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?
solvaĵo
Heat engine efficiency = 50%.
