The First Law of Thermodynamics
Joule’s Law or Law of Conservation of Energy
- When a closed system undergoes a complete cycle, net work is done on or by the system. The only other energy involved in the cycle is heat which is supplied or rejected during various processes.
- The first law relates these two forms of energy in the following statement:
“For a closed system undergoing a cycle the net energy interaction in the form of heat is equal to the net energy interaction in the form of work”
Let us consider a closed system which consists of a known mass of water, m, contained in an adiabatic vessel having a
thermometer and a paddle wheel, as shown in figure (a). Let a certain amount of work W1-2 be done upon the system by the paddle wheel.
The quantity of work can be measured by the fall of weight which drives the paddle wheel through a pulley. The system was initially at temperature t1, the same as that of atmosphere, and after work transfer let the temperature rises to t2.
The pressure is always 1 atm. The process 1-2 undergone by the system is shown in figure (b) in generalized thermodynamic coordinates x-y.
Let the insulation now be removed. The system and the surroundings interact by heat transfer till the system returns to the original temperature t1, attaining the condition of thermal equilibrium with the atmosphere. The amount of heat transfer Q2-1 from the system during this process, 2-1 as shown can be estimated from Q2-1 = mcp(t2 – t1). The system thus executes a cycle, which consists of a definite amount of work input W1-2 to the system followed by the transfer of an amount of heat Q2-1 from the system. It has been found that this W1-2 is always proportional to the heat Q2-1, and the constant of proportionality is called the Joule’s equivalent or the mechanical equivalent of heat.
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Must Read: What is thermodynamics in Mechanical Engineering?
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