Laws Of Thermodynamics

There are four laws of thermodynamics which are developed into basic equations describing property relationships, energy balances, entropy balances, and mass balances. An excellent way of stating the four laws of thermodynamics can be demonstrated by visualizing three bodies, A, B, and C (1). A and C are identical rigid hot bodies at the same temperature and body B is a cold body below the temperature of A and C. In discussing the laws and the applications, especially in food processing, it is useful to visualize the transfer of heat between these bodies.

Zeroth Law

When bodies A and C are placed in contact with each other, no change will occur. This is a relatively recent law, added to the original three laws, to emphasize the fact that when two bodies or systems are at the same state of energy, no useful work can be obtained by putting the two systems in contact with each other. From this, it follows that the zeroth law also states that two systems in thermal equilibrium with a third are in thermal equilibrium with each other.

First Law

If A is placed in contact with B, there will be an increase in the internal energy of B equaling a decrease in the internal energy of A. The first law as originally stated is that the sum of all the energies in an isolated system is constant or that energy may be transformed from one form into another, but it cannot be created or destroyed.

This law is particularly important as applied to food processing in which the basis for operations is the transfer of mass and energy. Food processing involves heating, cooling, changing state (freezing and evaporating), adding chemicals that may cause internal energy changes, and irradiation.

Second Law

After A has been in contact with B, the initial conditions will never be restored without outside influence. The cooling of A and heating of B constitute a spontaneous, irreversible process resulting in loss of ability to do work. Simply stated the second law is that all systems tend to approach a state of equilibrium.

Third Law

It is impossible by any procedure, no matter how idealized, to reduce any system to the absolute zero of temperature in a finite number of operations. As in the case of the zeroth law, the third law has little practical application in determining energy relationships involving food processing. It is primarily applied to the physics of very low temperatures.

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