Abstract
This essay briefly describes the meanings of the first law and the second law of thermodynamics. Thermodynamics, as the name illustrates, is the field of science where the dynamics of quantities including but not limited to heat and temperature and their relationship with energy, work, entropy etc are dealt with. The first law of thermodynamics is a version of the principle of conservation of energy. It establishes a theoretical limit on how much work can be output when a certain amount of energy is available and vice versa. The second law of thermodynamics is a bit more subtle and it deals with the practical limit of the magnitude of work that can be actually extracted given a certain amount of energy as input in real-world boundary conditions and vice versa. The second law also has many other corollaries and implications. High quality fuel sources such as fossil fuels can only be used and cannot be recycled as their content of useful energy is irreversibly depleted for all practical purposes.
Keywords: Thermodynamics, First law, Second law, Entropy, Heat, Work, Energy
The scientific field that deals with heat and temperature and their relationship with energy and work is named thermodynamics. According to the theory of thermodynamics, the behavior of the aforementioned physical quantities is governed by the four fundamental laws of thermodynamics. In this essay, the definitions of the first and second laws will be explained and their meanings will be explored.
In its 19th century definition by Clausius, the first law asserts that, in all cases where work is produced by the action of heat, the measure of heat expended is proportional to the volume of work done; and conversely, the quantity of heat generated equals the magnitude of work that is applied (Truesdell, 1980). The first law is an adaptation of the law of conservation of energy that is applied to thermodynamic systems. In the context of a closed system, the first law postulates that the change in the system’s internal energy is the exactly equal to the difference between the amount of heat supplied to it and the quantity of work done by it on its surroundings. Machines that violate the first law are called perpetual motion machines of the first kind. Such a machine would output work without the input of energy and they are impossible.
The second law of thermodynamics has numerous definitions; however the statement by Planck is most relevant to this essay as it deals with irreversibility and states that “Every process occurring in nature proceeds in the sense in which the sum of the entropies of all bodies taking part in the process is increased. In the limit, i.e. for reversible processes, the sum of the entropies remains unchanged.” (Uffink, 2003) While the first law explains how energy can be neither created nor destroyed but instead only converted from one form to another, the second law elucidates how much of the total available energy is actually useful and can be converted to work. Energy always moves from higher concentrations to lower concentrations. Every time this transfer of energy occurs, it becomes less useful. The amount of useful energy in a system is also determined by its surroundings. As an example, the maximum amount of useful heat/work that can be extracted from a system is directly proportional to the difference in their temperatures. A machine that violates the second law and is able to spontaneously convert all of the thermal energy input into mechanical work output with no other side effects is termed perpetual motion machine of the first kind, and is impossible.
According to the second law, entropy of an isolated system increases, except when it undergoes a reversible process where it stays unchanged. A reversible process is only a theoretical concept wherein the process takes place very slowly in infinitesimally small steps while maintaining constant entropy; but it does not occur naturally in the real world. Burning fuel, to obtain useful energy to perform work, is an irreversible process. In this process, the energy contained in the fuel is transformed into other forms of energy. However, because of losses and heat rejection to the surroundings, the amount of useful energy obtained is lesser than the total amount of energy available in the fuel to begin with. Entropy is increased in this process, making the energy available at the end of the process less useful in generating further work. This is the reason why, fossil fuels such as oil can only be used once as an energy source. This is also why other energy sources of high quality are not recyclable, as the useful energy in them degrades with every use.
References
Truesdell, C. (1980). The tragicomical history of thermodynamics, 1822-1854 (pp. 188-189). New York: Springer-Verlag.
Uffink, J. (2003). Bluff Your Way in the Second Law of Thermodynamics. Studies In History And Philosophy Of Science Part B: Studies In History And Philosophy Of Modern Physics, 32, 131.
