A simplified version of the Second Law of Thermodynamics is that, whenever events occur, the amount of entropy increases. Entropy can best be understood as disorder, or, as one of the founders of thermodynamics, chemist J. Willard Gibbs, described it, "mixedupness." The natural tendency is for orderliness to decay into disorder. This occurs because, for any system, there are far more possible disordered states than there are ordered states. The process of diffusion allows the Second Law to produce many of its effects. Diffusion occurs from the individual movements of atoms or molecules toward a less ordered, or more uniform, arrangement.

In most events with which humans are familiar, both the First and Second Laws operate. In nearly every event, energy changes from one form to another and entropy increases. When no energy input occurs from outside a system, events tend to occur in one direction only: They continue until equilibrium is reached in which energy is uniform throughout the system (First Law), and maximum disorder has been reached (Second Law). Consider the following examples:

Diffusion of heat. Heat diffuses from regions of higher temperature to regions of lower temperature. Warm molecules move faster (have more kinetic energy) than cold molecules and can transfer their energy to the cold molecules by colliding into them. As a result, heat energy diffuses from regions of high temperature to regions of low temperature. Diffusion of heat is also called conduction. Convection occurs when a mass parallel movement of molecules, such as those in the air, carry heat from a warm region to a cool region. If conduction and convection go to completion, an equilibrium of lukewarm molecules will result. This is what happens when a cup of hot coffee, or a recently dead mammal, cools off to environmental temperature. (The room actually becomes slightly warmer from the heat lost by the coffee cup.) The temperature of an object can increase when heat is conducted to it from another source that is warmer. Energy must be expended, for instance by a refrigerator, to make a relatively cool place even cooler; the coils in the back of the refrigerator disperse the heat, from the space inside the refrigerator and from the machinery, into the air. The First Law indicates that the total amount of energy is unchanged, and the Second Law indicates that the energy has reached a maximum state of disorder: The energy is no longer concentrated in any one location.

Movement of air. Air moves from regions of high pressure to regions of low pressure. Gas molecules in air that has high pressure (high potential energy) flow toward regions of air that have lower pressure, producing wind. Because wind involves the parallel movement of many gas molecules, it is not an example of diffusion. Air movement continues until an equilibrium is reached in which all regions have equal pressure. Since warm air has a lower pressure than cool air, temperature differences can cause air to move. Energy must be expended, by a fan or a pump, to force air to move in the absence of pressure differences. Other fluids, such as water, also move from regions of high to regions of low pressure. The First Law indicates that the total amount of energy has remained unchanged, even though it changed from potential to kinetic forms; and the Second Law indicates that pressure has reached maximum uniformity, when equilibrium is reached. . .

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