Energy Cannot Be Created or Destroyed
In 1865, the German physicist Rudolf Clausius investigated energy use by steam engines. The steam engine was a key driving force behind the Industrial Revolution because it was the first practical, affordable, and reliable machine that could convert the chemical energy in fossil fuels to mechanical energy.
In addition to mechanical work, the engine also generates waste heat, energy released from the boiler and other engine parts in the form of heat. The term "waste" refers to the fact that some of the chemical potential energy in the fuel is not converted to useful work, but instead is released to the general surroundings of engine.
Clausius' experiments demonstrated two important laws of energy use. We now know that these laws of thermodynamics (from the Greek therme, meaning "heat" and dynamis, meaning "power") apply to every type of energy conversion. The first law of thermodynamics states that there is no increase or decrease in the quantity of energy in any energy conversion. The total energy input to an energy converter and the total energy output always are equal. In the case of the steam engine, the quantity of chemical potential energy in the fuel is equal to the sum of the mechanical energy and waste heat produced by the engine. This law applies to all energy conversions. For example, the amount of chemical potential energy in the food energy you consume is equal to the sum of the work done by your body (walking, thinking, etc.) plus the heat generated. Similarly, the amount of chemical potential energy in gasoline is equal to the sum of the work done by a car engine, its gadgets (e.g., air conditioner and radio), plus the waste heat generated.
The first law of thermodynamics means that the quantity of energy remains constant in every conversion process. But Clausius' experiments with steam engines revealed another fundamental law of energy conversion: the quality of energy also changes. The second law of thermodynamics states that in all energy conversion processes energy loses its ability to do work and is degraded in quality. The energy that the steam engine converts from chemical potential energy to waste heat has lost its ability to do useful work; it is degraded in quality. The chemical potential energy in these fuels is converted to converted to a low quality heat, so their ability to do work literally is "used up" when they are burned in power plants and cars.
The second laws also tells us that there is a direction to real world processes, namely, things move from a highly ordered state to a disordered state. This is the principle of entropy. Entropy is the degree of order or organization in a system. Clausius’ work applied the concept to energy, but since then scientists have found that the same principle also applies to all changes in materials. Matter or energy that are highly organized or highly ordered have low entropy. Matter or energy that are highly disorganized or random have high entropy.
The entropy laws describes the tendency for all objects to rust, break, fall apart, wear out, and otherwise move to a less ordered stateThe entropy laws describes the tendency for all objects to rust, break, fall apart, wear out, and otherwise move to a less ordered state The second law of thermodynamics means that orderly structures, patterns, and arrangements of energy and materials tend to drift towards disorder by themselves. This movement towards a greater state of entropy occurs without outside interference. Thus, the tendency for energy and materials to move from an ordered, low entropy state to a disordered, high entropy state is a spontaneous process. An automobile engine spontaneously converts the low entropy, chemical potential energy of gasoline to forward motion and waste heat. Similarly, the low entropy, chemical potential energy of wood is spontaneously converted to heat in your fireplace.
The entropy law and the notion of spontaneous changes also applies to materials. If you place a single drop of ink in a glass of water, the ink will disperse spontaneously throughout the water. Eventually, the ink will be evenly mixed and the water will take on a dark hue. If you leave a bicycle outside, it spontaneously crumbles into a pile of rust as the molecules of iron begin to flake off, fall to the ground, or are dispersed by the wind. Junkyards and landfills are testaments to the spontaneous tendency towards disorder described by the second law. They are full of materials that have broken down or worn out due to the forces of entropy.
Disordered, high entropy energy and materials do not organize back to a low entropy, ordered state by themselves; human coaxing or some other outside intervention is required to achieve this. The movement towards a greater state of organization is called a nonspontaneous process. We know that the heat generated by the fireplace will not spontaneously reorganize itself into wood. The atoms of ink will not reassemble themselves into a single drop, nor will a pile of rust re-assemble itself back into a new bicycle. However, you could vacuum up the rust particles and transport them back to a factory where they could be melted, refined, and ultimately fashioned into a new bicycle. That process is nonspontaneous because it would not occur without a significant investment of time and energy.
The laws of thermodynamics are a cornerstone of modern science and engineering. They are essential to the design of nearly every industrial process and device, they inform us about the structure and function of ecosystems, the direction of chemical reactions, the evolution of life, and they underpin our understanding of how energy from the sun and heat from the Earth's core drive the planet's biogeochemical cycles. The laws of thermodynamics have also been applied to economic systems where they set broad but immutable constraints on human economic aspirations.
This article draws on material adapted from Chapter 2 of: Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Debuke, IA).