Energy

Energy has different meanings in different contexts. In layperson's terms energy can be defined as the measure of potential to bring changes in a system. In physics parlance, energy refers to capacity of doing work. Energy can occur in various forms: kinetic, potential, electromagnetic, sound, and so on. Energy of a moving car is kinetic energy, where [Page 567]as the energy of water stored in a dam is potential energy. Energy can be converted from one form to another. For example, when water falls from a dam, the potential energy of water gets converted into kinetic energy, which drives turbines to produce electrical energy. When a car crashes into a wall, the kinetic energy of the car is converted into heat and sound energy. When such a conversion occurs, some of the useful energy is lost. As a result, not all energy can be converted to useful work. However, energy can neither be created nor destroyed (First Law of Thermodynamics). In SI units, energy is measured in Joule (J), which is equivalent to the work done when one Newton force is applied to move an object by 1 meter. Maintaining vital cellular activities that are necessary for survival requires a minimum of 4,000 kJ/day; whereas 20,000 kJ/day are required for activities such as bicycle riding, jogging, or construction work.

Energy sources have been broadly categorized as renewable and nonrenewable. Renewable energy refers to the energy that can not be depleted either due to its short-time frame of regeneration (e.g., biomass, ethanol from corn) or a source itself is inexhaustible for a considerable period of time running into millions of years. Traditionally, energy from sources such as solar, wind, geothermal, and biomass are considered to be renewable. Nonrenewable energy, on the other hand, can be depleted faster than it is regenerated, which usually occurs over a geologic time frame, that is, millions of years. Examples of nonrenewable energy sources are coal, oil, and natural gas.

Energy is very critical for industry, economy, and ecosystems since without energy, none of these can function and would not have existed today. The primary energy driving the earth system is solar energy, with tidal (lunar) and crustal energies being the next two most prominent sources. One example is the hydrological cycle. Solar energy heats up the oceans evaporating water into the atmosphere. Water vapors rise up due to lower density and eventually cool down to form clouds. Precipitation in the form of ice, snow and rain occurs from the clouds, which feed rivers, lakes, and groundwater, providing much-needed fresh water for humans and other living organisms. In ecosystems, primary producers (plants, green algae, diatoms, etc.) capture solar energy through photosynthesis and store it in carbohydrates, ATP, and acetate in the form of chemical energy. This stored chemical energy meets the energy demand of all other species higher up in the food chain or at higher trophic levels, including detritus. In addition, producers also release oxygen, an important component of cellular respiration that provides energy for all life functions of living organisms including movement, growth, and reproduction. Detritus plays an important role in an ecosystem by breaking down the dead plants and animals and releasing nutrients back into the ecosystems. In doing so, detritus derives energy from dead plants and animals. These nutrients, in turn, support the growth of primary producers. Industry derives materials and energy from the ecosystems that fuel growth and economic development. Fuels such as coal, oil, and natural gases, which are predominantly used by economy, are derived from energy stored in dead plants and animals through a series of transformations over a period of millions of years. Many of valuable materials we derive from ecosystems such as medicines, timber, foods, and biofuels are all products of biochemical pathways involving solar energy.

...