energy flow

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Energy flow


Understanding energy flow is vital to many environmental issues. One can describe the way ecosystems function by saying that matter cycles and energy flows. This is based on the laws of conservation of matter and energy and the second law of thermodynamics, or the law of energy degradation.

Energy flow is strictly one way, such as from higher to lower or from hotter to colder. Objects cool only by loss of heat. All cooling units, such as refrigerators and air conditioners, are based on this principle: they are essentially heat pumps, absorbing heat in one place and expelling it to another.

This heat flow is explained by the laws of radiation, as seen in fire and the color wheel. All objects emit radiation, or heat loss, but the hotter the object the greater the amount of radiation, and the shorter and more energetic the wavelength. As energy intensities rise and wavelengths shorten, the radiation changes from infrared to red, then orange, yellow, green, blue, violet, and ultraviolet. A blue flame, for example, is desired for gas appliances. A well-developed wood fire is normally yellow, but as the fire dies out and cools, the color gradually changes to orange, then red, then black. Black coals may still be very hot, giving off invisible, infrared radiation. These varying wavelengths are the main differences seen in the electromagnetic spectrum.

All chemical reactions and radioactivity emit heat as a by-product. Because this heat radiates out from the source, the basis of the second law of thermodynamics, one can never achieve 100% energy efficiency . There will always be a heat-loss tax. One can slow down the rate of heat loss through insulating devices, but never stop it. As the insulators absorb heat, their temperatures rise and they in turn lose heat.

There are three main applications of energy flow to environmental concerns. First, only 10% of the food passed on up the food chain/web is retained as body mass; 90% flows to the atmosphere as heat. In terms of caloric efficiency, more calories are obtained by eating plant food than meat. Since fats are more likely to be part of the 10% retained as body mass, pesticides dissolved in fat are subject to bioaccumulation and biomagnification . This explains the high levels of DDT in birds of prey like the peregrine falcon (Falco peregrinus ) and the brown pelican (Pelecanus occidentalis ).

Second, the%age of waste heat is an indicator of energy efficiency. In light bulbs, 5% produces light and 95% heat, just the opposite of the highly efficient fire fly. Electrical generation from fossil fuels or nuclear power produces vast amounts of waste heat.

Third, control of heat flow is a key to comfortable indoor air and solving global warming. Well-insulated buildings retard heat flow, reducing energy use. Atmospheric greenhouse gases , such as anthropogenic carbon dioxide and methane , retard heat flow to space, which theoretically should cause global temperatures to rise. Policies that reduce these greenhouse gases allow a more natural flow of heat back to space.

See also Greenhouse effect

[Nathan H. Meleen ]


RESOURCES

PERIODICALS

"Energy." National Geographic 159 (February 1981): 2-23.

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energy flow (in ecology) The flow of energy that occurs along a food chain. Energy enters the food chain at the level of the producers (usually plants) in the form of solar energy. The plants convert solar energy into chemical energy in the process of photosynthesis. Chemical energy is passed from one trophic level to the next through feeding. Since a large proportion of energy is lost at each trophic level, mostly in the form of heat energy due to respiration, a food chain does not normally consist of more than five trophic levels: the fifth trophic level does not contain enough energy to support further levels. Energy is also lost from the food chain in excretory products and the remains of dead organisms; this is converted into heat energy by the action of decomposers. See also productivity; pyramid of energy.

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energy flow The exchange and dissipation of energy along the food-chains and food-webs of an ecosystem.

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energy flow The exchange and dissipation of energy along the food-chains and food-webs of an ecosystem.