Energy Budgets

Energy is defined as the ability, or potential ability, of a body or system to do work. Energy can be measured in various units, such as the calorie, defined as the amount of energy required to raise the temperature of one gram of pure water from 59–61°F (15–16°C). (Note that the dietician's calorie is equivalent to one thousand of these calories, or one kilocalorie.) The Joule is another unit of energy, defined as the amount of work required to lift a weight of 1 kg by 10 cm, and equivalent to 0.24 calories.

Energy can exist in various states, all of which are interchangeable through various sorts of physical/chemical transformations. The basic categories of energy are: electromagnetic, kinetic, and potential, but each of these can also exist in various states, as is described below:

1. Electromagnetic energy is the energy of photons, or quanta of energy that have properties of both particles and waves, and that travel through space at a constant speed of 3 × 10⁸ meters per second (that is, at the speed of light). The components of electromagnetic energy are characterized on the basis of wavelength ranges, which ordered from the shortest to longest wavelengths are known as: gamma, x ray, ultraviolet, light or visible, infrared, and radio. All bodies with a temperature greater than absolute zero (that is, -459°F [-273°C], or zero degrees on the kelvin scale) emit electromagnetic energy at a rate and spectral quality that is strictly determined by their surface temperature. Relatively hot bodies have much larger emission rates and their radiation is dominated by shorter wavelengths, compared with cooler bodies. The Sun has a surface temperature of about 11,000°F (6,093°C) and most of its radiation is in the wavelength range of visible light (0.4-0.7 æm or micrometers) and shorter-wave infrared (0.7-2 æm), while Earth has a surface temperature of about 77°F (25°C) and its radiation peaks in the longer-wave infrared range at about 10 æm.
2. Kinetic energy is the energy of dynamic motion, of which there are two basic types, the energy of moving bodies, and that of vibrating atoms or molecules. The later is also known as thermal energy, and the more vigorous the vibration, the greater the heat content.
3. Potential energy has the capacity to do work, but it must be mobilized to do so. Potential energy occurs in various forms, including the following: (a) Chemical potential energy is stored in the inter-atomic bonds of molecules. This energy can be liberated by so-called exothermic reactions, which have a net release of energy. For example, heat is released when the chemically reduced sulfur of sulfide minerals is oxidized to sulfate, and when crystalline sodium chloride is dissolved into water. All biochemicals also store potential energy, equivalent to 4.6 kilocalories per gram of carbohydrate, 4.8 Kcal/g of protein, and 6.0-9.0 Kcal/g of fat. (b) Gravitational potential energy is stored in mass that is elevated above some gravitationally attractive surface, as when water occurs above the surface of the oceans, or any object occurs above the ground surface. Unless obstructed, water spontaneously flows downhill, and objects fall downwards in response to gradients of gravitational potential energy. (c) Other types of potential energy are somewhat less important in terms of ecological energy budgets, but they include potential energies of compressed gases, electrical potential gradients associated with voltage differentials, and the potential energy of matter, which can be released by nuclear reactions.