Energy



Energy, generally and qualitatively speaking, is the property (or the quantity of the property) of changing the state of a system or doing work. The expressions energy and power have different meaning in different scientific and non-scientific fields. Physics aims to explain quantitatively this property and gives a definition that makes it possible to consider energy as a description of the whole state and the different ways jobs are done are unified in this treatment.

Energy is a fundamental quantity that every physical system possesses; it allows us to predict how much work the system could be made to do, or how much heat it can exchange. In the past, energy was discussed in terms of easily observable effects it has on the properties of objects or changes in state of various systems. Basically, if something changes, some sort of energy was involved in that change. As it was realized that energy could be stored in objects, the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms; examples are the electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a hot water heater, or the kinetic energy of a moving train. To simply say energy is "change or the potential for change", however, misses many important examples of energy as it exists in the physical world.

Energy can be used not only to produce observable change, it also is used to prevent change in which case unaided observation of this kind of energy can be difficult. For example, looking at a statue holding a 50 pound weight, the presence of energy needed to do so may not be observable. However, if you are holding up the fifty pound weight instead of the statue the need for energy to accomplish this becomes apparent. You can feel the gravitational force on you both when you are moving the weight up and when you are not moving it. Energy can be readily transformed from one form into another; for instance, using a battery to power an electrical heater converts electrical energy into thermal energy.

In the previous example of holding the fifty pound weight, the work you perform to raise the weight is observed as kinetic energy of motion which is converted to potential energy and added to the weight's potential energy as you continue to hold the weight up against the pull of gravity. Letting go of the weight once again transforms this stored potential energy back into kinetic energy as the weight falls under the force of gravity. The law of conservation of energy states that the total amount of energy, corresponding to the sum of a system's constituent energy components, remains constant. Scientists have also defined several forms of energy that are not easily measured by the unaided observer.

Other units of energy

In cgs units, one erg is 1 g cm2 s?2, equal to 1.0×10?7 J. Another obsolete metric unit is the litre-atmosphere (101.325 J).

The imperial/US units for both energy and work include the foot-pound (1.3558 J), the British thermal unit (Btu) which has various values in the range of 1055 J, and the horsepower-hour (2.6845 MJ).

The energy unit used for everyday electricity, particularly for utility bills, is the kilowatt-hour (kW h), and one kW h is equivalent to 3.6×106 J (3600 kJ or 3.6 MJ).

The calorie is mainly used in nutrition and equals the amount of heat necessary to raise the temperature of one gram of water by 1 degree Celsius, at a pressure of 1 atm. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units sharing the name of "calorie" but having slightly different energy values. It is approximately equal to 4.186 J.

The calories used for food energy in nutrition are the large calories based on the kilogram rather than the gram, often identified as food calories. These are sometimes called kilocalories with that calorie being the small calorie based on the gram, and as a result the prefixes are generally avoided for the large calories (i.e., 1 kcal is 4.184 kJ, never 4.184 MJ, even if "calories" are also used for the other, larger unit in the same document or the same nutrition label). Food calories are sometimes noted as Calories (1000 calories) or simply abbreviated Cal with the capital C, but that convention is more often found in chemistry or physics textbooks?which do not use these large calories?than it is in real-world applications by those who do use these calories. (This convention is also, of course, useless when the word calorie appears in a location where it would ordinarily be capitalized, as at the beginning of a sentence or in the first column of a nutrition label as a substitute for the quantity being measured, which is energy, when all the other quantities such as "Iron" and "Sugars" are also capitalized.)