Energy
Energy transformation; In a typical lightning
strike, 500 mega joules of electric
potential energy is converted into the same amount
of energy in other forms, most notably light energy,
sound energy
and thermal energy.
In physics, energy is one of the basic quantitative properties
describing a physical system or object's state. Energy can be transformed
(converted) among a number of forms
that may each manifest and be measurable in differing ways. The law of conservation of energy states that the (total) energy of a system can increase or
decrease only by transferring it in or out of the system. The total energy of a
system can be calculated by simple addition when it is composed of multiple
non-interacting parts or has multiple distinct forms of energy. Common energy
forms include the kinetic energy of a moving object, the radiant energy
carried by light and other electromagnetic
radiation, and various types of potential energy such as gravitational and elastic.
Energy is measured in SI units of joules (J). Common types of energy transfer and transformation
include processes such as heating a material, performing mechanical work
on an object, generating or making use of electric energy,
and many chemical reactions.
Units of measurement for energy are usually defined via a
work process. The work performed by a given body on another is defined in
physics as the force
(SI unit: newton) applied by the given body, multiplied by the distance
(SI unit: metre) of movement against the opposing force exerted by the
other body. Thus, the energy unit is the newton-metre, which is called the joule.
The SI unit of power (energy per unit time) is the watt, which is simply a joule per
second. Thus, a joule is a watt-second, so 3600 joules equal a watt-hour. The CGS energy unit is the erg, and the imperial and US customary
unit is the foot pound. Other energy units such as the electron volt,
food calorie
or thermodynamic kcal (based on the temperature change of water in a heating
process), and BTU are used in specific areas of science and commerce and have
unit conversion factors relating them to the joule.
Potential energy is energy stored by virtue of the position of an object in
a force field, such as a gravitational, electric
or magnetic field. For example, lifting an object against gravity performs
work on the object and stores gravitational potential energy; if it falls,
gravity does work on the object which transforms the potential energy to
kinetic energy associated with its speed. Some specific forms of energy include
elastic energy due to the stretching or deformation of solid objects, chemical energy
such as is released when a fuel burns, and thermal energy,
the microscopic kinetic and potential energies of the disordered motions of the
particles making up matter.
Not all of the energy in a system can be transformed or
transferred by a work process; the amount that can is called the available energy. In particular the second
law of thermodynamics limits the amount thermal energy
that can be transformed into other forms of energy. Mechanical and other forms
of energy can be transformed in the other direction into thermal energy without
such limitations.
Any object that has mass when stationary (thus called rest mass),
equivalently has rest energy
as can be calculated using Albert Einstein's
equation E = mc2. Being a form of energy, rest energy
can be transformed to or from other forms of energy, while the total
amount of energy does not change. From this perspective, the amount of matter
in the universe contributes to its total energy.
Similarly, all energy manifests as a proportionate
amount of mass. For example, adding 25 kilowatt-hours (90 megajoules) of any
form of energy to an object increases its mass by 1 microgram. If you had a
sensitive enough mass balance or scale,
this mass increase could be measured. Our Sun (or a nuclear bomb) transforms nuclear
potential energy to other forms of energy; its total
mass doesn't decrease due to that in itself (since it still contains the same
total energy even if in different forms), but its mass does decrease when the
energy escapes out to its surroundings, largely as radiant energy.
A new form of energy can't be defined arbitrarily. In order
to be valid, it must be shown to be transformable to or from a predictable
amount of some known form(s) of energy, thus showing how much energy it
represents in the same units used for all other forms. It must obey
conservation of energy, so it must never decrease or increase except via such a
transformation (or transfer). Also, if an alleged new form of energy can be
shown not to change the mass of a system in proportion to its energy,
then it is not a form of energy.
Living organisms require available energy to stay alive; humans get such energy from food along with the oxygen needed to
metabolize it. Civilization requires a supply of energy to function; energy resources such as fossil fuels
are a vital topic in economics and politics. Earth's climate
and ecosystem
are driven by the radiant energy Earth receives from the sun, and are sensitive
to
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