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This article is about the scalar physical quantity. For other uses, see
is the electric breakdown of air by strong electric fields and is a flow of energy.
The electric potential energy in the atmosphere changes into thermal energy, light, and sound,
which are other forms of energy.
, "activity, operation", from
, "active, working"
) is a quantity that can be assigned to every particle, object, and
system of objects as a consequence of the state of that particle, object or system of objects.
Different forms of energy include
energy. The forms of energy are often named after a related force.
Hermann von Helmholtz
established that all forms of energy are equivalent
- energy in one form can disappear but the same amount of energy will appear in another
Energy is subject to a
. Energy is a
physical quantity. In the
International System of Units
(SI), energy is measured in
, but in some fields other
units such as
are also used.
Any form of energy can be
into another form. When energy is in a form other
than thermal energy, it may be transformed with good or even perfect efficiency, to any other
type of energy. With thermal energy, however, there are often limits to the efficiency of the
conversion to other forms of energy, due to the
second law of thermodynamics
. As an
is reacted with oxygen, potential energy is released, since new
are formed in the products which are more powerful than those in the oil and oxygen. The
released energy resulting from this process may be converted directly to electricity (as in a
fuel cell) with good efficiency. Alternately it may be converted into thermal energy, if the oil
is simply burned in order to heat the combustion gases to a certain temperature. In the latter
case, however, some of the thermal energy can no longer be used to perform work at that
temperature, and is said to be "degraded." As such, it exists in a form unavailable for further
transformation. The remainder of the thermal energy may be used to produce any other type
of energy, such as electricity.
In all such
processes, the total energy remains the same. Energy may
not be created nor destroyed. This principle, the
conservation of energy
, was first postulated
in the early 19th century, and applies to any
. According to