"There is a fact, or if you wish, a law, governing all natural phenomena that
are known to date.
There is no known exception to this law--it is exact so far as we know. The
law is called the conservation of energy.
It states that there is a certain quantity, which we call energy that does not
change in manifold changes which nature undergoes.
That is a most abstract idea, because it is a mathematical principle; it says
that there is a numerical quantity which does not change when something
It is not a description of a mechanism, or anything concrete; it is just a
strange fact that we can calculate some number and when we finish
watching nature go through her tricks and calculate the number again, it is
The Feynman Lectures on Physics
What is energy?
A brief history of energy
Types of energy
Measurement of energy
Uses of energy
"Energy is the capacity of a physical system to perform work. Energy
exists in several forms such as heat, kinetic or mechanical energy,
light, potential energy, electrical, or other forms.
According to the law of conservation of energy, the total energy of a
system remains constant, though energy may transform into another
form. Two billiard balls colliding, for example, may come to rest, with
the resulting energy becoming sound and perhaps a bit of heat at the
point of collision."
The SI unit of energy is the joule (J) or newton-meter (N * m).
The joule is also the SI unit of work."
"The word energy derives from the Greek energeia, which possibly appears for the first time in the work of
Aristotle in the 4th century BCE. Thomas Young - the first to use the term "energy" in the modern sense.
The concept of energy emerged out of the idea of vis viva (living force), which Gottfried Leibniz defined as the
product of the mass of an object and its velocity squared; he believed that total vis viva was conserved.
To account for slowing due to friction, Leibniz theorized that thermal energy consisted of the random motion of the
constituent parts of matter, a view shared by Isaac Newton, although it would be more than a century until this was
generally accepted. In 1807, Thomas Young was possibly the first to use the term "energy" instead of vis viva, in its
Gustave-Gaspard Coriolis described "kinetic energy" in 1829 in its modern sense, and in 1853, William Rankine coined
the term "potential energy". It was argued for some years whether energy was a substance (the caloric) or merely a
physical quantity, such as momentum.
William Thomson (Lord Kelvin) amalgamated all of these laws into the laws of thermodynamics, which aided in the
rapid development of explanations of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, and Walther
Nernst. It also led to a mathematical formulation of the concept of entropy by Clausius and to the introduction of laws
of radiant energy by Joef Stefan.
Since 1918 it has been known that the law of conservation of energy is the direct mathematical consequence of the
translational symmetry of the quantity conjugate to energy, namely time. That is, energy is conserved because the
laws of physics do not distinguish between different instants of time (see Noether's theorem)."
There are several different types of energy and these are the main ones that should be understood:
Mechanical: "the energy that is possessed by an object due to its motion or due to its position.
Mechanical energy can be either kinetic energy (energy of motion) or potential energy (stored energy
of position). Objects have mechanical energy if they are in motion and/or if they are at some position
relative to a zero potential energy position (for example, a brick held at a vertical position above the
ground or zero height position). A moving car possesses mechanical energy due to its motion (kinetic
energy)." Physics Classroom
Electric: " the presence and flow of an electric charge. The energy portion of electricity is found in a
variety of phenomena such as static electricity, electromagnetic fields and lightning. Humans have
found the ability to harness these phenomena and store the electrical charge for later use. The
concept of electrical energy is defined using a variety of different terminologies such as charge,
current and potential" Wisegeek
Thermal: "A specialized term that refers to the part of the internal energy of a system which is the
total present kinetic energy resulting from the random movements of atoms and molecules." Thermal
Because energy is defined via work, the SI unit for energy is the same as the unit of work - the joule (J), named in
honour of James Prescott Joule and his experiments on the mechanical equivalent of heat. In slightly more
fundamental terms, 1 joule is equal to 1 newton-metre and, in terms of SI base units:
An energy unit that is used in atomic physics, particle physics and high energy physics is the electronvolt (eV).
One eV is equivalent to 1.60217653x10-19 J. In spectroscopy the unit cm-1 = 0.000123986 eV is used to represent
energy since energy is inversely proportional to wavelength from the equation E = h = hc / .
In discussions of energy production and consumption, the units barrel of oil equivalent and ton of oil equivalent are
Gcal (109) MBtu (106) GJ (109)
MWh (106) toe
Source IEA/Unit Converter
When discussing amounts of energy released in explosions or bolide impact events, the TNT equivalent unit is often
used. 1 ton of TNT equivalent is equal to 4.2 x 109 joules. Therefore, 1 kt TNT is 4.2 x 1012 joules, and 1 Mt TNT is 4.2 x
Note that torque, the "rotational force" or "angular force" which causes a change in rotational motion is typically
expressed in newton-metres. This is not a simple coincidence: a torque of 1 newton-metre applied on 1 radian
requires exactly 1 newton-metre = 1 joule of energy.
There are various energy suppliers who have different clientele
who they sell energy to for different uses.
Energy can be used for:
Heating (such as radiators and under floor heating)
Heat extraction (such as air conditioning)
Residential power sourcing
Energy is the capacity of a physical system to
There are various types of energy and
different ways to measure it.
Depending on what energy is needed for, it
may be used differently
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