- Commonly, energy is energy development, the field concerned with providing abundant and
accessible energy to all humans.
Energy is a quantifiable state function of every
physical system. Energy allows one to predict how much work a physical system could be made to do, or how much heat it
can exchange. In general, the presence of energy is detected by an observer or
system any time there is a change in the properties of another
object or system. This is where the early exploration of the nature of energy began. As
our understanding of the nature of energy progressed, scientists found it to exist in many forms not readily observable by the
average unaided observer. Empirical observations have shown that the total quantity of energy is conserved. This makes the concept of energy very important in physics.
Units
The SI unit for both energy and work is the joule
(J), named in honor 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, 1 J is equal to 1 kg m²s−2.
In cgs units, one erg is 1 g cm² s-2, equal to 1.0×10−7 joules. The imperial/US unit for both energy and work is
the foot-pound, and one foot-pound is approximately 1.3558 joules (although the energy foot-pound can be confused with the
imperial/US unit for torque, which is also the foot-pound).
The unit used for energy in the form of electricity, particularly for
utility bills, is the kilowatt-hour (kW h), and one kW h
is equivalent to 3.6×106 J = 3600 kJ.
A unit that is used in particle physics is the electronvolt (eV). One eV is equivalent to
1.602176462×10−19 J.
Transfer of energy
Work
Main article: mechanical work.
Work is a measure of energy expended in applying force over a distance. Performing work requires energy, and thus the
amount of energy in a system limits the maximum amount of work that a system could conceivably perform.
For example, in the one-dimensional case of applying a force
through a distance, the energy E required is given by the integral:
-
where f(x) gives the amount of force being applied as a function of the distance moved, x.
Note, however, that not all energy in a system is stored in a recoverable form: for example, energy may be converted into heat
which cannot then be converted into another useful form of energy. Thus, in practice, the amount of energy in a system available
for performing work may be much less than the total amount of energy in the system.
Heat
Main article: Heat.
Heat is an amount of energy which is usually linked with a change in temperature or in a change in phase of matter.
In chemistry, heat is the amount of energy which is absorbed or released by a given chemical reaction. The relationship between
heat and energy is similar to that between work and energy. Heat flow from areas of high temperature to areas of low temperature.
All objects (matter) have a certain amount of internal energy that is related to the random motion of their atoms or molecules.
This internal energy is directly proportional to the temperature of the object. When two bodies of different temperature come in
to thermal contact, they will exchange internal energy until the temperature is equalized. The amount of energy transferred is
the amount of heat exchanged. It is a common misconception to confuse heat with internal energy, but there is a difference: the
change of the internal energy is the heat that flows from the surroundings into the system plus the work performed by the
surroundings on the system.
Conservation of energy
The first law of thermodynamics says that the total inflow of energy
into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system.
This law is used in all branches of physics. Noether's theorem
relates the conservation of energy to the time invariance of physical laws.
Kinetic energy
Main article: Kinetic energy.
Kinetic energy is that portion of energy associated with the motion
of a body.
-
The equation above says that the kinetic energy (Ek) is equal to the integral of the dot product of the velocity
(v) of a body and the infinitesimal of the body's
momentum (p).
For non-relativistic velocities, we can use the Newtonian approximation
-
where Ek is kinetic energy, m is mass of the body, v is velocity of the body
At near-light velocities, we use the relativistic formula:
-
-
where v is the velocity of the body, m is its rest mass, and c is the speed of light in a
vacuum.
The first term, γmc2, is the total energy of the body, and the second term,
mc2, is again the rest mass energy.
Potential energy
Main article: Potential energy.
Potential energy is energy associated with being able to move to
a lower-energy state, releasing energy in some form. The potential energy can be stored as gravitational energy, elastic energy,
chemical energy, rest mass energy or electrical energy.
For example a mass released above the Earth has energy resulting from the gravitational attraction of the Earth which is transferred in to kinetic energy.
Equation:
-
where m is the mass, h is the height and g is the value of
acceleration due to gravity at the Earth's surface.
Internal energy
Main article: Internal energy.
Internal energy is the kinetic energy associated with the
motion of molecules, and the potential energy associated with the rotational, vibrational, and electric energy of
atoms within molecules. Internal
energy, like energy, is a quantifiable state function of a
system.
Total energy (as a series)
In the form of a Taylor series, the relativistic formula for total
energy can be written:
-
Hence, the third and higher terms in the series correspond with the "inaccuracy" of the Newtonian approximation for kinetic
energy in relation to the relativistic formula.
Examples
An example of the conversion and conservation of energy is a pendulum. At its
highest points the kinetic energy is zero and the potential gravitational energy is at its maximum. At its lowest point the
kinetic energy is at its maximum and is equal to the decrease of potential energy. If one unrealistically assumes that there is
no friction, the energy will be conserved and the pendulum will continue swinging
forever.
Another example is a chemical explosion in which potential
chemical energy is converted to kinetic energy and heat in a very short time.
See also
External links
Further reading
- Feynman, Richard. Six Easy Pieces: Essentials of Physics
Explained by Its Most Brilliant Teacher. Helix Book. See the chapter "conservation of energy" for Feynman's explanation of
what energy is, and how to think about it.
References
- Einstein, Albert (1952). Relativity: The Special and the
General Theory (Fifteenth Edition). ISBN 0-517-88441-0
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