In general, the concept energy refers to "the
potential for causing a change". The word is used in several
different contexts. The scientific use has a precise, well- defined
meaning, whilst the many non- scientific uses often do not.
In physics the energy of a system in a certain
state is defined as the work needed to bring the system to that
state from some reference state. Because work is defined via force
involved, forms of energy are usually classified according to that
force (elastic, gravitational, nuclear, electric, etc). Energy is a
conserved quantity: it is neither created nor destroyed, but only
transfered from place to place or from one form to another.
Ultimately, this is because the laws of nature do not change with
time.
Energy is the ability to do work (work is,
simplistically, a force applied through a distance), and has
several different forms. However, no matter what the form, physical
energy uses the same units as work: a force applied through a
distance. For example, kinetic energy is the amount of work to
accelerate body, gravitational potential energy is the amount of
work to elevate mass, etc. Because work is frame dependent (= can
only be defined relative to certain initial state or reference
state of the system), energy also becomes frame dependent. For
example, a speeding bullet has plenty of kinetic energy in the
reference frame of non- moving observer, but it has zero kinetic
energy in proper (co- moving) reference frame - because it
takes zero work to accelerate a bullet from zero speed to zero
speed. Of course, the selection of a reference state (or reference
frame) is completely arbitrary - and usually is dictated to
maximally simplify the problem to be dealt with.
Forms of energy
- Kinetic
energy is the energy of motion (an object which has speed can
perform work on another object by colliding with it).
-
Potential energy or unreleased kinetic energy. This sort of energy
arises when work is done on an object to move it somewhere against
an opposing force. For instance, stretching a rubber band increases
the elastic potential energy stored within the band. When the band
is released, this energy is converted into kinetic energy, and work
is performed. Other forms of potential energy include gravitational
potential energy (from moving masses apart), electrical potential
energy (from moving charges against a field), and chemical
potential energy (energy stored within chemical bonds).
- Thermal
energy the kinetic energy associated with the various motions of
microscopic particles. The average thermal energy within a sample
of matter is referred to as the sample's temperature (work is
required to accelerate the particles and raise the
temperature).
- Light
energy the energy that composes photons and is responsible for the
various sorts of electromagnetic radiation (work is required to
create photons).
- Nuclear
energy, the energy stored within the nuclei of atoms.
- Mass is
also considered as a form of energy, (or in lay terms, the
manifestation of energy,) because during annihilation or other mass
change, the equivalent amount of energy (E = mc²) is always
released.
Conservation of
energy
Energy is subject to the law of conservation of
energy (which is a mathematical restatement of shift symmetry of
time). Thus, energy cannot be made or destroyed, it can only be
converted from one form to another, that is, transformed. In
practice, during any energy transformation in (macroscopic) system,
some energy is converted into incoherent microscopic motion of
parts of the system (which is usually called heat or thermal
motion), and the entropy of the system increases. Due to
mathematical impossibility to invert this process (see statistical
mechanics), the efficiency of energy conversion in a macroscopic
system is always less than 100%.
The first law of thermodynamics states 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. In other words, energy is neither created nor
destroyed, only converted between forms. This law is used in all
branches of physics, but frequently violated by quantum
mechanics.
1. Matter
