Lasted edited by Andrew Munsey, updated on June 14, 2016 at 9:43 pm.

  • 2 errors has been found on this page. Administrator will correct this soon.
  • This page has been imported from the old peswiki website. This message will be removed once updated.

Energy, "the potential for causing changes", is a concept used to understand and describe processes.

Introduction and terms

The etymology of the term "energy" is from Greek ????????, ??- means "in" and ????? means "work" the -?? suffix forms an abstract noun. The compound ??-?????? in Epic Greek meant "divine action" or "magical operation" it is later used by Aristotle in a meaning of "activity, operation" or "vigour", and by Diodorus Siculus for "force of an engine."

The word energy is used in several different contexts. The scientific use has a defined meaning, whilst the manyother technical uses often vary. Radiant energy is the energy of electromagnetic waves. Free energy may be categorised as renewable energy, although most renewable energy sources would not normally be called free energy sources or sources of perpetual motion. Free energy is energy which may be directly utilized (and returned) by a device from the surroundings (electromagnetic free energy is sometimes referred to as radiant energy). Free energy can also mean a primary energy source that is free (i.e. does not cost anything) for consumption. Examples include Wind power, There was an error working with the wiki: Code[1], There was an error working with the wiki: Code[2], and Solar power.

In physics, energy is the ability to do work and has many different forms (potential, kinetic, electromagnetic, etc.) No matter what its form, physical energy has the same units as work a force applied through a distance. The SI unit of energy, the joule, equals one newton applied through one meter, for example. The conservation of energy is a fundamental law in science. It states that the total amount of energy (including potential energy) in a closed system remains constant (in contrast to a system whose border is permeable to energy and/or mass). In other words, energy can be converted from one form to another, but it cannot be created or destroyed. In modern physics, all forms of energy exhibit mass and all mass is a form of energy.

Energy is work

Energy is defined in terms of work and there are a variety of energy forms which are defined via type of work. Because of this, a definition of work is critical to the understanding of energy. Work is a defined by a generalization of force over distance evaluated to exactly one output along a curve. This is mathematically stated as:

: W = \int {F} \cdot {d}s

The equation defines that the work (W\,) is equal to the integral of the dot product of the force (F\,) on a body and the infinitesimal of the body's translation ({s}\,). Depending on the kind of force F\, involved, work of this force results in corresponding kind of energy (gravitational, electrostatic, kinetic, etc).

Science and energy

The energy of a closed 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 transferred from place to place or from one form to another. The concept of energy change from one form to another, as a "driver" for natural processes, is useful in explaining many phenomena. In particular, since energy cannot be created or destroyed, the driver of energetic processes is not creation of energy per se, but rather the transformation of energy in such a way that the energy can diffuse in space toward areas of less energy concentration (that is, toward areas of less energy per volume). Such changes are associated with increases in entropy.


In physics, 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 a body to a given velocity, gravitational potential energy is the amount of work to elevate or move a mass against a gravitational pull, etc. Because work is frame dependent it 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 kinetic energy in the reference frame of non-moving observer, but it has zero kinetic energy in its 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. However, when a certain amount of total energy cannot be removed from a system by simple choice of frame, that energy is associated with an invariant mass in the system.


In chemistry, the spontaneous exchange and transformation of energy with the environment is the cause and effect of all chemical transformations that a substance can undergo. These transformations can be a decomposition, synthesis or a reaction of molecules or atoms. A chemical transformation is possible only if so-called free energy considerations are fulfilled. The concept of free energy is a synthesis of energy and entropy, and in practice is entirely driven by entropy increases as energy is transferred to (or from) a reaction to its environment. Free energy is important in the context of chemistry, because energy considerations alone are not sufficient to decide whether a (net) chemical reaction will occur. Instead, this is determined by the total entropy of reactants and surroundings before and after the reaction, with the heat evolved or absorbed by the reaction taken into account only as it creates or destroys entropy (respectively). According to the second law of thermodynamics, the entropy of the universe must increase in all spontaneus processes (including chemical processes), and energy may be transmuted from any form to any other form (including from heat to any other form) so long as the second law is not violated. For example, a gas may expand and thus allow some of its heat to do work, but this is only possible because the net entropy of the universe increases due to the gas expansion, more than it decreases due to the disappearance of heat. The speed of a permitted spontaneous chemical reaction is also determined by another concept, activation energy. It refers to the minimum energy reactant molecules must have in order to be able to produce product molecules.

Basic forms of energy

There are various forms of energy and there exists relations between the different forms.

In science, energy has different forms, forexample thermal, chemical, electrical, radiant, nuclear etc. They can all be, in fact, reduced to kinetic energy or potential energy. Thus energy can be divided into two broad categories.

Kinetic energy

Kinetic energy (Movement) is energy that a body possesses as a result of its motion. It is formally defined as the work needed to accelerate a body from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. Negative work of the same magnitude would be required to return the body to a state of rest from that velocity. Kinetic energy is the energy of motion (an object which has speed can perform work on another object by colliding with it). The formula for kinetic energy is E_k= {1\over{2}} mv^2, v