Energy

Understanding Energy

Rakesh Mohan Hallen

ENERGY is often confusing. While we often come across phrases like "Save Energy"; "Energy Crisis", "Energetic Person", in physics texts energy is said to be conserved. Students often ask: If energy is conserved, why is there an energy crisis?

One needs to delve into the etymology of the word energy to get a satisfactory answer. The word energy has its roots in the Latin word ergo which meant work, with the addition of the prefix en- it leads to another word energia which meant activity. So energy from very ancient times has been associated with activity and work. But what we mean by the term has also simultaneously undergone changes. Initially it was only associated with motion, then with the developments in technologies and science during past few centuries heat , light and chemicals also became associated. The word "energy" in the above phrases, then, refers only to the resources available to do activities. These resources as we all know are depleting and need to be conserved by our conscious efforts. What, then is the energy which is conserved naturally?

The word "energy" used by the physicist today has a different connotation. In most contemporary texts and those of past several decades it is defined as "the capacity or ability to do 'work', which can be in several different forms, and is conserved, i.e. it can neither be created nor destroyed". Work, in physics, is said to be done when a mass is moved under the influence of a force. A small stone with zillions of atoms vibrating within it has energy according to this definition. But, as we all know, this energy cannot be used in the sense electricity, or a piece of coal can be used. Thus the definition of energy in physics does not refer to resources available for doing useful work alone.

The Energy Concept

The definition of energy mentioned above leads to several questions from youngsters:

* What is energy?

* How can we say that energy is neither created nor destroyed?

* If energy can neither be created nor destroyed, then who created it and how?

Energy is not an entity or a pattern. It is a concept, like time, gravity or God. An attribute we ascribe to matter, just like mass. Therefore it cannot be created or destroyed like an object. If at all it is a creation, then it is a creation of the human intellect. Initially this concept was only limited to a property of moving objects, but as our knowledge of heat and the structure of matter has advanced, it now includes molecular motions and electromagnetic interactions also. We can calculate the quantity of this attribute in a system according to the relations derived from other fundamental laws of nature concerning motion, position or fields. We have found that in any interaction we so far know, the value of this property for a closed system does not change; hence we say that it is conserved. It is not that during the development of our knowledge we never came across instances where this conservation law seemed to be in question, but in all such instances we could find or define additional fields, which could account for the discrepancy. Professor Richard Feynman, the famous American Nobel laureate in physics, has said in his Lectures in Physics:

"There is a fact, or if you wish, a law, governing natural phenomena that are known to date. There is no known exception to this law -- it is exact so far we know. The law is called 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 happens. 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 same."

Energy and Mass

Scientists indeed faced a dilemma over the conservation of energy was when they went deep into nuclear reactions. In several nuclear reactions it is found that the reactions are accompanied by the evolution of energy and there is a discrepancy in the total mass of products as compared to that of reactants. This anomaly can be explained by the famous Einstein's mass energy equivalence relation normally expressed as

E=mc2

where E and m are the energy and mass respectively and c is a universal constant known as the velocity of light.

This relation has also raised several questions amongst many youngsters:

* Can energy be converted into matter?

* In the Sun energy is generated from mass, which means that the law of conservation of mass is being violated, if it is so will the Universe come to an end, or is the above energy equation not valid in this case ?

* According to the equation E=mc2 a small stone is a store house of immense energy. Can we somehow tap this energy?

* Can we consider mass as a form of energy?

The concept of mass -- as an attribute of matter -- like energy, has also undergone several changes. What was initially an attribute signifying the bulk of matter, was later modified to a constant relating the concept of force to acceleration -- by Newton's laws of motion and gravitation. Till our knowledge was limited to large bodies moving at velocities much lower than the velocity of light, energy and mass were distinct concepts. However, as our knowledge progressed into the realm of atomic and then subatomic and cosmic interactions and as we started studying motion at velocities comparable to the speed of light, this concept underwent a revolution.

The energy and mass in Einstein's energy mass relation are different from the Newtonian concepts of energy and mass. The energy in the Einstein's equation is the total energy of the system, which includes besides kinetic and potential energy, the energy due to various molecular atomic subatomic and subnuclear interactions involving the gravitational field, the electric field; the magnetic field; and the electromagnetic field etc. The term mass is no longer