Principle of conservation of energy

PRINCIPLE OF CONSERVATION OF ENERGY

In physical and chemical processes, a change in mass is very difficult to determine since it is too small. It is assumed that the mass and energy of the substances involved in a process are constant. This assumption is emphasized by the Law of Conservation of Energy.

One of the most fundamental laws of Nature is the Conservation of Energy Principle. It simply states that during an interaction, energy can change from one form to another, but the total amount of energy always remains constant. That is, energy cannot be created or destroyed.

Examples

(i) Water is flowing from an overhead tank through a pipe. Potential energy is converted into kinetic energy.

(ii) A rock falling off a hill picks up speed, as its potential energy is converted to kinetic energy.

 (iii) The conservation of energy principle also forms the backbone of the diet industry: A person, who has a greater energy input (food) than energy output (exercise), will gain weight (store energy in the form of fat). A person who has a smaller energy input than output will lose weight.

(iv) You are applying brake to your two-wheeler. In this process, kinetic energy of the wheel is converted into heat due to friction.

1. FIRST LAW OF THERMODYNAMICS

First Law of Thermodynamics is an expression of Law of Conservation of Energy. It states that energy is a thermodynamic property. According to this Law, the total energy of an isolated system in all its forms remains constant.

Consider a system where there is energy transfer. The net amount of energy transferred in various forms should be equal to the change in internal energy of the system, with the possibility of conversion of one form of energy into any other form.

2. SECOND LAW OF THERMODYNAMICS

If the First Law of Thermodynamics states that all forms of energy are convertible to one another, the Second Law puts a limitation on the conversion of some forms of energy to others

Second Law of Thermodynamic states that “It is impossible for heat energy to flow from a body at a lower temperature to a body at a higher temperature without the aid of external work.”

Example: A cup of hot coffee left on a table eventually cools. The high temperature energy of o the coffee is degraded (i.e., transformed into a less useful form at a lower temperature), once it is transformed to the surrounding air. But, a cup of cool coffee in the same room never gets hot by itself.

The second law indicates that heat cannot be entirely converted into work. The portion of heat that cannot be converted into work is called Unavailable Energy, which is rejected as low grade heat after the work is done.

Reversible and Irreversible Processes in a Thermal System

Entropy Entropy is defined with respect to a reversible process. A Reversible Process is one that after being executed on a system can be reversed. The same system can be brought back to its initial state without any change to the surrounding. Example: Condensation or boiling of liquids.

Any process, which is not reversible, is called an Irreversible Process. All natural processes are irreversible types. The important sources of irreversibility in nature are: friction, heat transfer, throttling and mixing. Examples:

(i) Friction created by a rotating shaft in a bearing generates heat. Is it possible to rotate the shaft by supplying the same amount of heat? No. The heat transfer from a high temperature source to a low temperature source cannot reverse itself without external work.

(ii) Process of combustion.

(iii) Free unrestricted expansion of gases.

We must have some measure of quality of energy as well as a measure of the irreversibility of a process. A property used to measure the quality of energy or irreversibility of the process is known as Entropy,