Laws of Thermodynamics
Revised 18 August 2021
- All forms of energy are inter-convertible and when conversions occur, they do so according to rigorous laws of exchange.
Work & Heat
- Energy is defined as 'the ability to do work on other bodies’ or as ‘stored work’.
- Work is said to be done on a system if that system experiences a displacement as the result of a force parallel to and in the same direction as that force.
- Heat is defined as that energy which is transferred between a system and its surroundings solely by virtue of a temperature difference
- The unit of heat energy is the calorie which is defined as that amount of heat whose absorption by 1 gram (gm) of water at constant atmospheric pressure is accompanied by a temperature rise from 14.5 to 15.5 °C
- The mechanical equivalent of heat is the joule where 4.185 joules is the amount of mechanical work required to effect the same change in state as that produced by the absorption of one calorie of heat
- One watt (W) is a unit of power as opposed to energy, being the rate of one joule of energy flow per second.
- The degree of convertibility of energy - stored work - into applied work is often called availability.
First Law of Thermodynamics - Conservation of Energy
- The First Law of Thermodynamics: Energy may be transformed from one form into another, but energy is neither created nor destroyed.
Second Law of Thermodynamics
- The Second Law of Thermodynamics: All physical processes proceed in such a way that the availability of the energy involved decreases.
- In a low entropy system the energy is free in the sense that it is available for producing mechanical work, whereas in a high entropy system the energy is said to be bound.
- The law of entropy: When all systems taking part in a process are included, the entropy S of the total system either remains constant or increases.
Entropy as a Measure of Order and Disorder
- An open system may exchange both energy and matter with the outside, whereas a closed system exchanges only energy and not matter with the outside.
- Contrary to being a violation to the law of entropy, life forms comprise systems that hasten the increase of entropy in the universe
- The Law of Entropy can be stated as: In spontaneous processes, concentrations tend to disperse, structure tends to disappear, and order becomes disorder.
- The First Law of Thermodynamics does not contradict the laws of mechanics, but the Second Law of Thermodynamics, the law of entropy, is in direct contradiction with the laws of classical mechanics in that the law of entropy introduces the element of an irrevocable qualitative change when systems undergo any process.