Atmospheric pressure: The atmospheric air exerts a normal pressure upon all surfaces with which it is in contact and it is known as atmospheric pressure. Atmospheric pressure is also known as barometric pressure. The atmospheric pressure at sea level (above absolute zero) is called standard atmospheric pressure and its value is given as follows:

Standard atmospheric pressure = 101.3 kN/m² or kPa (1 kN/m² = 1 kPa)
= 10.3 m of water
= 760 mm of Hg
Atmospheric pressure = Absolute pressure - Gauge pressure

First law of thermodynamics: First law of thermodynamics states that, Heat and mechanical work are mutually convertible. According to this law, a definite amount of mechanical work is needed to produce a definite amount of heat and vice versa. The first law of thermodynamics is the law of conservation of energy.
First law of thermodynamics also states that energy can neither be created nor destroyed, through it can be transformed from one form to another. According to this law, the energy due to heat supplied must be balanced by external workdone plus the gain in internal energy due to rise in temperature.

Different thermodynamics laws are

1. Zeroth law of thermodynamics
2. First law of thermodynamics
3. Second law of thermodynamics

Zeroth law of thermodynamics : Zeroth law of thermodynamics states that, when two bodies are in thermal equilibrium with a third body. They are also in thermal equilibrium with each other.

First law of thermodynamics: First law of thermodynamics states that, Heat and mechanical work are mutually convertible. According to this law, a definite amount of mechanical work is needed to produce a definite amount of heat and vice versa.
First law of thermodynamics also states that energy can neither be created nor destroyed, through it can be transformed from one form to another. According to this law, the energy due to heat supplied must be balanced by external workdone plus the gain in internal energy due to rise in temperature.

Second law of thermodynamics: Second law of thermodynamics states that there is a definite limit to the amount of mechanical energy, which can be obtained from a given quantity of heat energy.
According to Claussius, this lay may be stated as ” It is impossible for a self acting machine working in a cyclic process, to transfer heat from a body at a lower temperature to a body at a higher temperature without the aid of an external agency”. The second law of thermodynamics has also been stated by Kelvin Planck as ” It is impossible to construct an engine working on a cyclic process, whose sole purpose is to convert heat energy into work.” According to this statement, the second law of thermodynamics is sometimes called as law of degradation of energy.