First Law of Thermodynamics

Energy interactions between a system and its surroundings across the boundary in the form of heat and work have been discussed separately in the previous chapter.
So far, no attempt has been made to relate these interactions between themselves and with the energy content of the system.

First law of thermodynamics, often called as law of conservation of energy,  relating work, heat, and energy content of the system will be discussed in detail in this chapter.

3.1   First Law of Thermodynamics

In its more general form, the first law may be stated as follows

“When energy is either transferred or transformed, the final total energy present in all forms must precisely equal the original total energy”.

It is based on the experimental observations and can not be proved mathematically.  All the observations made so far, confirm the correctness of this law.

3.2   First Law of Thermodynamics for a Closed System

Undergoing a Process

First law can be written for a closed system in an equation form as

           

For a system of constant mass, energy can enter or leave the system only in two forms namely work and heat.

Let a closed system of initial energy E1 receives Q units of net heat and gives out W units of work during a process.  If E2 is energy content at the end of the process.

            Q - W = (E2 - E1)                                                                                      

Where the total energy content

                   E    = Internal Energy     +     Kinetic energy     +     Potential energy

                       

The term internal energy usually denoted by the letter U is the energy due to such factors as electron spin and vibrations, molecular motion and chemical bond.

Kinetic energy term is due to the system movement with a velocity C.  For stationary systems this term will be zero.  The term gc is a constant of value 1 in SI unit.  It will be dropped here after since SI unit is followed throughout the book.

Potential energy term is due to the location of the system in the gravitational field.  It remains constant for a stationary system.  The unit of energy in SI is kJ.