FIRST LAW OF THERMODYNAMICS

Statement

" Although energy assumes many forms, the total quantity of energy is constant, and when energy disappears in one form it appears simultaneously in other form"

• First law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes.
• The first law makes use of the key concepts of internal energy, heat, and system work.
• For any process, the first law can be written as 

Energy balance for the closed system

• In closed system no streams enter or leave a system, and no energy associated with the matter is transported across the boundary.
• All energy exchange between a closed system and its surroundings then appears as heat and work.
• And hence total energy change of surroundings equals the net energy transferred to or from it as heat and work

                                                                  where Q= heat of system
                                                                          W= work done

Sign convention of heat and work

• The work done by the system on the surroundings is treated as positive quantities similarly the energy transferred as heat to the system from surroundings is also treated as positive quantity.

• Hence with reference to surroundings, heat transferred from the system to the surrounding and work done by the surroundings taken as follows
• Energy of the surroundings becomes 

• From equation of first law we can say that

• From these equation we can say that all energy exchange between a closed system and its surroundings then appears as heat and work, and the total energy change of surroundings equal the net energy transferred to or from it as heat and work.
• Closed system often undergo process during which only internal energy of the system changes. hence equation becomes

• Where Ut is the total internal energy of the system . for differential changes

Hence first law of thermodynamics can be stated as the change in the internal energy of a system is equal to the sum of the heat gained or lost by the system and the work done by or on the system.