What is the First law of thermodynamics

Statement of the First law of thermodynamics

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

• The 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

Δ (Energy of the system) + Δ ( Energy of surroundings ) = 0

Energy balance for the closed system

• In a 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 appear as heat and work.
• And hence total energy change of surroundings equals the net energy transferred to or from it as heat and work

Δ ( Energy of surroundings ) = ± Q ± W

Sign convention of heat and work

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

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

Δ ( Energy of surroundings ) = Q _surr +W _surr = - Q -W

• From the equation of first law, we can say that

 Δ ( Energy of the system ) =  Q + W

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

Δ U^t = Q + W

• Where U^t is the total internal energy of the system.
• For differential changes

dU^t = dQ + dW

 Hence the 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.