First Law of Thermodynamics

The first law of thermodynamics is a statement about universal law of conservation energy and identifies heat transfer as a form of energy transfer. The most common statement from the first law of thermodynamics reads:

"The increase in internal energy of a thermodynamic system is proportional to the amount of heat energy that is added to the system minus the work that done by system to its environment."

The first law of thermodynamics in the application can be divided as the first law of thermodynamics for closed system and the first law of thermodynamics for open system.

1. First law of thermodynamics for closed system

In this law is a system whereby there is no mass transfer of the system so that the fluid is always in the system boundary.

Q_{net, in} - W_{net, out} = ΔE_system

in isolated system, energy in the system remain same so  Q = W = 0. Thus can be obtained the equation:

E_in - E_out = ΔE_system

2. First law of thermodynamics for open system

According to the first law of thermodynamics in closed system:

E_in - E_out = ΔE_system

Or energy balance:

E_in  =  E_out

The energy can be transferred by heat, work and mass is a balance in the stationary states and can be written as follow:

Q_in + W_in + Σm_iθ_i  =  Q_out + W_out + Σm_eθ_e

energy in the flowing fluid per unit mass is:

θ = h + ke + pe

θ = h + v²/2 +gz

with the substitution of the equations above, then:

Q_in + W_in + Σm_i(h_i + v²/2 +gz_i) = Q_out + W_out + Σm_e(h_e + v²/2 +gz_e)  

Then can be obtained the mass balance equation

ΔQ = Σm_i(h_i + v²/2 +gz_i) - Σm_e(h_e + v²/2 +gz_e) + ΔW

If the work is ignored ΔW = 0 then:

ΔQ = Σm_i(h_i + v²/2 +gz_i) - Σm_e(h_e + v²/2 +gz_e)