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Monday, 25 July 2016


We have seen the basic concepts of work and heat transfer in the field of thermal engineering and simultaneously we have also seen the sign conventions used in thermodynamics for heat and work transfer. We were discussing various forms of work such as displacement work or PdV work, paddle wheel work, flow work and shaft work in thermodynamics in our previous posts.

Today we will focus here over a very important topic of thermodynamics and that is enthalpy. So let us see what is enthalpy?


Enthalpy is basically defined as the property of the system which will be basically the summation of internal energy (U) and the product of pressure and volume i.e. PV. So we can write here the equation of enthalpy as mentioned here.

H = U + PV

Enthalpy (H) will be measured in joules
Internal Energy (U) will be measured in joules
Pressure (P) will be measured in Pa
Volume (V) will be measured in m3
We must note it here that U, P, H and V all are state functions.

Enthalpy will be dependent over the quantity of mass of the system and hence will be considered as the extensive property of the system. As we have written above enthalpy is a state function because it will not be dependent over the path. Enthalpy will be applicable for both type of the system i.e. for open system and for close system too.

In thermodynamics, we focus over the calculation of change in enthalpy as absolute enthalpy calculation is quite difficult and we do not do calculation for absolute enthalpy.

Let we have a thermodynamic system and heat Q is added to the system and system changes its state from state 1 to state 2. Let pressure is constant here.

According to first law of thermodynamics, we will have
∆U = Q - W
Q = ∆U + P dV
Q = U2- U1 + P (V2-V1)
Q = (U2 + PV2) – (U1 + PV1)
Q = H2 – H1
Therefore, change in enthalpy will be equal to the heat interacted with the system. If change in enthalpy is positive then its means heat has been supplied to the system as it will be positive value if change in enthalpy is positive.  

We will come again with new topic i.e."Specific heat of a substance" in our next post in area of thermodynamics.
Do you have suggestions? Please write in comment box.


Engineering thermodynamics by P. K. Nag
Basic thermodynamics by Prof. S.K. Som
Image courtesy: Google

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