Please Read :
Enthalpy, Entropy and Internal Energy .
What is system. boundary ans surroundings in thermodynamics ?
What is substance in thermodynamics?
What is the objective of thermodynamics ? Difference between thermodynamics and heat transfer
First law of thermodynamics-definitions
Q-W= de ………………. (1)
All quantities in Eq. (1) may be regarded as those referring to unit mass of the system. (In thermodynamics texts it is customary to denote quantities per unit mass by lowercase letters, and those for the entire system by uppercase letters. This will not be done here.)
The internal energy (also called “thermal energy”) is a manifestation of the random molecular motion of the constituents. In fluid flows, the kinetic energy of the macroscopic motion has to be included in the term ‘e’ in Eq. (1) in order that the principle of conservation of energy is satisfied. For developing the relations of classical thermodynamics, however, we shall only include the “thermal energy” in the term e in explaining 1st law of thermodynamics.So in this section we see how energy is conserved in the first law of thermodynamics.
It is important to realize the difference between heat and internal energy. Heat and work are forms of energy in transition, which appear at the boundary of the system and are not contained within the matter. In contrast, the internal energy resides within the matter. If two equilibrium states 1 and 2 of a system are known, then Q and W depend on the process or path followed by the system in going from state 1 to state 2.
The change de = e2 – e1, in contrast, does not depend on the path. In short, e is a thermodynamic property and is a function of the thermodynamic state of the system.
Thermodynamic properties are called state functions, in contrast to heat and work, which are path functions.
First law of thermodynamics-equation
Frictionless quasi-static processes, carried out at an extremely slow rate so that the system is at all times in equilibrium with the surroundings, are called reversible processes. The most common type of reversible work in fluid flows is by the expansion or contraction of the boundaries of the fluid element. Let v = I/p be the specific volume, that is, the volume per unit mass. Then the work done by the body per unit mass in an infinitesimal reversible process is -pdv, where du is the increase of u.
Equations of State for thermodynamics first law
In simple systems composed of a single component only, the specification of two independent properties completely determines the state or the system. We can write relations such as
Specific Heats explaining the 1st law of thermodynamics
Cp = (dh/dT)p ... (5)
Above mentioned equations mean that we regard h as a function of p and T, and find the partial derivative of h with respect to T, keeping p constant. Equation (6) has an analogous interpretation. It is important to note that the specific heats as defined are thermodynamic properties, because they are defined in terms of other properties of the system. That is, we can determine Cp and Cv when two other properties of the system (say, p and T) are given. Thus in the understanding of the first law of thermodynamics specific heat certainly have some significance.