First law of thermodynamics
The first law of thermodynamics states that:
For a cyclic process
ΣQ = ΣW
Q = Heat interaction
W = Work interaction
For a finite non-cyclic process
Q1-2 = W1-2 + ΔE
E = Internal energy
E = U + KE + PE + any other kind of stored energy
U = Intermolecular energy
KE = Kinetic energy
PE = Potential energy
For a closed system in equilibrium KE, PE and other kinds of stored energy are zero.
E = U
Hence for a finite non-cyclic process first law of thermodynamics becomes
Q1-2 = W1-2 + ΔU
If we consider only P*ΔV work, above equation becomes
Q1-2 = P*ΔV + ΔU
Note: Conventionally work done by the system and the heat given to the system are always taken positive.
A property of a system whose change in a process executed by the system equal to the difference between the heat and work interactions by the system with its surrounding.
Enthalpy is a thermodynamic quantity which is equal to total heat content in a system.
H = U + PV
According to the first law of the thermodynamics
Q1-2 = P*ΔV + ΔU
Q1-2 = P(V2-V1) + U2 – U1
Rearranging the above equation
Q1-2 = U2 + P2V2 – (U1 + P1V1)
From the equation of enthalpy, it implies
Q1-2 = H2 – H1
Specific heat is the quantity of heat which is required to raise the temperature of unit mass by one degree Celsius.
There are two types of specific heat
Specific heat at constant volume
Cv = ( ∂u/ ∂T)v=constant
Specific heat at constant pressure
Cp = ( ∂h/ ∂T)p=constant
First law applied to the open system (or control volume)
Unlike a closed system mass flows in and out of an open system. Here we have to take conservation of mass into account.
Conservation of mass
(dm1/dt) – (dm2/dt) = dmcv/dt
dm1/dt = Rate of mass entering to the system
dm2/dt = Rate of mass leaving from the system
dmcv/dt = Rate of mass stored in the system
Conservation of energy
e = u + p*v + g*z + (V2/2)
e = stored energy in the stream of fluid
u = internal energy stored in the stream of fluid
V2/2 = kinetic energy of the stream of fluid
g*z = potential energy stored in the stream of fluid
p*v = pressure work
Mathematical expression of first law for open system
(dm1/dt)*e1 + (∂Q/∂t) – (dm2/dt)*e2 – (∂W/∂t) = dEcv/dt
At steady state
m1 = m2 = m
dEcv/dt = 0
Hence the equation becomes
(dm/dt)*e1 + (∂Q/∂t) – (dm/dt)*e2 – (∂W/∂t) = 0
Above equation is also known as steady flow energy equation.