Modern Engineering Thermodynamics

3.6 Enthalpy 63
WHO WAS AMALIE EMMY NOETHER?
PART 1
Emmy Noether was born on March 23, 1882, the first of four children. Her first name was Amalie, after her mother and paternal
grandmother, but she began using her middle name at a young age. Emmy was taught to cook and clean—as were most
girls of the time—and she took piano lessons. She pursued none of these activities with passion, although she loved to dance.
Emmy attended the Höhere Töchter Schule in Erlangen, Germany, from 1889 until 1897. She studied German, English,
French, and arithmetic and was given piano lessons. In 1900, became a certificated teacher of English and French in Bavarian
girls’ schools.
We use the simpler Eq. (3.10) when writing general expressions and the more complete Eq. (3.9) when calculations
are required. Total internal energy is an all-inclusive concept that includes chemical, nuclear, molecular,
and other energies within the system. Since all mass has internal energy, the only systems that have zero internal
energy are devoid of matter.
We define a system’s specific internal energy u as
u = U/m (3.11)
where m is the system mass. We can now write Eqs. (3.9) and (3.10) as
e = E m = u + V2 + gZ
(3.12)
2g c g c
and
e = u + ke + pe (3.13)
where ke = V 2 /2g c and pe = gZ/g c .
The specific internal energy u is an intensive property, like pressure, temperature, and specific volume; so, it too
can be written as a function of any other two independent properties. For a simple (i.e., homogeneous and pure)
substance, the temperature and specific volume are independent thermodynamic properties. So we can write
then
du =
u = uðT, vÞ
∂u
∂T v
dT +
∂u
dv (3.14)
∂v T
The first term in Eq. (3.14) describes the temperature dependence of u, and the coefficient of dT is written as c v ,
where
∂u
= c v = constant volume specific heat (3.15)
∂T v
Then Eq. (3.14) becomes
du = c v dT +
∂u
dv
∂v T
Many of the equations of thermodynamics have groupings of similar terms. It is convenient to simplify the writingoftheseequationsbyassigningasinglesymbolandnametosuchagrouping.Thisiswhatwasdonein
Eq. (3.9) in defining the total system energy as the sum of three other energy terms. Also, it should be quite
clear that any function of a system’s thermodynamic properties is also a thermodynamic property itself.
3.6 ENTHALPY
When we introduce the open system energy balance later in this text, we find that the properties u and pv are
consistently grouped together. For simplicity, then, we combine these two properties into a new thermodynamic
property called enthalpy, whose total and specific forms are defined as
and
H = U + pV = total enthalphy (3.16)
h = H/m = u + pv = specific enthalpy (3.17)