Direct Energy, 2018a

8 THERMOELECTRICS 179
kPa, and the mass of the neon in the container is 3000 g. Find the volume
of the container. The temperature is 298.15 K. From a periodic table, the
atomic weight of a neon atom is 20.18
g . Thus, the container holds 148.7
mol
mol of neon atoms. Next, we use the ideal gas law to nd the volume.
V = NRT
P = 148.7 mol · 8.314 J
mol·K · 298.15 K
10 4 Pa
=36.86 m 3 (8.20)
8.5 First Law of Thermodynamics
The idea of energy conservation was introduced in Sec. 1.3. Most discussions
of thermodynamics also begin with the same idea. The rst law
of thermodynamics is a statement of energy conservation. Energy can be
stored in the material polarization of a capacitor, the chemical potential
of a battery, and in many other forms. People studying thermodynamics
and heat transfer, however, often make some drastic assumptions. They
classify all energy conversion processes as heat transfer or other where the
primary component of the latter is mechanical work. At the beginning
of introductory thermodynamics courses, all forms of energy besides heat
transfer and mechanical work are ignored. Charging a capacitor, discharging
a battery, and all other energy conversion processes are grouped in with
mechanical work when writing the rst law of thermodynamics. The rst
law of thermodynamics is typically written as
(change in int. energy) =(heat in) − (work and other forms) . (8.21)
ΔU = Q − W (8.22)
Each term of the Eq. 8.22 has the units of joules. The symbol Q represents
the energy supplied in to the system by heating, and −W , with the minus
symbol, represents the mechanical work in to the system as well as all
other forms of energy into the system. The quantity ΔU represents the
change in internal energy of the system. In a closed system the total energy
is conserved. In a closed system, energy is either stored in the system
(for example as potential energy or another form of internal energy), is
transfered in or out as heat, or is transfered in or out as another form such
as mechanical work [109, p. 51].