Direct Energy, 2018a

180 8.6 Thermoelectric Eects
heater
heater
+-
Figure 8.1: Illustration of closed systems containing energy conversion devices.
As an example, consider the closed system shown on the left part of
Fig. 8.1 comprised of a cylinder with a piston and a heater. Assume the
cylinder contains a xed volume of gas inside. Suppose the heater is used
to transfer 100 J of energy into the piston in an hour while the piston is
forced to remain in a xed position. After the hour, the internal energy of
the gas will be 100 J greater than before. Again suppose the heater is used
to transfer 100 J of energy into the gas, but this time assume the piston
is allowed to move thereby expanding the gas volume. After the hour, the
internal energy of the gas will be the original energy of the gas, plus the
100 J supplied by the heater, and minus a factor due to the mechanical
work done by the piston.
The rst law of thermodynamics says two things. First, energy is conserved.
Second, energy can be stored, converted to mechanical work, or
converted to heat. We know energy can be converted to other forms too,
like electrical or electromagnetic energy. While introductory thermodynamics
classes do not usually do so, we can add other devices to the piston
as shown on the right part of Fig. 8.1. We can include a battery and put a
resistor inside to convert the chemical energy to electrical energy, and the
resistor can heat the air in the piston. We can put a mass and a spring in
the piston and convert potential energy of a compressed spring to kinetic
energy by removing a clip which holds the spring compressed. In a closed
system when all energy conversion processes are considered, energy must
be conserved.
8.6 Thermoelectric Eects
8.6.1 Three Related Eects
In the 1800s, three eects were experimentally observed. At rst, it was
not obvious that these experiments were related, but soon they were found