University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
Second law of thermodynamics
Thermodynamics is concerned with transformations of energy . The first law
reflects the observation that energy is conserved, but it imposes no restriction on
the process direction.
The differences between the two forms of energy, heat and work, provide some
insight into the second law.
Work is readily transformed into other form of energy :
® Potential due to elevation change of weight
® Kinetic due to acceleration of a mass
® Electrical energy e.g. generator
Conversion efficiency can be approach 100 by elimination friction or by some
other means.
First law: showed the equivalence of work and heat
Second law:
· Puts restrictions on useful conversion of work and heat
· Follows from observation of a directionality to natural or spontaneous
processes
· Provides a set of principles for
- determining the direction of spontaneous change
-determining equilibrium state of system
Statements of the 2nd law
Statement 1: No device can operate in such a way that its only effect (in system
and surroundings) is to convert heat absorbed by a system completely into work
done by the system.
® Statement 1 a: It is impossible by a cyclic process to convert the heat
absorbed by a system completely into work done by the system.
Statement 2: No process is possible which consists solely in the transfer of heat
from one temperature level to a higher one.
Heat engine
The device that convert heat into work is called "heat engine" e.g. power plant , In
such a power plant the cycle (in its simplest form) consists of the following steps:
® Liquid water at ambient temperature is pumped into a boiler at high
pressure.
® Heat from a fuel (heat of combustion fuel) or heat from a nuclear reaction is
transferred in the boiler to the water, converting it to high-temperature steam
at the boiler pressure.
University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
® Energy is transferred as shaft work from the steam to the surroundings by a
device such as a turbine, in which the steam expands to reduced pressure and
temperature.
® Exhaust steam from the turbine is condensed by transfer of heat to the
surroundings , producing liquid water for return to the boiler , thus
completing the cycle.
Essential to all heat-engine cycles are the absorption of heat at high temperature ,
the rejection of heat at lower temperature and the production of work.
Heat reservoir Definition: A very large system of uniform T, which does not change
regardless of the amount of heat added or withdrawn.
In heat engine there are two temperature level
® High temperature TH reservoir where the working fluid of heat engine
absorbs heat ½QH½ and produce work.
® Low temperature TC where the working fluid of heat engine discard
heat½
½QC½ and return to its original state .
The first law therefore reduce to :
W = Q = QH - QC
Thermal efficiency h :ratio between net work output to heat input
h =
Q - QC
Q
W
= H
=1- C
QH
QH
QH
University of Babylon /College Of Engineering
Electrochemical Engineering Dept.
Second Stage /Thermodynamics
Absolute value signs with heat to make equations independent of sing convention
for Q .
h equal unity (100%) when ½ QC½ = 0 .No engine ever been built for which his is
true.