Limitations
of First Law of Thermodynamics
If a well insulated tank of fluid is stirred by a rotating paddle
wheel, the energy of the fluid increases. If the stirrer is stopped, however
the energy of the fluid will not decrease and cause the stirrer to rotate in
the opposite direction. The possibility of this process proceeding in the
opposite direction is not excluded by the first law of Thermodynamics. Hence
first law of thermodynamics does not allow us to predict whether a proposed
conceived energy conversion is possible or not.
In all the internal combustion engines fuel and air mixture is
supplied at room temperature. This mixture undergoes combustion inside the
engine and gives out work. Exhaust gases coming out of the engine are always at
higher temperature, indicating that some heat is taken away into atmosphere.
Hence, in all the IC engines only a part of the heat is converted into work.
From our experience we know that if any attempt is made to convert all the heat
into work, our effort will go in vain. This limitation in the extent of
energy conversion has also not been addressed in first law of thermodynamics.
4.2 The Second law of Thermodynamics
Kelvin
Planck’s statement : It is impossible to construct a device
that, operating continuously, will produce no effect other than transfer of
heat from a single thermal reservoir and performance of an equal amount of
work.
The term thermal reservoir refers
to a very large system in stable equilibrium, to which or from which, any
amount of heat can be transferred at constant temperature.
A thermal reservoir supplying heat
continuously at constant temperature is known as source. (Example : Sun)
A thermal reservoir receiving heat
continuously at constant temperature is known as sink. (Examples : River, Sea)
From Kelvin-Planck statement it is clear that
for any system to operate in a cycle and to give out work continuously it
should interact with a minimum of two reservoirs at different temperatures. The
system will receive heat from the high temperature reservoir and reject heat to
the low temperature reservoir. Such devices are known as heat engines.
Performance (or) Efficiency of a heat engine can be expressed as the ratio of
desired output to the required input. In a heat engine the desired output is
net work output and the required input is total heat input
...(4.1)
From first law of thermodynamics
SQ = SW
Qin - Qout = Wnet
...(4.2)
Clausius statement : Unaided by an external agency
heat can not be transferred from a body at lower temperature to a body at
higher temperature.
