Second law of thermodynamics
- There are two ways in which the second law of thermodynamics is expressed
1. Kelvin- Planck statement
The Kelvin - Planck statement of the second law of thermodynamics tells that it is impossible to construct a cyclically operating device which produces no other effect than the extraction of energy as heat from a single thermal reservoir and delivers an equivalent amount of work.
- A diagram of a device is shown in
figure
- Considering the working fluid as a
system as shown in the figure, application of the first law of
thermodynamics gives Q = W
- That is all the energy absorbed as
heat by the device is completely converted into work
- Hence the thermal efficiency of
such an engine is η = W/ Q = 1
- Hence the Kelvin -Planck statement
thus implies that no heat engine can have an efficiency equal to one.
2. Clausius statement
The Clausius statement of the second law of thermodynamics tells that it is impossible to construct a self-acting device which working cyclically will produce no other effect than the transfer of energy as heat from a low-temperature body to high-temperature body.
- A schematic of a device which is
impossible to construct according to the Clausius statement of the second law
of thermodynamics is shown in fig.
- As we know that the transfer of
energy as heat from a high-temperature body to a low-temperature body
occurs very fast.
- The Clausius statement of the
second law of thermodynamics denies the possibility of self-reversal of
such a spontaneous process
- The coefficient of performance of
the device is given by
COP = Q/W = Q/0 = ∞
Thus the second law of thermodynamics implies that the COP of a heat pump or refrigerator cannot be infinity
No comments:
Post a Comment