Second law of Thermodynamics: Kelvin-Planck Statement, Clausius Statement

 Second law of thermodynamics

1. Kelvin-Planck Statement 

It is impossible to devise a cyclically operating device, which produces no other effect than the extraction of heat from a single thermal  reservoir  and delivers an equivalent amount of work. 


Heat engine with single thermal reservoir is not possible.


For a 1-T engine the thermal efficiency h=W/Q=1. No heat engine can have efficiency equal   to unity.


Second  law not only identifies the direction of process, it also asserts that energy has quality   as well as quantity.

Thermal Reservoir


A thermal reservoir is a large system (very high mass x specific heat value) from which a quantity of energy can be absorbed or added as heat without changing its temperature. The atmosphere and sea are examples of thermal reservoirs.


Any physical body whose thermal energy capacity is large relative to the amount of energy it supplies or absorbs can be modeled as a thermal reservoir.

A reservoir that supplies energy in the form of heat is called a source and one that absorbs energy in the form of heat is called a sink.

Heat Engine

 It is a cyclically operating device which absorbs energy as heat from a high temperature reservoir, converts part of the energy into work and rejects the rest of the energy as heat to a thermal reservoir at low temperature.

Schematic Diagram of Thermal Power Plant

Schematic Diagram of Thermal Power Plant
Schematic Diagram of Thermal Power Plant

The working fluid is a substance, which absorbs energy as heat from a source, and rejects energy as heat to a sink.

Q2 – Heat rejected to cooling water in condenser

WP – Work done on the pump

Wnet=WT-W

W = Q1 – Q2

Thermal Efficiency,


Schematic Diagram of Heat Engine

Schematic Diagram of Heat Engine
Schematic Diagram of Heat Engine




Schematic Diagram of Refrigerator and Heat Pump


Schematic Diagram of Refrigerator and Heat Pump
Schematic Diagram of Refrigerator(a) and Heat Pump (b)



QL – Heat absorbed from low temperature thermal reservoir

QH – Heat rejected to a high temperature thermal reservoir when work (W) is done on it.



In a reversible, isothermal expansion of an ideal gas, all the energy absorbed as heat by the system is converted completely into work. However this cannot produce work continuously  (not a cycle).

Single reservoir heat engine (1 T engine) is not possible.

2. Clausius
Statement

 It is impossible to construct a device
that operates in a cycle and produces no effect other than the transfer of heat
from a lower-temperature body to higher- temperature body.

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