Second Law of Thermodynamics | Mechanical Engineering SSC JE (Technical) PDF Download

SECOND LAW OF THERMODYNAMICS

• Work is said to be high grade energy and heat is low grade energy. The complete conversion of low grade energy into high grade energy in a cycle is impossible while the complete conversion of high grade energy into low grade energy is possible.
•  A heat engine works on a thermodynamic cycle in which there is net heat transfer to the system and a net work transfer from the system.

• Boiler, turbine, condenser and pump, all four together constitute a heat engine.

Thermal Efficiency

Wnet = Qnet (First law of thermodynamics)

•  A thermal energy reservoir (TER) is defined as a large body of infinite heat capacity which is capable of absorbing or rejecting an unlimited quantity of heat without any appreciable changes in its thermodynamic properties.

PMM2 

•   It is a device which produces work while exchanging heat with single reservoir in a cycle.

Q₂ = 0 , W_net = Q₁ , h_th = 1
Such a heat engine is called as PMM2.

• Second Law of Thermodynamics:
  • Kelvin Plank Statement: It is impossible for a heat engine to produce net work in a complete cycle if it exchange heat only with a single reservoir.
  • PMM2 is impossible according to Kelvin-Plank statement.
  • Clausis statement: it is impossible to transfer heat from lower temperature to higher temperature without any external input.
  • Heat always flow from a body at higher temperature to a body at lower temperature but it can not flow in reverse direction on its own. Some work must be done to achieve this.
•  Refrigeration plant and heat pump constitute of
  •  Evaporator
  •  Compressor 
  •  Condenser
  •  Expander

(Figure: Schematic diagram of Refrigeration plant and heat pump.)

Coefficient Of Performance(COP) =
(COP) _ref =

(COP) _{H.P =} 

(COP) _{H.P =} (COP) _ref  + 1

• The basic cycle in refigerator and heat pump is same. But the main purpose of refrigerator is to maintain temperature which is less than that of surrounding while in heat pump, the main purpose is to maintain temperature which is more that of surrounding.
• PMM–3 : Continual motion of a movable device in complete absence of friction is known as PMM-3.

Carnot Cycle:

• It is a reversible cycle.
• It is an ideal cycle.

Carnot Theorem: It states that all heat engines operating between a given constant temperature source and given constant temperature sink, none has higher efficiency than a reversible engine.

• The efficiency of all reversible heat engines operating between the same temperature levels is the same.
• Third law of thermodynamics : It is impossible by using any procedure, no matter how idealised the procedure is. to reduce any system to absolute zero of temperature in a finite no. of operations.
• The effect of decreasing T₂ to increase thermal efficiency of Carnot cycle is more than the effect of increasing T₁ by same amount.
• First law of thermodynamics gave origin to internal energy and second law leads to entropy.