SECOND LAW OF THERMODYNAMICS

SECOND LAW OF THERMODYNAMICS

• KELVIN PLANK STATEMENT: - It is impossible for a heat engine working on cyclic process which can convert all the heat supplied to it into equivalent amount of mechanical work.

• CLAUSIUS  STATEMENT: - It is impossible for a self acting machine, working in a cyclic process, to transfer heat from a body at lower temperature to a body at higher temperature without the external aid.

Work is a high grade of energy and Heat is a low grade of  energy.

• EFFICIENCY OF HEAT ENGINE: - 

A heat engine works on a thermodynamic cycle.

The efficiency is the ratio of  net work transfer to the net heat transfer to the system.

Fig.1 |  Efficiency of Heat Engine

η_th = W_net/ Q₁  = (Q₁ - Q₂) / Q₁

Where Q₁  = Heat flow from source to the heat engine.

  and  Q₂ = Heat rejected from heat engine to the sink.

η_th = (T_H– T_L) / T_H

=(1- T_L/ T_H)

Where T_H = Higher Temperature

  and T_L  = Lower Temperature

• COEFFICIENT OF PERFORMANCE OF REFRIGERATOR: -

It is defined as the ratio of Refrigerating Effect to the work input.

Fig.2 | COP of Refrigerator

COP_R = Q₂/ (Q₁ – Q₂) [FOR BOTH REVERSIBLE AND IRREVERSIBLE]

COP_{R (REV. CYCLE)}= T_L / (T_H – T_L) = (1 – ηth) / ηth

 

• COEFFICIENT OF PERFORMANCE OF HEAT PUMP: -

It is defined as the ratio of Heating Effect to the work input.

Fig.3 | COP of Heat Pump

COP_HP = Q₁/ (Q₁ – Q₂) [FOR BOTH REVERSIBLE AND IRREVERSIBLE]

COP_{HP (REV. CYCLE)}= T_H / (T_H – T_L) =1 / ηth

 

• COP_HP = 1+ COP_R

• PERPETUAL MOTION MACHINE OF SECOND KIND (PMM II): -

The machine which can continuously absorb heat from a single thermal reservoir and convert this completely into work.

The thermal efficiency of search machine is 100%

This is impossible.