Laws of Thermodynamics & Heat Engine | Physics Class 11 - NEET PDF Download

Zeroth Law of Thermodynamics

According to this law, two systems in thermal equilibrium with a third system separately are in thermal equilibrium with each other. Thus, if A and B are separately in equilibrium with C, that is
If T_A = T_C and T_B = T_C, then 
⇒  T_A = T_B
 i.e., the systems A and B are also in thermal equilibrium.

Zeroth Law

First Law of Thermodynamics

• Heat given to a thermodynamic system (ΔQ) is partially utilized in doing work (ΔW) against the surrounding and the remaining part increases the internal energy (ΔU) of the system.
  ΔQ = ΔU + ΔW
• First law of thermodynamics is a restatement of the principle conservation of energy.
• In an isothermal process, change in internal energy is zero (ΔU = 0), ΔQ = ΔW
• In an adiabatic process, no exchange of heat takes place, i.e., Δθ = O, ΔU = – ΔW
• In an adiabatic process, if gas expands, its internal energy and hence, temperature decreases and vice-versa.
• In an isochoric process, work done is zero, i.e., ΔW = 0, ΔQ = ΔU

Limitations of First Law of Thermodynamics

The first law of thermodynamics is important because it helps determine how much work can be done by transferring a certain amount of heat energy in a thermodynamic process. However, it has some limitations:

• It doesn't specify the direction in which heat will transfer.
• It doesn’t provide any details about the conditions necessary for heat to be transformed into work.
• It doesn't explain why all the heat energy cannot continuously be converted into mechanical work.

Second Law of Thermodynamics

The second law of thermodynamics gives a fundamental limitation to the efficiency of a heat engine and the coefficient of performance of a refrigerator. It says that efficiency of a heat engine can never be unity (or 100%). This implies that heat released to the cold reservoir can never be made zero.

Kelvin’s Statement

 It is impossible to obtain a continuous supply of work from a body by cooling it to a temperature below the coldest of its surroundings.

Clausius’ Statement

It is impossible to transfer heat from a lower temperature body to a higher temperature body without use of an external agency.

Planck’s Statement

It is impossible to construct a heat engine that will convert heat completely into work. All these statements are equivalent as one can be obtained from the other.

Entropy

 Entropy is a physical quantity that remains constant during a reversible adiabatic change.

Change in entropy is given by dS = δQ / T
where, δQ = heat supplied to the system
and T = absolute temperature.
Entropy of a system never decreases, i.e., dS ≥ 0.
Entropy of a system increases in an irreversible process.

Here is the table for significance, limitations and applications for first and second law of thermodynamics:

Heat Engine

A heat energy engine is a device which converts heat energy into mechanical energy.

Heat Engine

• A heat engine consists of three parts:
  (i) Source of heat at higher temperature
  (ii) Working substance
  (iii) Sink of heat at lower temperature
• Thermal efficiency of a heat engine is given by
  where Q₁ is heat absorbed from the source,
  Q₂ is heat rejected to the sink and T₁ and T₂ are temperatures of source and sink.
• Heat engine are of two types:
  (i)External Combustion Engine: In this engine fuel is burnt a chamber outside the main body of the engine. e.g., steam engine. In practical life thermal efficiency of a steam engine varies from 12% to 16%.
  (ii)Internal Combustion Engine: In this engine, fuel is burnt inside the main body of the engine. e.g., petrol and diesel engine. In practical life thermal efficiency of a petrol engine is 26% and a diesel engine is 40%.

Example 1: 

Example 2: