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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 TA = TC and TB = TCthen 
  TA = TB
 i.e., the systems A and B are also in thermal equilibrium.

Zeroth LawZeroth 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

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

Question for Laws of Thermodynamics & Heat Engine
Try yourself:
According to the Zeroth Law of Thermodynamics, two systems that are separately in thermal equilibrium with a third system are also in thermal equilibrium with each other. Which of the following statements best describes this law?
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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.

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

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.

Question for Laws of Thermodynamics & Heat Engine
Try yourself:
Which statement is equivalent to Kelvin's statement of the second law of thermodynamics?
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Here is the table for significance, limitations and applications for first and second law of thermodynamics:

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

Heat Engine

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

Heat EngineHeat 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 Laws of Thermodynamics & Heat Engine | Physics Class 11 - NEET
    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: 

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

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

Example 2: 

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

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

Question for Laws of Thermodynamics & Heat Engine
Try yourself:
Which type of engine consists of fuel being burnt outside the main body of the engine?
View Solution

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FAQs on Laws of Thermodynamics & Heat Engine - Physics Class 11 - NEET

1. What is the Zeroth Law of Thermodynamics and why is it important?
Ans. The Zeroth Law of Thermodynamics states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This law is important because it establishes the concept of temperature and allows for the measurement of temperature in a consistent manner across different systems.
2. How does the First Law of Thermodynamics relate to energy conservation?
Ans. The First Law of Thermodynamics, also known as the Law of Energy Conservation, states that energy cannot be created or destroyed in an isolated system. It can only change forms, such as from kinetic energy to potential energy or from heat to work. This principle is fundamental in understanding energy transfer in thermodynamic processes.
3. What is the Second Law of Thermodynamics and what does it imply about energy transfer?
Ans. The Second Law of Thermodynamics states that the total entropy of an isolated system can never decrease over time. This implies that energy transfer is not 100% efficient; some energy is always lost as heat, leading to increased disorder in the system. It explains why heat engines cannot convert all absorbed heat into work.
4. What is a heat engine and how does it operate according to the laws of thermodynamics?
Ans. A heat engine is a device that converts thermal energy into mechanical work by exploiting the temperature difference between a hot and a cold reservoir. According to the laws of thermodynamics, particularly the First and Second Laws, a heat engine absorbs heat from the hot reservoir, performs work, and expels waste heat to the cold reservoir, with efficiency limited by the Second Law.
5. How do the laws of thermodynamics apply to real-world applications, such as refrigeration?
Ans. The laws of thermodynamics are crucial in real-world applications like refrigeration. The Second Law governs the refrigeration cycle by requiring work input to remove heat from a cold area and expel it to a warmer area. The efficiency of refrigeration systems is analyzed using the principles of the First Law to ensure energy conservation while minimizing entropy production.
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