Second Law of Thermodynamics: Overview, Questions, Preparation

Updated on May 11, 2021 03:19 IST

Introduction

Can a book lying on the table jump to a certain height by itself? Certain things happen only in one direction. This is entropy. Entropy is the quantity of thermal energy in a system that requires it to be in a particular direction for a specific time. According to the second law, the entropy of an isolated system increases as time increases.

Table of Contents

The Second Law of Thermodynamics
Second Law of thermodynamics for Class 11
Illustrative Examples
FAQs

The Second Law of Thermodynamics

All spontaneous processes produce an increase in the entropy of the universe. This law helps find the theoretical limits for the performance of engineering systems such as refrigerators and heat engines.

Derivation

We consider changes in the entropy to understand why it increases or decreases. Entropy change in the universe is the sum of the changes in a system's entropy and surroundings.

For reversible process △S univ = 0 and for irreversible process >0. In reality, the reversible process never happens.

Heat Engines

Heat engines are the devices that undergo a cyclic process to convert heat to work.

Kelvin-Planck statement

It is impossible to operate in the thermodynamic cycle, whose sole result is the absorption of heat from the reservoir and the complete conversion of heat into work.

The statement describes the unattainable perfect engine. No heat engine has a thermal efficiency of 100%.

Refrigerators and Heat pumps

In nature, we observe the transfer of heat from high-temperature regions to low-temperature regions. The reverse cannot occur by itself. Refrigerators are cyclic devices that can transfer heat from low to high temperatures.

The performance of refrigerators and heat pumps is measured in terms of the coefficient of performance(α).

Clausius Statement

It is impossible for a system whose sole result is the transfer of heat from a colder object to a hotter object. For instance, a refrigerator will not operate unless it has a compressor.

Kelvin-Planck and Clausius statements are negative statements and cannot be proved.

The second law gives a limitation to the efficiency of the heat engine and the coefficient of performance.

Second Law of thermodynamics for Class 11

This topic's questions focus more on the problems of heat engines and refrigerators' efficiency, the relation between enthalpy and entropy. The second law has a weightage lower than 10 marks.

Illustrative Examples

1. If the coefficient of performance is 5 and operates at room temperature 27०c. Find the inside temperature of a refrigerator?

Temperature in kelvin 27+ 273 K= 300K

5= T/300-T

1500-5T = T

6T= 1500;

T =250K ; 250-273 = -23०c.

2. If 1 kg of ice at 0℃ (273K) is in 0.5kg water at 20℃ (293K). If the ice melts at 0℃, what is the change in the entropy?

1(3.33×10⁵ J/kg)/273K = +1219 J/K

3. A heat engine receives 2000 KJ of heat from a constant temperature source at 450 K. If the surroundings are at 250 K, determine the availability of heat energy.

△S = Q1/T1+T0

2000/450+250 =2.85KJ/K

Availability of Heat energy A =Q1-T0(△S)

2000-250(2.85)=1287.5KJ

FAQs

1. What are the applications of the second law?

It identifies the direction of the process. It helps determine the quality of energy and the theoretical performance limits of engineering systems.

2. What device violates the second law of thermodynamics?

The device that violates the second law is called a perpetual‐motion machine of the second kind (PMM2).

3. What is the thermal efficiency of a heat engine?

It is the ratio of work done by the engine to the energy absorbed at a higher temperature.

4. Which is the essence of the Second Law of Thermodynamics?

The crux of the second law is the entropy principle. It says that adiabatic processes can quantify, by an entropy function of all equilibrium states, the increase essential and enough for this action to take place.

5. What are the properties of heat engines operating between two reservoirs?

A reversible engine operating between two reservoirs will have greater efficiency than an irreversible engine. All reversible engines have the same efficiency.