Understanding heat and entropy

Heat and entropy are the two most important concepts in physics and chemistry. These two concepts will help us understand the different thermodynamic processes happening around us.

There are many different forms of energy such as electric, kinetic, potential and magnetic energy. Heat is also a type of energy. Heat energy plays an important role in various phenomena in nature. Heat is also responsible for the convection of water and air. This convection causes rain and wind flow. 

Entropy is defined as the disorderliness caused by a system. More disorderliness implies high entropy of the system.   

Heat

Heat is a particular type of energy that is caused due to molecular vibrations in a material. Temperature is a measure of the heat difference between the two bodies. A body is considered hot or cold depending on the temperature difference between the two.

Whenever the temperature of a body is raised, the molecular vibrations increase inside the material. The vibrational energy is converted into heat energy which in turn causes the rise in temperature of the body.

Whenever the heat is exchanged between the two bodies, the temperature of the body either rises or falls or remains constant.

• If heat is absorbed by the body, then the temperature rises. 
• If heat is released by the body, then the temperature decreases.
• A material can absorb or release heat without rising or falling in temperature. This happens whenever heat exchange causes a change in the physical state of the system.

Heat can be converted to other forms of energy and vice versa. For example, heat energy is converted into electricity in thermal power plants. Heat is converted into kinetic energy in vehicles. Heat is converted into light energy in electric bulbs.    

 Unit of heat

The SI unit of heat is joules.

The CGS unit is calories.

Unit of temperature

SI unit of temperature is Kelvin (K)

CGS unit is ℃ (degree celsius)

Entropy

In simple terms, entropy is the measure of disorderliness in a system. Apart from physics and chemistry, this concept finds its importance in other fields such as economics, life sciences and atmospheric sciences. Entropy is often associated with uncertainty in a system.

The more disordered a system is, the more is its entropy. For example, water exhibits three different phases namely ice, liquid and steam. In the case of ice, the molecules are close together and do not randomly move in any direction. In the case of the liquid state of water, the molecules can move slightly as compared with ice molecules. In the case of steam, all molecules move rapidly in different directions. Thus, disorderliness is more in the case of steam than liquid and ice. Thus, the entropy of steam is higher than that of liquid water and the entropy of liquid water is higher than that of solid ice.

Quantitatively, entropy is defined as the amount of heat exchanged reversibly and isothermally divided by the absolute temperature. Thus,

     S=Qrev/T

The total change in entropy is the sum of the change in entropy of the system and the change in entropy.

Stotal=Ssystem+Ssurrounding 

Change in entropy in different processes 

• Isothermal process:

In an isothermal process, the temperature change is zero (T=0). According to the first law of thermodynamics,

Q=U+W=W …….(∵ U=0)

Thus entropy change for the isothermal process is given as,

S=W/T

• Reversible adiabatic process:

For the reversible adiabatic process, 

Qrev=0 

Thus, the change in entropy is given as,

Ssys=0

• Isochoric process:

For the isochoric process, the volume change is zero (V=0).

Thus according to the first law of thermodynamics,

Q=U+W=U

The entropy is given as,

S=U/T

• Isobaric process:

For isobaric processes, change in pressure is equal to zero

Thus the change in entropy is given as,

S=Q/T=(U+W)/T

∴ S=Cvln(T2/T1)+R ln(V2/V1)

Where,

Cv = Specific heat capacity of a system at constant volume

T2 = Final temperature of the system

T1 = Initial temperature of the system

V2 = Final volume of the system

V1 = Initial volume of the system

Conclusion

In this article, we studied the concept of heat and entropy and their importance in daily life. We also discussed the change in entropy in different processes.