Thermal energy is the energy that a system has because of the energy present in its moving particles. Several particles interact amongst themselves in complicated ways, but if they meet the right criteria, the system is said to be in equilibrium. This leads to the creation of the first, second, and third laws of thermodynamics.

Introduction

The branch of science that deals with the quantitative relationship between heat and other forms of energy is called thermodynamics. 

The chemical energy stored by the molecules generates heat in the chemical reactions when a fuel burns in the air. Herein, several energies are interrelated and may transform into each other, and the study of this energy transformation forms the basis of thermodynamics.

The first, second, and third laws of thermodynamics apply only when a system is in equilibrium or moves from one equilibrium state to another.

Thermodynamic Terms

1. System is the representation of the universe where the observations are held.
2. Surroundings are the parts of the universe other than the system.
3. Thermodynamic equilibrium is a system where bigger properties remain unchanged with time.
4. Boundary is the wall separating the surroundings and the system.
5. Thermal equilibrium occurs when the flow of heat does not take place.
6. Mechanical equilibrium occurs if no mechanical work is done by one part of the system on another part of the system. It is assumed to be in mechanical equilibrium if the pressure remains constant.

Thermodynamic Processes

1. Isothermal process is the process in which the temperature is constant, i.e., (dT = 0, Δ U = 0).
2. Isochoric process is the process in which the volume is constant, i.e., (Δ V = 0).
3. Isobaric process is the process in which the pressure is constant, i.e., (Δp = 0).
4. Adiabatic process is the process in which there is no exchange of heat by the system with the surroundings, i.e., (Δq = 0).
5. Cyclic process is the process in which a system returns to its original state after undergoing a series of changes, i.e., Δ Ucyclic = 0; Δ Hcyclic = 0.
6. Reversible process is the process that occurs in infinite steps and can be reversed by changes in the state function.
7. Irreversible process is the process in which the energy increases, and it cannot be reversed. All-natural processes are irreversible.

Types of Systems

1. Open system is the system in which both the energy and matter can be exchanged with the surroundings.
2. Closed system is the system in which only the energy can be exchanged with the surroundings.
3. Isolated system is the system in which neither energy nor matter can be exchanged with the surroundings.

Internal Energy (E or U) is the total energy within the substance. It is the sum of several types of energies like vibrational energy, translational energy, etc.

For an exothermic process, ΔU = -ve, whereas for an endothermic process, ΔU = +ve.

The work done by a system is the quantity of energy that is exchanged between a system and its surroundings.

The heat in thermodynamics is the kinetic energy of the molecules of the substance.

The Zeroth Law of Thermodynamics or the Law of Thermal Equilibrium

This law states that if the two systems are in thermal equilibrium with a third system, they are in turn in thermal equilibrium with each other. The temperature is recorded to find out whether the system is in thermal equilibrium or not.

The laws of thermodynamics are as follows:

• The first law of thermodynamics: According to this law, when energy moves in or out of a system, the internal energy of the system changes according to the law of conservation of mass.
• The second law of thermodynamics: According to this law, as an isolated system, the state of the entropy of the entire universe will always increase over time.
• Third law of thermodynamics: According to this law, the entropy of a perfect crystal at absolute zero temperature is zero.

The First Law of Thermodynamics

The first law is often formulated as ΔU = Q − W, where:

• ΔU refers to a change in the internal energy of a closed system.
• Q refers to the amount of energy supplied to the system as heat.
• W denotes the amount of thermodynamic work done by the system on its surroundings.

Now if,

• Q is positive: There is a net heat transfer into the system. As a result, it adds energy to the system.
• W is positive: There is work done by the system. As a result, it takes energy from the system. 

The limitations of the first law of thermodynamics are as follows:

1. There is no information about the direction of the flow of heat.
2. There is no report on the spontaneity of the process.
3. The reverse process is not possible.

The first law of thermodynamics example:

Question: Determine the internal energy of a system that has constant volume, and the heat around the system is increased by 50 J.

Solution: Given, q = 50 J

Since the gas has constant volume, ΔV = 0

So work done 

W = PΔV = 0

The equation for internal energy is ΔU = q + W

ΔU= q + 0

ΔU = q = 50 J

The Second Law of Thermodynamics

The second law is also known as the “law of increased entropy.”

Mathematically, 

ΔSuniv > 0

where ΔSuniv is the change in the entropy of the universe.

Some factors may cause an increase in the entropy of the closed system. These are as follows:

1. In a closed system, as the mass remains constant, there occurs an exchange of heat with the surroundings which may bring about a change in the heat content that creates a disturbance in the system, thereby increasing the entropy of the entire system.
2. Internal changes may occur in the movements of the molecules of the system. This may lead to disturbances that may further cause irreversibilities inside the system, resulting in the increment of its entropy.

The second law of thermodynamics example:

Question: A heat pump uses 300 J of work to remove 400 J of heat from the low-temperature reservoir. How much heat is being delivered to a higher temperature reservoir?

Solution:

W = 300 J

QC = 400 J

QH = W + QC

QH = 300 J + 400 J

QH = 700 J

The heat delivered to the higher temperature reservoir is 700 J.

The Third Law of Thermodynamics

The temperature at which all particle motion almost stops is absolute zero. It is the lowest possible temperature, and is equivalent to -273.15 degrees Celsius, -459.67 degrees Fahrenheit, and 0 Kelvin.

These laws are being observed regularly in everyday life.

Every day, ice needs to be maintained at a temperature below the freezing point of water to remain solid. The first and second laws of thermodynamics act in this process. It is the total amount of heat in the system that is the same and has just moved towards the equilibrium at the same temperature.

An example of a closed system in thermodynamics can be a room full of sweaty people where no heat is lost and is only transferred, thus approaching equilibrium with maximum entropy.

Conclusion

Thermodynamics deals with energy changes in chemical or physical processes which enable us to study these changes quantitatively to make successful predictions. For such purposes, the universe is divided into systems and surroundings. The chemical or physical processes lead to the formation of heat (q), part of which may be converted into work (w). Entropy will increase with softer and less rigid solids, that is, solids that contain larger atoms and solids with a complex molecular structure.

In a thermodynamic sense, a living cell can be viewed as a low-entropy system that is not in equilibrium with the surroundings and is capable of replicating itself. A constant input of energy is needed to maintain the cell’s highly organised structure, its wide range of biomolecules, and its intricate system of thousands of chemical reactions.