The Laws Of Thermodynamics

Table of Contents

The laws of thermodynamics are the cornerstone of energy science. They provide a framework for understanding how energy transfers and transforms within systems. These universal principles govern everything from the smallest atomic interactions to the largest cosmic phenomena. By studying these laws, we gain insights into processes like heat transfer. We understand engine efficiency and entropy. These insights are critical in fields ranging from engineering to environmental science. This blog delves into the fundamental laws of thermodynamics, exploring their meaning, implications, and practical applications in everyday life.

Zeroth Law of Thermodynamics

Suppose there are three bodies, Body “A,” Body “B,” and Body “C.”

Body “A” is in equilibrium with Body “B,” and Body “B” is in equilibrium with Body “C.” So we can say Body “A” is also in equilibrium with Body “C.” As shown in the figure below.

Illustration of the Zeroth Law of Thermodynamics, showing three systems in thermal equilibrium with each other, symbolizing that if two systems are in equilibrium with a third, they are in equilibrium with each other.

The zeroth law is crucial for creating thermometers. We can also say that a thermometer operates on the principle of the zeroth law.

First Law of Thermodynamics

Energy cannot be created. It cannot be destroyed. It can only convert from one form of energy to another.

One form of energy is often produced in a certain quantity. To achieve this, another kind of energy must be used in exactly the same amount.

“Energy of the universe is conserved.”

The first law of thermodynamics is also known as the Law of Conservation of Energy.

The First Law of Thermodynamics for a Cyclic Process

A system undergoing a cycle of changes has an algebraic summation of all work effects. This summation exactly equals the summation of all heat effects.

$\boxed{\sum W = \sum Q}$

Where Q is heat added to the system. Q is positive when heat is added to the system. It is negative when heat is rejected from the system.

W is work done by the system. W is positive when work is done by the system. It is negative when work is done on the system.

The First Law of Thermodynamics for a Closed System

The total energy change within the system equals the heat transferred into the system minus the work done by the system.

$\Delta E_{sys}= \Delta KE + \Delta PE + \Delta U = Q - W$

Where,

$\Delta E_{sys} =$ The total energy change within the system,
$\Delta KE =$ Change in Kinetic Energy,
$\Delta PE =$ Change in Potential Energy,
$\Delta U =$ Change in Internal Energy,
Q = Heat,
W = Work.

For a steady-state non-flow process in which there are no changes in the Kinetic Energy and Potential Energy, the above equation simplifies to

$\boxed{\Delta U = Q - W}$

For differential changes in the thermodynamic state of a closed system,

$\boxed{dU = dQ - dW}$

For an isolated system, $dQ = 0, dW = 0$

Thus, $dU = 0$

The First Law of Thermodynamics for Flow Processes or Open Systems

Consider an idealized flow system as shown in the figure below.

Diagram illustrating the First Law of Thermodynamics for an open system, showing the energy flow into and out of the system, emphasizing energy conservation where the change in internal energy equals the heat added minus the work done by the system.

Fluid is flowing through the apparatus from section 1 to section 2.

Here, Velocity = u, Specific Volume = V, Pressure = P, Height above the datum level = Z, Internal Energy = U, Mass of the fluid = m, Suffix 1 = Conditions at sections 1, Suffix 2 = Conditions at sections 2.

Heat (Q) is added per unit mass of the fluid through the heat exchanger. Shaft work (W_S) is extracted through a turbine or any other suitable device.

The internal energy of incoming fluid = mU₁, outgoing fluid = mU₂.

The potential energy of fluid at inlet = mgZ₁, and at outlet = mgZ₂.

Kinetic energy of incoming fluid = $\frac{1}{2} mu_1^2$ and outgoing fluid = $\frac{1}{2} mu_2^2$

Flow energy required to push in liquid at inlet = mP₁V₁. and at outlet = mP₂V₂

Total energy at section 1 = $mU_1 + mgZ_1 + \frac{1}{2} mu_1^2 + mP_1V_1$

Total energy at section 2 = $mU_2 + mgZ_2 + \frac{1}{2} mu_2^2 + mP_2V_2$

Net energy imparted to the fluid $= m Q - m W_S$

For unit mass of fluid, energy balance gives,

$U_1 + gZ_1 + \frac{1}{2} + u_1^2 + P_1V_1 + Q - W_S = U_2 + gZ_2 + \frac{1}{2}u_2^2 + P_2V_2$

$\Delta U + \Delta (PV) + g \Delta Z + \frac{1}{2} \Delta u^2 = Q + W_S$

$\Delta H + g \Delta Z + \frac{1}{2} \Delta u^2 = Q - W_S$

For most applications in thermodynamics, the Kinetic Energy and Potential Energy terms are negligible compared to the other terms. Thus the first law of thermodynamics for flow processes is reduced to

$\boxed{\Delta H = Q - W_S}$

Limitation of First Law of Thermodynamics

The first law of thermodynamics can not tell about the direction of the process.
It can not tell us about equilibrium conditions.
It can not give a qualitative difference between Heat and Work.

Second Law of Thermodynamics

Various Statements of Second Law of Thermodynamics.

Heat cannot pass by itself from a cold body to a hot body.

All spontaneous processes are, to some extent, irreversible and are accompanied by a degradation of energy.

Every system, when left to itself, will on an average, change toward a system of maximum probability.

Kelvin-Planck Statement

It is impossible to construct an engine that operates continuously in a cycle. Such an engine will produce no effect other than transferring heat from a single thermal reservoir. It will do this at a uniform temperature and perform an equal amount of work.

This statement can also be stated in the following way.

No apparatus can operate so its only effect is to convert heat absorbed by a system into work. It cannot do this completely within the system and surroundings.

Clausius Statement

Constructing a heat pump that operates continuously in a cycle is impossible. It cannot produce any effect other than transferring heat from a lower temperature body to a higher temperature one.

This statement can also be stated in the following way.

No process is possible which consists solely in the transfer of heat from one temperature level to a higher one.

A cyclic process cannot completely convert the heat absorbed by a system into work done by the system.

The mathematical statement of the second law of thermodynamics.

Consider the adiabatic closed system.

Path AB shows irreversible adiabatic operation.

Curve AB shows the pressure-volume relationship of the irreversible process. This process is spontaneous and occurs in a closed adiabatic system. There is no heat interaction between the system and its surroundings.

Work done by the system in this process is W_AB.

Let the system be brought back to its original state by a reversible path BA.

In this cyclic process, the overall energy and entropy changes must be zero.

So the change in internal energy $\Delta U$ must be zero.

From the first law of thermodynamics, we can say that net heat interaction must be equal to net work interaction.

$Q_{BA} = W_{AB} + W_{BA}$

If Q_BA is Positive, it means the system receives heat and converts completely into work. Which is a violation of the second law of thermodynamics.

So the Q_BA is either Zero or Negative.

Q_BA = 0 is not possible, because it represents a reversible cyclic process, but then here we considered it irreversible.

Q_BA is negative. The entropy change $\Delta S_{BA}$ for a reversible change can be calculated by $\frac{Q_{BA}}{T}$ , which is also negative.

$\Delta S < 0$

Therefore entropy change for the path (AB) must be positive. Because entropy change for the cycle as a whole is zero.

$\Delta S > 0$

The entropy change of an isolated system (adiabatic closed system) must be greater than or equal to zero. We can conclude this statement from the observations. This holds true in any process.

$\boxed{\Delta S_{\text{Isolated System}} \geq 0}$

It is the general mathematical statement of the second law of thermodynamics.

In nature, an Isolated system is made of a combination of system and surroundings. So, we can write,

$(\Delta S)_{ \text{System} } + (\Delta S)_{ \text{Surroundings} } \geq 0$

We can conclude that a spontaneous process occurs in a closed adiabatic system. This process is accompanied by an increase in entropy.

From the value of entropy change, we can determine the direction of change.

Process for which entropy change is Positive, Process is possible.

Process for which entropy change is Negative, Process is not possible.

Validity of the above equations can be verified for the process. In this process, an amount of heat (Q) is transferred from a heat source at temperature T₁. It moves to a heat sink at temperature T₂.

Entropy change of Heat Source $= - \frac{Q}{T_1}.$

Entropy change of Heat Sink $= \frac{Q}{T_2 }$

The total entropy change of heat source and sink is given by

$\Delta S_{ \text{Total} } = - \frac{Q}{T_1} + \frac{Q}{T_2} = Q \frac{T_1 - T_2}{T_1 T_2}$

If (Q) is positive and the heat transfer is carried out irreversibly when there exists a finite difference in the temperature of heat source and heat sink, $\Delta S_{\text{Total}}$ would be positive.

The process can be made reversible by lowering the temperature T₁ to a value only slightly greater than T₂.

In this case, the $\Delta S_{\text{Total}}$ approaches zero. For a true reversible process, the value becomes zero.

So, $\boxed{ (\Delta S)_{\text{Total}} \geq 0}$

It is also referred to as the general mathematical statement of the second law of thermodynamics.

The above equation is also known as the principle of increase in entropy.

Entropy increases in spontaneous processes within an isolated system. Therefore, these processes occur because they lead to a rise in entropy.

The universe is the perfect example of an isolated system, so all naturally occurring processes increase the entropy.

Therefore we can say that the entropy of the universe goes on increasing.

Combine statement of first and second law of thermodynamics is:-

The energy of the universe is conserved but the entropy of the universe keeps on increasing.

Read More: Use of second law of thermodynamics in Heat Transfer

Third Law of Thermodynamics

The absolute entropy is zero for all perfect crystalline substances at absolute zero temperature.

Entropy is the reference property and is absolute like pressure, volume, and temperature.

The third law of thermodynamics can be useful to calculate the absolute value of entropy.

Assign zero value of the entropy to perfect crystalline substance at absolute zero temperature.

Let the substance be in the vapor phase at temperature T.

Measure the heat capacity at different temperatures. Also, measure the latent heat of phase change from absolute zero temperature to T temperature.

Let the melting point of the substance is T_f, and the boiling point of the substance is T_b.

The entropy at temperature T can be calculated as follows,

$S = \int_{0}^{T_f}\frac{C_{PS}dT}{T}+\frac{\Delta H_f}{T_f}+\int_{T_f}^{T_b}\frac{C_{PL}dT}{T}+\frac{\Delta H_V}{T_b}+ \int_{T_b}^{T}\frac{C_{PG}dT}{T}$

Where, C_PS = Specific heat of solid, C_PL = Specific heat of liquid, C_PG = Specific heat of gas, $\Delta H_f =$ Latent heat of fusion, $\Delta H_V =$ Latent heat of vaporisation.

Conclusion

The laws of thermodynamics are more than just abstract scientific concepts. They are the rules that dictate the behavior of the universe. From improving energy efficiency to designing cutting-edge technologies, these principles have profound implications in science and engineering. Understanding and applying these laws empower us to tackle global challenges like energy sustainability and climate change. As we continue to innovate, the timeless wisdom of thermodynamics will remain a guiding force in shaping our world.

FAQs

What are the 4 laws of thermodynamics?

The four laws of thermodynamics are:

Zeroth Law: If two systems are in thermal equilibrium with a third, they are in equilibrium with each other, forming the basis for temperature measurement.
First Law: Energy cannot be created or destroyed; it can only change forms, emphasizing energy conservation.
Second Law: Entropy, a measure of disorder, always increases in an isolated system, leading to irreversible processes.
Third Law: As a system approaches absolute zero, the entropy approaches a constant minimum, implying absolute zero is unattainable.

Why is the second law of thermodynamics important?

The second law of thermodynamics is crucial because it explains the natural direction of energy flow, from ordered to disordered states, and sets the limit on efficiency for machines and processes. It provides the foundation for concepts like entropy, heat engines, refrigeration, and the irreversibility of natural processes.

How are the laws of thermodynamics applied in daily life?

The laws are applied in various everyday scenarios:
Zeroth Law: Thermometers measure temperature.
First Law: Energy conservation in power systems and household appliances.
Second Law: Cooling in refrigerators and inefficiencies in energy conversion.
Third Law: Cryogenics and reaching near-zero temperatures for scientific experiments.

What is entropy in simple terms?

Entropy is the measure of disorder or randomness in a system. It reflects the number of possible arrangements of particles and energy. High entropy means greater disorder, such as a melting ice cube spreading water and heat into its surroundings.

How does the first law of thermodynamics relate to energy conservation?

The first law states that energy cannot be created or destroyed, only transformed or transferred. For example, in a car engine, chemical energy from fuel converts into kinetic energy and heat, ensuring total energy remains constant.

Can entropy ever decrease?

Entropy can locally decrease in a system when energy is added or organized, like freezing water into ice. However, the total entropy of an isolated system or the universe always increases, aligning with the second law of thermodynamics.

What is absolute zero in the third law of thermodynamics?

Absolute zero (0 Kelvin or -273.15°C) is the theoretical temperature where all particle motion stops, and entropy reaches its minimum. Achieving absolute zero is impossible due to quantum mechanical constraints.

Why is the zeroth law important in thermodynamics?

The zeroth law establishes the concept of temperature and thermal equilibrium, enabling the development of thermometers and consistent temperature measurement across different systems.

What is the relationship between thermodynamics and energy efficiency?

Thermodynamics determines the efficiency of energy conversion processes. The first and second laws limit how much energy can be converted into useful work, guiding the design of efficient engines, power plants, and refrigeration systems.

How do the laws of thermodynamics affect the environment?

The laws help understand energy consumption, waste heat production, and entropy increase, critical in designing sustainable systems. For example, renewable energy technologies and waste heat recovery are grounded in these principles.