Understanding Thermodynamics

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Thermodynamics literally means “moving of heat”. The science of thermodynamics is concerned with heat and its transformation to mechanical energy. In the course of our study of thermodynamics, we shall encounter terms such as:

System

The object or collection of objects under study is called a system. This could be anything from steam, an engine, or even the human body. The system is the focus of thermodynamic analysis.

Universe

The universe in thermodynamics refers to the combination of the system and its surroundings. It encompasses everything that interacts with the system.

Surrounding

The surrounding includes everything outside the system that can exchange energy or matter with it. This can include the environment, other systems, or any external factors that influence the system's behavior.

Internal Energy

Internal energy is the total energy contained within a system. It is the sum of the kinetic energies of the molecules of a body and the potential energy due to intermolecular forces. Internal energy is a key concept in understanding how energy is transferred and transformed within a system.

Reversible and Irreversible Processes

• A reversible process is one in which the system and its surroundings can be returned to their initial states before the process occurred. Examples include phase changes like fusion, vaporization, and sublimation.
• An irreversible process, on the other hand, cannot return to its original state. An example of this is the aging process, where changes are permanent and cannot be undone.

These definitions provide a foundational understanding of the key concepts in thermodynamics, essential for further study in the field.

The Four Laws of Thermodynamics

Zeroth Law

The Zeroth Law states that if object A is in thermal equilibrium with object B, and object A is in thermal equilibrium with a third object C, then object B must be in equilibrium with object C. This law is similar to the intransitive law of elementary algebra.

First Law of Thermodynamics

The First Law states that when heat is added to a system, some of it remains in the system, increasing its internal energy, while the rest leaves the system as the system does work. This is essentially the law of conservation of energy; energy can be transformed from one form to another but cannot be created or destroyed. The first law may be expressed in symbols as:

H = ΔU + W

Where:
H = heat, ΔU = change of internal energy, W = work. In using this equation, the following sign convention must be applied: H is + when added to the system and – when removed from the system. W is + if done by the system and – if done on the system.

Example: In a car engine, part of the energy stored in gasoline is transformed into useful work in moving the car. Some of the energy heats up the car, while the rest is given off as heat in exhaust gases.

SAMPLE PROBLEM: 

The internal energy decreases by 700 J when it absorbs 2,000 J of heat. How much work is done during the process? Is the work done by the system or on the system?

Second Law of Thermodynamics

The Second Law of Thermodynamics highlights the limitations of energy transformations and the directional nature of physical processes. It can be expressed in three distinct statements:

1. Kelvin-Plank Statement

This statement asserts that no heat engine can completely convert heat energy into work. In practical terms, this means that there will always be some energy that cannot be transformed into useful work, indicating that 100% efficiency is impossible for heat engines.

2. Clausius Statement

The Clausius statement emphasizes the direction of heat transfer, stating that heat flows naturally from hot objects to cold objects. This implies that heat cannot spontaneously flow from a colder body to a hotter body without external work being performed on the system.

3. Entropy Statement

The entropy statement introduces the concept of entropy as a measure of disorder. It states that:

• When a reversible process occurs, the total entropy of the universe remains constant.
• When an irreversible process occurs, the total entropy of the universe increases.

This illustrates the natural tendency of systems to evolve towards a state of greater disorder or entropy over time.

Each version of the Second Law of Thermodynamics underscores the inherent inefficiencies in energy transformations and the fundamental principles governing thermodynamic processes.

The thermodynamic measure of disorder is entropy, represented by S. Examples where entropy increases include:

• When heat is added to an object, causing the molecules to move faster.
• When gas flows from a container under high pressure to a space under low pressure, similar to spraying air fresheners in a big room.
• When ice melts, allowing the molecules to move freely throughout the liquid.

Changes in Entropy and Its Calculation

The changes in entropy (ΔS) for a process occurring at constant temperature can be defined using the following equation:

Entropy Change at Constant Temperature

$$\mathrm{\Delta }S=\frac{H}{T}$$

Where:

• ΔS = Change in entropy
• H = Heat added or released during the process
• T = Temperature in Kelvin

Entropy Change for a Solid or Liquid

When heat is added or removed from a solid or liquid with mass $m$ and specific heat $c$, and its temperature changes from $T_1$ to $T_2$, the change in entropy can be expressed as:

$$\mathrm{\Delta }S=mc\ln \left(\frac{T_2}{T_1}\right)$$

Where:

• ΔS = Change in entropy
• m = Mass of the substance
• c = Specific heat capacity of the substance
• $T_1$ = Initial temperature
• $T_2$ = Final temperature

SI Unit of Entropy

The SI unit of entropy is Joules per Kelvin (J/K), which reflects the amount of energy dispersed per unit temperature.

These equations provide a fundamental understanding of how entropy changes in response to heat transfer and temperature variations in thermodynamic processes.

SAMPLE PROBLEM: Find the change in entropy when 3.5 kg of ice melts at °C.

Third Law of Thermodynamics

The Third Law states that it is impossible to attain absolute zero temperature. It is often expressed as: “You cannot win, you cannot break even, nor can you get out of the game.” It can also be stated regarding the properties of systems in equilibrium at absolute zero temperature: the entropy of a perfect crystal at absolute zero is exactly equal to zero.

The following table summarizes the governing laws for various thermodynamic processes involving a perfect gas:

Name of Process	Governing Law
Constant Volume	$\frac{p_1}{T_1}=\frac{p_2}{T_2}$
Constant Pressure	$\frac{v_1}{T_1}=\frac{v_2}{T_2}$
Adiabatic	$p{v}^{\gamma }=C$
	where $\gamma =\frac{c_p}{c_v}$
	Also, $\frac{T_2}{T_1}={\left(\frac{p_2}{p_1}\right)}^{\frac{\gamma -1}{\gamma }}{\left(\frac{v_2}{v_1}\right)}^{1-\gamma }$
Polytropic	$p{v}^n=C$
	$\frac{T_2}{T_1}={\left(\frac{p_2}{p_1}\right)}^{\frac{n-1}{n}}={\left(\frac{v_2}{v_1}\right)}^{1-n}$
Isothermal	$p_1{v}_1=p_2{v}_2$

This table provides a concise overview of the relationships between pressure, volume, and temperature for different thermodynamic processes involving a perfect gas.

READ MORE ON : AREA 3 (AB STRUCTURES & ENVIRONMENT ENGINEERING & BIOPROCESS ENGINEERING &  ALLIED SUBJECTS)