Basics of Thermodynamics

Thermodynamics is the study of energy, energy transformations, and energy’s relationship to matter. Thermal systems are analysed using the governing conservation equations, which are Conservation of Mass, Conservation of Energy (first law of thermodynamics), the second law of thermodynamics, and the property relations. Energy can be defined as the ability to effect change.

Study of thermodynamics can be done through two different approaches:

Macroscopic approach:

• In this approach, time average behaviour of molecules is taken into consideration.
• This approach results in Classical Thermodynamics.
• This approach is applied till the continuum concept is valid

Microscopic approach:

• In this approach, individual molecular behaviour is under consideration
• This approach results in Statistical Thermodynamics
• It is applied where the gases have low density

Know more about the Forms of Energy here.

System and Surroundings

System:

A thermodynamic system is defined as a quantity of matter or a region in space that has been chosen for investigation. Everything external to the system is known as surroundings. Laws of thermodynamics are applied on systems only.

Surroundings:

The surroundings are the masses or regions that exist outside of the system.

Boundary:

• A boundary is a physical or fictitious surface that separates a system from its surroundings.
• A system’s boundaries can be fixed or movable.
• The boundary has no thickness, no mass, and no volume.

Types of System

• Closed systems: It has a fixed amount of mass and no mass can pass through its boundary. However, energy can cross the boundary in the form of heat or work, and the volume of a closed system wouldn’t have to be fixed.

• Open system: A system that allows both mass and energy to be transferred. It is also referred to as a control volume system. Pumps, compressors, turbines, and other similar devices are examples.

• Isolated system: A closed system that has no way of communicating with its surroundings.

• Rigid system: A closed system that only communicates with its surroundings through heat.
• Adiabatic system: It’s a closed or open system in which heat is not used to exchange energy with the environment.

Know more about Nuclear Energy here.

Properties of a System

• A property is a term used to describe any characteristic of a system. The substance is assumed to be a continuous, homogeneous matter with no microscopic holes in classical thermodynamics.
• As long as the volumes and length scales are large in comparison to the intermolecular spacing, this assumption holds.
• Intensive properties: Temperature, pressure, and density are examples of variables that are independent of a system’s size (mass). They aren’t interchangeable.
• Extensive properties: Properties like mass, volume and total energy U are all quantities that are dependent on the size of the system. They complement each other.

Thermodynamic Equilibrium

 

• Thermodynamic Equilibrium is achieved when all of a system’s properties have fixed values at any given state. As a result, even if the value of one attribute changes, the state will change.
• There are no imbalanced potentials (or driving forces) within the system in an equilibrium condition. When a system in equilibrium is isolated from its environment, it does not change.

Types of Equilibrium

• Mechanical equilibrium: No unbalanced force within the system and between the system and the surroundings. Pressure of the system is constant everywhere.
• Chemical equilibrium –:Chemical reaction potential should be absent within the system.
• Thermal equilibrium : A system will be in thermal equilibrium with the surroundings when the temperature of the system and the surroundings is uniform throughout.
• Phase equilibrium: Mass of each phase remains constant with time within the system.
• Electrostatic equilibrium: No unbalanced charge within the system.

Read more about Types of Thermodynamic Process, here

Processes and Cycles

A process is any transition a system makes from one equilibrium state to another, and a path is the set of states that a system passes through during that transition.

When a system returns to its initial condition at the end of a process, it is said to have completed a cycle.

Types of Process

Quasi‐equilibrium process: It can be thought of as a sufficiently slow process that allows the system to self-adjust and stay infinitesimally close to equilibrium at all times. The quasi-equilibrium process is an idealised version of the actual process. We use quasiequilibrium processes to model real processes.

The prefix iso- is used to denote a process that has a constant property.

• Isothermal process: the temperature remains constant
• Isobaric process: the pressure remains constant
• Isometric process: the specific volume remains constant.

Reversible process: In a reversible process the system as well as its surrounding returns back to its initial state, without any change in the universe.

Irreversible process: A thermodynamic process that does not return back to its initial state is termed as an irreversible process.

Temperature

• The direction of energy transfer as heat is indicated by temperature.
• Thermal equilibrium is achieved when the temperatures of two bodies are equal. The only prerequisite for thermal equilibrium is temperature equality.

Zeroth Law of Thermodynamics

If systems A and B are in thermal equilibrium with system C separately, they must also be in thermal equilibrium with one another.

The 0th law makes a thermometer possible.

Thermometers and its Types

• A thermometer is a device that is used to measure temperature. It may determine the temperature of a solid, like food, a liquid, like water, or a gas, like air.
• Celsius, Fahrenheit, and Kelvin are the three most used temperature measuring units.

Types of thermometers:

• Mercury glass tube thermometer
• Thermistor or resistance thermometer
• Constant volume gas thermometer
• Constant pressure gas thermometer
• Thermocouple

Important Thermodynamics Topics for Competitive Exams

Please find the following list of articles. Get access to the below-mentioned Thermodymanics articles through the links given in the table. As we work on more articles, this list will get updated, so don’t forget to regularly check out the Testbook website!

Topic Name	Explanation
Laws of Thermodynamics	Zeroth Law of Thermodynamics, First Law of Thermodynamics, Entropy, Second law of Thermodynamics, Third Law of Thermodynamics
Application of Thermodynamics	Heat Engine, Thermal Efficiency of Heat Engine, Heat Pump, Refrigerator, Carnot Cycle, Carnot Theorem
Types of Thermodynamic Process	Cyclic Process, Isothermal Process, Adiabatic Process, Reversible and Irreversible Process
Entropy	What is Entropy, Entropy Change For a System, Implementation of Principle of Entropy, Application in First and Second Law of Thermodynamics, Different Processes In Terms Of Entropy

Thermodynamics Objective Questions

1. ___ is the condition for a reaction to occur spontaneously :

1. ΔH must be negative
2. ΔS must be negative
3. (ΔH-TΔS) must be negative
4. (-ΔH+TΔS) must be negative

2. The thermal efficiency of a petrol engine is…….

1. 15%
2. 30%
3. 50%
4. 45%

3. In thermodynamics, a process is called reversible when

1. Surroundings and systems change into each other
2. There is no boundary between the system and surroundings
3. The surroundings are always in equilibrium with the system
4. The system changes into the surroundings spontaneously

4. Gas laws are applicable to……..

1. Gases as well as vapours
2. Gases alone and not to vapours
3. Gases and steam
4. Gases and vapours under certain conditions

5. The term N.T.P. stands for____

1. Natural temperature and pressure
2. Nominal temperature and pressure
3. Normal temperature and pressure
4. Neutral thermodynamic pressure

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