Thermodynamics - Interview Questions

What is Thermodynamics?


Thermodynamics is the branch of physical science that deals with the relationship between heat and other forms of energy such as mechanical, electrical, chemical etc.

The key concept of Thermodynamics is that heat is a form of energy that corresponds to a proportional amount of mechanical work.

Define a thermodynamic system.


A thermodynamic system is a body of matter and/or radiation that is of interest and is under study.

A thermodynamic system is separated from its surroundings by a boundary or wall, which defines the permeabilities between the thermodynamic system and its surroundings. The surroundings can be other thermodynamic systems, or can be systems that are not thermodynamic.

In certain cases the boundary or wall can be purely notional or imaginary, which means that the thermodynamic system is permeable to all matter.

What are the different types of thermodynamic systems?


There are three kinds of thermodynamic systems.

Open - An open thermodynamic system can exchange both energy and matter with its surroundings. Example - A vessel of boiling water on a stove is an open thermodynamic system, since both energy (heat) and matter (water vapor) is lost to the surroundings.

Closed - A closed thermodynamic system can exchange only energy with its surroundings. Example - A pressure cooker on a stove with its lid closed and whistle in position is a closed thermodynamic system, since energy (heat) can leave the cooker but matter (water vapor) cannot. (But once the whistle blows it becomes an open system since matter (water vapor) now leaves the cooker in addition to heat.

Isolated - An isolated thermodynamic system can exchange neither energy nor matter with its surroundings. Example - A perfectly insulated cooler is an isolated system since neither energy (heat) nor matter is exchanged between the cooler and its surroundings.

What is the zeroth law of Thermodynamics?


The zeroth law of thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

What is the first law of Thermodynamics?


The first law of thermodynamics states that energy can neither be created nor destroyed in a system of constant mass, but it may be converted from one form to another.

Formula - Consider a cylinder with a frictionless piston at one end, containing a mass of gas at constant temperature. Consider heat q provided to the cylinder, which results in work w - which moves the piston. Then we can say that...

ΔU = q - w

Where ΔU denotes the increase in internal energy of the system, q denotes the amount of energy supplied to the system as heat, and w denotes the amount of thermodynamic work done by the system on its surroundings.

This is the mathematical formula for the first law of thermodynamics.

What is the second law of Thermodynamics?


The second law of thermodynamics states that the entropy of an isolated system always increases.

Entropy is a physical property of a thermodynamic system that predicts the direction of spontaneous processes, and determines if they are irreversible.

There are various other ways to state the second law of thermodynamics.

Carnot's principle - Carnot's principle based on Carnot heat engine states that the efficiency of a Carnot cycle depends on the temperatures on the two heat reservoirs irrespective of the working substance.

Clausius statement Clausius stated the second law of thermodynamics based on the relationship between heat transfer and work. Clausius statement states that - Heat can never pass from a colder to a warmer body without an external work performed on the system.

Kelvin statement Kelvin stated the second law of thermodynamics as - It is impossible for a self-acting machine, unaided by any external agency, to convey heat from one body to another at a higher temperature.

What is the third law of Thermodynamics?


The third law of thermodynamics states that - 'The entropy of a system approaches a constant value as its temperature approaches absolute zero.'

The third law of thermodynamics can also be stated in terms of a perfect crystal of a pre substance as - 'The entropy of a perfect crystal of a pure substance approaches zero as the temperature approaches zero.'

What is the difference between classical thermodynamics and statistical thermodynamics?


Classical thermodynamics is based on describing the thermodynamic system macroscopically, concerning with the relationships between bulk properties of matter. Classical thermodynamics does not deal with atomic or molecular level considerations.

Statistical or Boltzmann thermodynamics is based on describing the thermodynamic system using probability theory, based on the average behavior of a large number molecules and constituent atoms making up the system.

What is an adiabatic process in Thermodynamics?


Adiabatic process in Thermodynamics is a Thermodynamic process in which change occurs within a system as a result of transfer of energy to or from the system in the form of work only. No heat or mass is transferred between the systems.

What is an Isothermal processes in Thermodynamics?


Isothermal process in Thermodynamics is a Thermodynamic process in which temperature remains constant within a system, primarily with heat exchange with an external system.

What is an Isobaric process in Thermodynamics?


Isobaric process in Thermodynamics is a Thermodynamic process in which the pressure remains constant within a system.

What is heat capacity, and what is specific heat of substance?


Heat capacity is the amount of heat required to change the temperature of a specific amount of matter by 1 degree Celsius.

Specific heat of a substance is the amount of heat required to raise the temperature of 1 gram of substance by 1 degree Celsius.

What is Entropy?


Entropy is a measure of unavailable energy in a closed thermodynamic system that indicates the degree of disorder or uncertainty in a system.

Entropy is usually considered to be a measure of the system's disorder and is a property of the system's state.

Entropy varies directly with any reversible change in heat in the system and inversely with the temperature of the system

What is Thermodynamic equilibrium?


Thermodynamic equilibrium is an internal state of a single thermodynamic system or of multiple connected thermodynamic systems, in which there is no net flow of energy either within a system or within the systems.

What is Otto cycle?


Otto cycle is a thermodynamic cycle that describes the functioning of a spark ignition internal combustion engine.

The Otto cycle is made up of the following four internally reversible processes.

Process 0-1 - Intake Stroke

Process 1-2 - Compression stroke

Process 2-3 - Ignition phase

Process 3-4 - Expansion stroke

Process 4-1 - Heat rejection phase

Process 1-0 - Exhaust stroke

What is Diesel cycle?


Diesel cycle is a thermodynamic cycle that describes the functioning of a reciprocating internal combustion engine.

The diesel cycle is made up of the following four distinct processes.

Process 1-2 - Isentropic compression of the fluid

Process 2-3 - Constant pressure heating

Process 3-4 - Isentropic expansion

Process 4-1 - Reversible constant volume cooling

What is Carnot cycle?


Carnot cycle is a theoretical and ideal thermodynamic cycle that is the most efficient. Carnot cycle is made up of the following four processes.

Isothermal expansion

Adiabatic expansion

Isothermal compression

Adiabatic compression

What is Hess's law?


Hess's law, also known as the law of constant heat summation, states that the heat evolved or absorbed in a chemical process is the same whether the process takes place in one step or in several steps. This law is also known as the law of constant heat summation.

What is Joule's law?


Joule's law is based on Joule heating, which is the heat produced when an electric current passes through a conductor.

Joule's law states that the power of heat generated by an electric conductor is proportional to the product of its resistance and the square of the current.

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