Thermodynamics is a fundamental topic in mechanical engineering that every aspiring engineer needs to master. You can expect to face challenging thermodynamics questions during an engineering interview so make sure you’re fully prepared!
In this article, we’ll cover the most common and important thermodynamics interview questions you need to know. I’ll explain each question in simple terms and provide examples to help reinforce your understanding. With practice and preparation, you’ll gain the thermodynamics knowledge to impress your interviewers.
What is a Thermodynamic System?
A thermodynamic system refers to a specific quantity of matter or region in space that is being studied. Everything outside of the thermodynamic system is called the surroundings.
Thermodynamic systems can be categorized as:
- Open system – Exchanges both energy and matter with surroundings
- Closed system – Exchanges energy but not matter with surroundings
- Isolated system – Exchanges neither energy nor matter with surroundings
Being able to identify the type of system is key to analyzing thermodynamic processes.
What is a State Function vs Path Function?
State functions depend only on the current state of the system, not how it reached that state Internal energy, enthalpy, entropy are examples of state functions.
Path functions depend on the specific path by which a system arrived at its state. Heat and work are examples of path functions.
This is an important distinction when evaluating thermodynamic properties.
What is the Zeroth Law of Thermodynamics?
The zeroth law of thermodynamics states that if two systems are in thermal equilibrium with a third system separately, then they are also in thermal equilibrium with each other.
This law helps define the concept of temperature and enables the creation of thermometers.
What is the First Law of Thermodynamics?
The first law of thermodynamics conservation of energy applied to thermodynamic systems. It states that the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system.
Mathematically: ΔU = Q – W
This is a fundamental relation you must memorize and understand.
What is the Second Law of Thermodynamics?
There are several ways to state the second law of thermodynamics:
- Heat flows spontaneously from hot to cold objects, not vice versa
- No heat engine can have 100% efficiency
- The entropy of an isolated system always increases
Ultimately, the second law describes the irreversibility of natural processes and the increase of disorder in the universe.
What is Entropy?
Entropy (S) is a measure of the disorder or randomness of a thermodynamic system. Entropy quantifies the number of specific ways atoms or molecules can be arranged in a system.
As a system becomes more disordered, its entropy increases. Entropy also increases as temperature increases.
What is Enthalpy?
Enthalpy (H) is a thermodynamic property equal to the internal energy of a system plus the product of pressure and volume.
It is useful for accounting for energy changes during constant pressure processes and chemical reactions.
What is Helmholtz Free Energy?
Helmholtz free energy (A) is a thermodynamic property that measures the “useful” work obtainable from a closed thermodynamic system at constant temperature and volume. It is defined as:
A = U – TS
Where U is internal energy, T is temperature, and S is entropy.
What is Gibbs Free Energy?
Gibbs free energy (G) is another useful thermodynamic property. It represents the maximum amount of non-expansion work that can be extracted from a closed system at constant pressure and temperature.
G = H – TS
Where H is enthalpy, T is temperature, and S is entropy. Gibbs free energy determines the spontaneity of chemical reactions.
What is the Difference Between Q and W?
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Q represents the heat transfer between a system and its surroundings. It depends on the path a system takes.
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W represents the work done by or on a system. It also depends on the path taken.
Heat and work are path functions, whereas internal energy is a state function.
What is the Carnot Cycle?
The Carnot cycle is a theoretical thermodynamic cycle proposed by Nicolas Léonard Sadi Carnot that provides an upper limit on the efficiency of heat engines.
It consists of:
- Isothermal expansion (heat added)
- Adiabatic expansion
- Isothermal compression (heat removed)
- Adiabatic compression
All reversible engines working between the same temperatures follow the Carnot cycle.
What is the Otto Cycle?
The Otto cycle describes the operation of high-compression spark ignition engines, such as petrol/gasoline engines. Its stages are:
- Isochoric heat addition
- Isentropic expansion
- Isochoric heat rejection
- Isentropic compression
This thermodynamic cycle provides a conceptual model for the workings of a real engine.
What is the Diesel Cycle?
The diesel cycle models the operation of a diesel engine. It consists of:
- Isentropic compression
- Constant-pressure (isobaric) heat addition
- Isentropic expansion
- Constant-volume heat rejection
Again, this cycle helps understand the thermodynamic processes taking place in a reciprocating internal combustion engine.
What is Pump Work?
Pump work (Wp) refers to the work required to drive a fluid through a centrifugal pump. It is given by:
Wp = ρ x Q x (Pout – Pin)
Where ρ is the fluid density, Q is the volume flow rate, Pout is discharge pressure, and Pin is suction pressure. Understanding pump work is key for analyzing pumping systems.
What is Isentropic Efficiency?
Isentropic efficiency measures how closely a real process matches an ideal isentropic process where entropy remains constant.
It compares the actual work for a process to the minimum theoretical work required for a reversible, adiabatic process between the same inlet and outlet states.
What is Coefficient of Performance (COP)?
For heat pumps and refrigerators, the COP measures efficiency and is defined as:
COP = Qh/W
Where Qh is the heat transfer to the hot reservoir and W is the work required to drive the process. The higher the COP, the more efficient the system.
What is the Difference Between Steady State and Quasi-Static Processes?
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Steady state processes have time-independent properties in the frame of reference of the process. But they are not necessarily reversible.
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Quasi-static (reversible) processes are sequences of equilibrium states so close to each other that the process is reversible. They are often hypothetical.
Steady state processes can occur quickly, whereas quasi-static processes take place infinitely slowly to maintain equilibrium.
What is the Clausius-Clapeyron Equation?
The Clausius-Clapeyron equation describes the variation of saturation pressure with temperature for a pure substance transitioning between liquid and vapor phases:
ln(P2/P1) = (ΔHvap/R) x (1/T2 – 1/T1)
Where ΔHvap is the enthalpy of vaporization, R is the gas constant, and T1 and T2 are equilibrium temperatures. This relation is useful for analyzing phase change processes.
What is Joule-Thomson Expansion?
Joule-Thomson expansion is an adiabatic, irreversible process where a gas or liquid experiences a temperature change as it flows through a valve or porous plug while expanding from high to low pressure.
This process can be used to liquefy gases and has applications in refrigeration systems.
What is Throttling in Thermodynamics?
Throttling is a specific type of Joule-Thomson expansion process where a fluid passes through a restriction like a partly closed valve. This results in a decrease of pressure and temperature.
Throttling has applications in refrigeration and air conditioning systems.
What is the Critical Point?
The critical point defines the conditions (critical temperature and pressure) above which a gas cannot be liquefied by increasing pressure alone. At this point, distinct liquid and gas phases do not exist.
Substances at conditions exceeding the critical point are called supercritical fluids.
Practice Common Thermodynamics Problems
In addition to conceptual questions, you may be asked to work through thermodynamics problems during an interview. Practice solving problems involving the first and second laws, Carnot and heat engine efficiency, enthalpy, entropy, and more.
Having strong quantitative skills will demonstrate your proficiency in thermodynamics.
Summing Up Thermodynamics Interview Prep
You made it to the end of this monster thermodynamics guide! With diligent study and practice, you’ll be fully equipped to tackle any thermo questions thrown your way.
Remember to review key concepts like the laws of thermodynamics, Carnot cycles, thermodynamic properties, and processes. Mastering both qualitative an
What is heat capacity, and what is specific heat of substance?
It takes a certain amount of heat to raise the temperature of a certain amount of matter by 1 degree Celsius.
It takes a certain amount of heat to raise the temperature of one gram of a substance by one degree Celsius.
Entropy is a way to measure how much disorder or uncertainty there is in a closed thermodynamic system by looking at how much energy is not available.
Entropy is a property of the system state and is usually thought of as a way to measure how disordered a system is.
If there is a change in heat that can be undone, entropy changes directly. If the temperature of the system changes, entropy changes in the opposite direction.
What is an adiabatic process in Thermodynamics?
In thermodynamics, an adiabatic process is when energy is transferred into or out of a system only in the form of work. No heat or mass is transferred between the systems.
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