Free Bachelor of Mechanical Engineering Thermodynamics Questions and Answers — Questions and Answers

Question 1: What exactly is thermodynamics?

study of the conversion of mechanical energy to other forms of energy
study of the relationship between mechanical energy to other forms of energy
study of the conversion of chemical energy to other forms of energy
study of the relationship between heat and other forms of energy (Correct answer)

Correct answer: study of the relationship between heat and other forms of energy

This area of physical science is focused on how heat interacts with many types of energy, including mechanical, electrical, chemical, and others.

Question 2: Which of the following statements best describes a thermodynamics law?

Boyle’s Law of thermodynamics
Ideal Gas Law of thermodynamics (Correct answer)
Zeroth law of thermodynamics
Faraday’s Law of thermodynamics

Correct answer: Ideal Gas Law of thermodynamics

The main principles of thermodynamics are based on a set of four laws that, when applied to systems that fall within each law's bounds, are universally valid. These are what they are: The first law of thermodynamics, the second law of thermodynamics, and the third law of thermodynamics all apply.

Question 3: A cooler body cannot voluntarily give off heat to a hotter one. Which of the following statements about thermodynamics is true?

First law of thermodynamics
Zeroth law of thermodynamics
Third law of thermodynamics
Second law of thermodynamics (Correct answer)

Correct answer: Second law of thermodynamics

The second law of thermodynamics establishes the idea of entropy as a physical characteristic of a thermodynamic system. Even though the first law of thermodynamics states that energy must be conserved, entropy predicts the course of spontaneous events and establishes whether they are irreversible or impossible.

Question 4: Which of the following is a thermodynamics application?

Power plants (Correct answer)
Refrigerators
Gas compressors
All of the mentioned

Correct answer: Power plants

All refrigerators, deep freezers, commercial refrigeration systems, all types of air conditioning systems, heat pumps, and other devices are subject to the second law of thermodynamics. All types of air and gas compressors, blowers, and fans run on thermodynamic cycles. An essential area of thermodynamics is the investigation of the viability of using various renewable energy sources for residential and commercial applications.

Question 5: Which of the following laws of thermodynamics defines enthalpy?

Second law of thermodynamics
First law of thermodynamics
Fourth law of thermodynamics
Third law of thermodynamics (Correct answer)

Correct answer: Third law of thermodynamics

While enthalpy (H = U + PV) is fundamentally defined and derived from the First Law of Thermodynamics, which relates heat, work, and internal energy, the Third Law of Thermodynamics establishes a reference point for entropy at absolute zero. This absolute entropy is crucial for determining absolute values of thermodynamic potentials (like Gibbs free energy, G = H - TS), which incorporate enthalpy, especially in low-temperature contexts. Thus, the Third Law contributes to the complete thermodynamic characterization that includes enthalpy.

Question 6: What does the absolute thermodynamic temperature scale represent?

4K
1K
3K
0K (Correct answer)

Correct answer: 0K

The absolute thermodynamic temperature scale, known as the Kelvin scale, defines 0K as absolute zero. This is the theoretical point where all thermal motion of particles ceases, and a system possesses its minimum possible energy. It serves as a fundamental and universal reference point for all thermodynamic calculations, representing the lowest possible temperature.

Question 7: Which of the following is a Carnot theorem application?

Gas compressors
Heat engines
Gas turbine engines
All of the mentioned (Correct answer)

Correct answer: All of the mentioned

The Carnot theorem establishes the maximum theoretical efficiency for any heat engine operating between two temperature reservoirs. Its principles are universally applicable to all devices that convert thermal energy into mechanical work or vice versa. This includes heat engines (like internal combustion engines), gas compressors, and gas turbine engines, as they all involve thermodynamic cycles and energy transformations governed by these fundamental limits.

Question 8: What should the water pressure be to lift a piston with an outside pressure of 100 kPa if the piston has a cross sectional area of 0.01 m2 and is resting on the stops?

168kPa
318kPa
218kPa
198kPa (Correct answer)

Correct answer: 198kPa

To lift the piston, the internal water pressure must generate an upward force that overcomes both the external pressure acting downwards and any additional downward force, such as the piston's weight or resistance from the stops. Given an external pressure of 100 kPa, the required internal pressure of 198 kPa implies an additional effective pressure of 98 kPa (198 kPa - 100 kPa) must be overcome. This additional pressure accounts for the force exerted by the piston's weight or the stops, allowing the piston to just begin lifting.

Question 9: A sealed rigid jar containing 2 kg of water and 1 m³ in volume has a volume of 1 m³ at 100 °C. Now that the ship has warmed up. If a safety pressure valve is installed, at what pressure should it be set to achieve a maximum temperature of 200°C?

131.3 kPa
831.3 kPa
231.3 kPa
431.3 kPa (Correct answer)

Correct answer: 431.3 kPa

The sealed rigid jar maintains a constant specific volume of 0.5 m³/kg (1 m³ / 2 kg). As the water is heated from 100 °C to 200 °C, its state changes. By consulting superheated steam tables for water at 200 °C and interpolating for a specific volume of 0.5 m³/kg, the corresponding pressure is found to be approximately 431.3 kPa. This is the pressure at which the safety valve should be set to prevent the temperature from exceeding 200 °C.

Question 10: Which of the following is selected as the reference material for thermometers?

Gas (Correct answer)
Solid
Liquid
None of the mentioned

Correct answer: Gas

Gas thermometers, particularly constant-volume gas thermometers, are chosen as primary reference standards for temperature measurement. This is because gases at low pressures exhibit highly predictable and nearly ideal thermal expansion behavior, which is very uniform and well-understood. This makes them ideal for defining temperature scales and accurately calibrating other types of thermometers.

Question 11: What is the mechanical work's magnitude?

sum of the force and distance travelled parallel to the force
sum of the force and distance travelled perpendicular to the force
product of the force and distance travelled parallel to the force (Correct answer)
product of the force and distance travelled perpendicular to the force

Correct answer: product of the force and distance travelled parallel to the force

Mechanical work is defined as the energy transferred when a force causes a displacement. Crucially, only the component of the force that acts in the direction of the displacement contributes to the work done. Therefore, the magnitude of mechanical work is calculated as the product of the force and the distance traveled parallel to the direction of that force.

Question 12: Which of the following determines the engine's mechanical efficiency?

BP/IP (Correct answer)
IP/BP
1/(BP*IP)
(BP*IP)

Correct answer: BP/IP

Mechanical efficiency quantifies how effectively the power generated inside an engine's cylinders (indicated power, IP) is converted into useful power delivered at the crankshaft (brake power, BP). It is calculated as the ratio of brake power to indicated power (BP/IP). This ratio indicates the fraction of power available for external work after accounting for internal frictional losses within the engine.

Question 13: Which of the subsequent motion types does Shaft use when working?

horizontal motion
rotational motion (Correct answer)
vertical motion
none of the mentioned

Correct answer: rotational motion

A shaft is a fundamental machine element designed to transmit power and torque. Its primary function involves transferring energy through rotation. Therefore, when a shaft is working, its characteristic motion is rotational, enabling it to drive other components like gears, pulleys, or wheels.

Question 14: A system's heat input and output are denoted by the symbols and ______, respectively.

positive, negative (Correct answer)
negative, positive
positive, positive
negative, negative

Correct answer: positive, negative

In thermodynamics, a standard sign convention is used for heat transfer. Heat added to a system (heat input) is considered positive, as it increases the system's internal energy. Conversely, heat removed from a system (heat output) is considered negative, as it decreases the system's internal energy, ensuring consistency in energy balance equations.

Question 15: What is (mgz) capable of?

microscopic potential energy
macroscopic potential energy (Correct answer)
macroscopic kinetic energy
microscopic kinetic energy

Correct answer: macroscopic potential energy

The expression (m*g*z) represents the gravitational potential energy of an object with mass 'm' at a height 'z' in a gravitational field 'g'. This is a form of macroscopic potential energy because it relates to the position of an entire body or system within a force field, rather than the microscopic kinetic or potential energies of its individual particles.

Question 16: In relation to temperature, the enthalpy and internal energy are a

ideal gas (Correct answer)
water
steam
all gases

Correct answer: ideal gas

For an ideal gas, both internal energy (U) and enthalpy (H) are solely functions of temperature. This unique characteristic arises because, in an ideal gas, intermolecular forces are negligible, and the energy content depends only on the kinetic energy of the molecules, which is directly proportional to temperature. For real substances, U and H also depend on pressure or specific volume.

Free Bachelor of Mechanical Engineering Thermodynamics Questions and Answers

What exactly is thermodynamics?