Test: Basic Concepts of Thermodynamics - 1 - Mechanical Engineering MCQ

A heat engine is a system that converts heat into work by taking heat from the reservoir ( hot body) to carry out some work. There is a discharge of some heat to the sink (cold body).

Hot Air Balloon is used to lift the weight using heat.

A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated. ... This effect is used in a range of mechanical and electrical devices.

Heat and work depend upon the path traversed to move from one thermodynamic state to another. This is why they are known as path functions.

It is free expansion. Since vacuum does not offer any resistance, there is no work transfer involved in free expansion.

Answer:
Definition of a Single Phase System:
A single phase system refers to a system in thermodynamics that consists of a single phase, meaning that the system has a uniform composition and properties throughout.
Distinguishing Characteristics of Different Systems:
To determine the type of system, it is important to understand the distinguishing characteristics of each type:
1. Closed System:
- A closed system is one that allows the transfer of energy (heat and work) but not mass across its boundaries.
- The mass of the system remains constant.
- Heat and work can be exchanged with the surroundings.
- Examples: A piston-cylinder system, a refrigeration cycle.
2. Open System:
- An open system is one that allows the transfer of both energy and mass across its boundaries.
- Both heat and work can be exchanged with the surroundings.
- Examples: A boiling pot of water, a flowing river.
3. Isolated System:
- An isolated system is one that does not exchange energy or mass with its surroundings.
- It is thermally and mechanically isolated.
- The total energy and mass of the system remain constant.
- Examples: The universe, a perfectly insulated container.
4. Homogeneous System:
- A homogeneous system is one that has a uniform composition throughout.
- It is characterized by a single phase.
- Examples: A pure substance in a single state (liquid, gas, or solid).
Identification of the Correct Answer:
Based on the definitions and characteristics mentioned above, the correct answer is option D: homogeneous system. A single phase system is a type of homogeneous system where the composition and properties are uniform throughout the system.

There are basically two reasons of irreversibility of a thermodynamic process.

1. Involvement of dissipative effect during the process.
2. Lack of thermodynamic equilibrium during the process.

Option (b) 2 and 3 is  correct. 

Specific enthalpy and pressure 

Are both intensive properties. 

Explanation:- 

 

{ Intensive properties are those that do not depend on the size of the system. Specific enthalpy is the enthalpy on a per gram basis. Since  this value is normalized to the per gram basis it does not depend on the size of the system being measured. } 
So, both are intensive properties.

Kinetic energy 1/2mv² depends on mass, Entropy kJ/k depends on mass so Entropy is extensive property but specific entropy kJ/kg K is an intensive property.

Extensive property: Whose value depends on the size or extent i.e. mass of the system (upper case letters as the symbols) e.g., Volume, Mass (V, M). If mass is increased, the value of extensive property also increases.

A control volume is a mathematical abstraction employed in the process of creating mathematical models of physical processes. In an inertial frame of reference, it is a volume fixed in space or moving with constant flow velocity through which the continuum (gas, liquid or solid) flows. The surface enclosing the control volume is referred to as the control surface.

 

At steady state, a control volume can be thought of as an arbitrary volume in which the mass of the continuum remains constant. As a continuum moves through the control volume, the mass entering the control volume is equal to the mass leaving the control volume. At steady state, and in the absence of work and heat transfer, the energy within the control volume remains constant. It is analogous to the classical mechanics concept of the free body diagram.

Isothermal Compression of Air in a Stirling Engine

The isothermal compression of air in a Stirling engine is an example of a closed system with a movable boundary. This can be explained in the following points:

Stirling Engine:
- A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas at different temperature levels.
- It works on the principle of the Stirling cycle, which involves the compression and expansion of a working fluid, typically air, at different temperature levels.
- The engine uses an external heat source to heat the gas and an external heat sink to cool the gas during the cycle.
Isothermal Compression:
- Isothermal compression refers to a compression process that occurs at a constant temperature.
- In a Stirling engine, the compression of air is carried out isothermally, meaning the temperature of the air remains constant during the compression process.
- This is achieved by transferring heat to the surroundings or through the use of regenerators or heat exchangers.
Closed System with a Movable Boundary:
- A closed system is a system that does not exchange matter with its surroundings, but it can exchange energy in the form of heat or work.
- In a Stirling engine, the air inside the engine is contained within a closed system, meaning it does not mix with the external environment.
- The boundary of the system is movable, as the air undergoes compression and expansion within the engine.
- The movable boundary allows the air to be compressed and expanded, enabling the conversion of heat energy into mechanical work.
Therefore, the isothermal compression of air in a Stirling engine is an example of a closed system with a movable boundary.

The energy transfer as heat and work during the forward process as always
identically equal to the energy transfer is heat and work during the reversal or the process is the correct reason for maximum efficiency because it is conservative system.

Isolated System - in which there is no interaction between system and the
surroundings. It is of fixed mass and energy, and hence there is no mass and energy transfer across the system boundary.

A thermodynamic system is considered to be an isolated one if there is no energy and mass transfer taking place.

All temperature measurements are based on Zeroth law of thermodynamics.

The Zeroth law of thermodynamics is the basis for measurement of temperature and setting its scale. In simple word, Zeroth law of thermodynamics says that “When two bodies are separately in thermal equilibrium with the third body, then the two are also in thermal equilibrium with each other."

We know that PV=mRT and it holds good at low pressure and high temperature. If volume is constant P is proportional to T (since R is constant and m depends on gas and is also constant). Therefore, at low pressure, the temperature of gas is proportional to its pressure at constant volume.

Because firstly they are of different material they have different thermal expansion etc and secondly no 2 things whether of same company can tel u the same.
Eg. if u purchase the weighing machine of 2 companies even they can' t tell the same weight .. i.e some amt of deflection will be there

Absolute zero, temperature at which a thermodynamic system has the lowest energy. It appeared that an “ideal gas” at constant pressure would reach zero volume at what is now called the absolute zero of temperature.

It is not possible to take a system from a given initial state to any final state by performing adiabatic work only.

The assertion is true since in an adiabatic process heat transfer is zero and work is equal to change in internal energy.