Basic Thermo Concepts

Question 1

System

Answer 1

A system is a finite quantity of matter or a prescribed region of space enclosed by a membrane or surface to separate it from the surrounding or environment.

Question 2

Boundary

Answer 2

The actual or hypothetical envelope enclosing the system is the boundary of the system.

Question 3

Closed system

Answer 3

If the boundary of the system is impervious to the flow of matter, it is called a closed system.

Question 4

Open system

Answer 4

An open system is one in which matter flows into or out of the system. Most of the engineering systems are open.

Question 5

Isolated system

Answer 5

An isolated system is that system which exchanges neither energy nor matter with any other system or with environment.

Question 6

Phase

Answer 6

A phase is a quantity of matter which is homogeneous throughout in chemical composition and physical structure.

Question 7

Homogeneous System with examples

Answer 7

A system which consists of a single phase is termed as homogeneous system. Examples : Mixture of air and water vapour, water plus nitric acid and octane plus heptane.

Question 8

Heterogeneous System with examples

Answer 8

A system which consists of two or more phases is called a heterogeneous system. Examples : Water plus steam, ice plus water and water plus oil.

Question 9

Differences between macroscopic and microscopic approach

Answer 9

1. macroscopic approach to thermodynamics is concerned with gross or overall behaviour of matter. it also known as classical thermodynamics.

• Microscopic approach to thermodynamics is concerned directly with the structure of the matter–known as statistical thermodynamics.
  2. The analysis and design of macroscopic system requires simple mathematical formulae.
• As the number of molecules is very large. Advanced statistical and mathematical methods are needed to explain the changes in the system.
  3. The values of the properties of the system are their average values. These properties like pressure and temperature can be measured very easily. The changes in properties can be felt by our senses.
• The properties like velocity, momentum, impulse, kinetic energy, force of impact etc. which describe the molecule cannot be easily measured by instruments. Our senses cannot feel them.
  4. In order to describe a system only a few properties are needed.
• Large number of variables are needed to describe a system

Question 10

Pure substance

Answer 10

A pure substance is one that has a homogeneous and invariable chemical composition even though there is a change of phase.

Question 11

Define thermodynamic Equilibriumo

Answer 11

A system is in thermodynamic equilibrium if the temperature and pressure at all points are same ; there should be no velocity gradient ; the chemical equilibrium is also necessary.

Question 12

PROPERTIES OF SYSTEMS, types

Answer 12

A property of a system is a characteristic of the system which depends upon its state, but
not upon how the state is reached. There are two sorts of property :
1. Intensive properties. These properties do not depend on the mass of the system.
Examples : Temperature and pressure.
2. Extensive properties. These properties depend on the mass of the system. Example :
Volume. Extensive properties are often divided by mass associated with them to obtain the intensive
properties.

Question 13

STATE

Answer 13

State is the condition of the system at an instant of time as described or measured by its
properties. Or each unique condition of a system is called a state.

Question 14

Quasi-static process

Answer 14

Quasi means ‘almost’. This process is a succession of equilibrium states and infinite slowness is its
characteristic feature.
A Quasi-static process is a reversible process

Question 15

CYCLE

with “cycle of operation” fig

Answer 15

Any process or series of processes whose end states are identical is termed a cycle. fig: pg 45

Question 16

POINT FUNCTION

Answer 16

When two properties locate a point on the graph (co-ordinate axes) then those properties
are called as point function.
Examples. Pressure, temperature, volume etc.

Question 17

Define path function

Answer 17

These are quantities which cannot be located on a graph by a point but are given by the area or so, on that graph. Examples. Heat, work etc.

Question 18

temperature

Answer 18

The temperature is a thermal state of a body which distinguishes a hot body from a
cold body. The temperature of a body is proportional to the stored molecular energy
i.e., the average molecular kinetic energy of the molecules in a system.
It has been found that a gas will not occupy any volume at the absolute zero temperature (273.15).

Question 19

Define superheated vapour

Answer 19

It is a vapour at a temperature higher than its vapourization (boiling point) at the absolute pressure where the temperature is measured.

Question 20

Define critical point

Answer 20

This is the limit of distinction between a liquid and a vapour.

Question 21

Define perpetual motion machine of the first kind

Answer 21

It is an imaginary device that produces energy without the input of energy. It violates the 1st law of thermodynamics (law of conservation of energy).

Question 22

Define a weak mixture

Answer 22

It is a fuel-air mixture in which the fuel is proportionally less than a chemically correct mixture i.e. (1:16)

Question 23

Define Refrigerator capacity

Answer 23

It is a measure of the effective cooling capacity of a refrigerator, expressed in Btu per hour, or in tons where one ton of capacity is equal to the heat required to melt 2,000 pounds of ice in 24 hours or 12,000 Btu per hour

Question 24

Define throttling process

Answer 24

It is a process in which a high pressure fluid is converted to low pressure by passing it through a narrow constriction.
The process is adiabatic and irreversible

Question 25

Reason why the carnot cycle is considered impracticable in real life

Answer 25

Because it is not possible to produce an engine that should work on carnot cycle as it would necessitate the piston to travel very slowly during the first portion of the first stroke (isothermal expansion) and to travel more quickly during the remainder of stroke (adiabatic).

Question 26

Distinguish between a nozzle and the diffuser

Answer 26

A nozzle increases velocity of a fluid

A diffuser decreases the velocity of a fluid

Nozzles can be used by jets and Rockets to provide extra Thrust. while many jet engines use diffuser to slow air coming into the engine for more uniform flow.

No friction in Nozzles while there is friction in diffusers

Question 27

0.92kg of certain perfect gas…

Answer 27

Yy

Question 28

What are the essentials requirement for a refrigerant needs to possessed to be considered for use in a vapour absorption system

Answer 28

Must be absorbed on learning the evaporator

Must be separated from the absorbent and subsequently condensed before being returned to the evaporator.

The suitability must be suitable

Question 29

Dryness fraction

Answer 29

Just the ratio of the actual dry steam to the mass of the same containing it. It is the term used for mass fraction of vapour in the mixture

Question 30

Adiabatic process

Answer 30

Is a process which no it is transferred to all from the system during the process. It is Thermally insulated from the surrounding but can exchange heat

Question 31

Entropy

Answer 31

He describes a degree of randomness and disorderliness of a system

Question 32

Refrigerant

Answer 32

It is any substance that absorbs heat through expansion or vaporisation and losses it through condensation in the refrigeration system

Question 33

Polytropic process

Answer 33

It is a process where specific heat is constant

Question 34

Examples of steady flow device is

Answer 34

Nozzle

turbine

compressor

diffuses

pump

Question 35

What is a heat engine

Answer 35

It is a system that converts heat or thermal or chemical energy to mechanical energy which can be used to do mechanical work. examples: thermal power station, internal combustion engine, steam engines.

It is powered by the expansion of heated gas. Hence, the name heat engine

Question 36

Efficiency of heat engine

Answer 36

It is how much useful work it outputs for a given amount of heat

Question 37

The zeroth law

Answer 37

‘Zeroth law of thermodynamics’ states that if two systems are each equal in temperature to a third, they are equal in temperature to each other.

Question 38

Process

Answer 38

A process occurs when the system undergoes a change in a state or an energy transfer at a
steady state. A process may be non-flow in which a fixed mass within the defined boundary is
undergoing a change of state. Example : A substance which is being heated in a closed cylinder
undergoes a non-flow process. Closed systems undergo non-flow processes.

A process
may be a flow process in which mass is entering and leaving through the boundary of an open
system.

Question 39

Corollary 1 of the first law

Answer 39

First law for a process: when is system executes a process the net heat interaction equals the net work interaction plus change in stored energy

dQ =dE + dE

Question 40

Corollary 2 of the first law

Answer 40

For an isolated system both heat and work interactions are absent. dE = 0

Question 41

Corollary 3

Answer 41

A motion machine of the first kind is impossible

Question 42

Draw and explain the otto cycle

Answer 42

Jpg

Question 43

Specific enthalpy

Answer 43

It is the total energy 1 kg of the steam made up of internal energy and pressure energy

Question 44

Specific volume

Answer 44

It is the volume in metre cube which one kg of the Steam occupies

Question 45

Control volume

Answer 45

It is an imaginary envelope that is said to exist around a piece of equipment and it is called a control surface. The space that the control surface occupies is called a control volume