upsc ese thermodynamics

8/13/2019 UPSC ESE Thermodynamics

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Himanshu Vasistha

Thermodynamics-1

for UPSC IES and GATE 2014 Mechanical Engineering

By


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Thermodynamics

The branch of science that deals with the relations

between heat and other forms of energy, and ofthe relationships between all forms of energy.


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System and Surroundings

!A system is a finite quantity of matter or a

prescribed region of space.

! The actual or hypothetical envelope

enclosing the system is called Boundary.

! Boundary may be fixed or moveable.

! Everything external to the system is

called Surroundings.

!A System and its Surroundings together

comprise a Universe.


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Types of System

! Open System- In this matter and

energy flow in or out of the

system. Most of the Engineering

Systems are Open Systems.

! Closed System- In this theboundary is impervious to the

flow of matter. A mass of gas and

vapour in an engine cylinder, with

a continuous boundary may be

regarded as a Closed System.

! Isolated System- In this the

system neither exchanges energy

nor matter with another system

or surroundings.


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Properties, Processes and Cycles

When properties like, volume, pressure, temperature etc. of

a system have definite values, then the system is said to

exist at a definite State.

Properties are the coordinates which describe the state of a

system.

An operation in which one or more properties of a system

changes is called change of state.

The succession of States passed during a change of state is

called aprocess.


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Process and Cycle


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Thermodynamic Equilibrium

Thermodynamic equilibrium exists in a system when no change of

macroscopic properties is registered, if the system is isolated from

its surroundings.

An isolated system always reaches in course of time a state ofthermodynamic equilibrium and can never depart from it

spontaneously.

If a system is in equilibrium, no spontaneous change in

macroscopic property can occur.

Thermodynamic equilibrium exists when, Mechanical, Thermal and

Chemical equilibrium exist simultaneously.


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Zeroth Law of Thermodynamics

When a body A is in thermal equilibrium with a body B, and

also separately with a body C, then B and C will be in thermal

equilibrium with each other.


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! Zeroth law is the basis of

temperature measurement.

! There are five kinds of

thermometer, each with its own

thermometric property.


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Thermometer Thermometric property

Constant vol. gas

thermometerPressure

Constant pressure gas

thermometerVolume

Electrical Resistance

ThermometerResistance

Thermocouple Thermal e.m.f

Mercury in glass

thermometerLength


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Work transfer

Work is done by a force as it acts upon a body moving in

the direction of the force.

When work is done by a system, it is taken to be positive,and if work is done on a system, it is taken to be negative.

Work is one of the forms in which a system and its

surroundings interact.


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Displacement work

! When the piston moves, the

volume changes, and the workdone can be calculated as

described in the figure.

! The magnitude of the work doneis the area under the P-V diagram.

! The path shown on the P-V

diagram must pass through

equilibrium states; i.e. must be

Quasi Static.

! The integration to calculate the

work done can be performed onlyif the process is quasi static.


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Path Function and Point Function

Area under the PV curve gives the amount of work done,

so it is not a function of the end states of a process, but

depends upon the path the process takes. Therefore,

work is called apath function.

Thermodynamic properties arepoint functions, since for a

given state, there is a definite value for each property.

Path Functions are called inexact or imperfect differential.

Point Functions are called exact or perfect differential.


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Work in various Quasi

Static Processes

PVk= constant


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Open and closed

system work

! The open system work is

calculated by plotting it on

volume axis.

! The closed system work is

calculated by plotting it on

pressure axis.


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Zero work transfer

Work transfer is ONLY identified at the boundaries of a

system. Therefore, it can be termed as a boundary

phenomenon.

The expansion of a gas against vacuum is called free

expansion. In this case the work done by the system is

zero, as no work crosses the boundary.

This free expansion process is not a quasi static process,

although the initial and end states are in equilibrium.


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Heat Transfer

It is defined as a form of energy that is transferred across

a boundary due to a temperature difference.

The direction of heat transfer is from a body at hightemperature to a body at lower temperature.

Heat flow into a system is taken to be positive, and heat

flow out of a system is taken to be negative.

Heat transfer is a transit, and occurs only at the boundary.


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Important points for a Heat and Work Transfer

Heat and work transfer are energy interactions.

The same e#ect in a closed system can be brought

about either by heat transfer or work transfer.

Both transfers are boundary phenomena, and both

represent energy crossing the boundaries of the system.

It is wrong to say 'total heat' of a closed system as heat and workare not a property of a system. They cannot be stored in the

system. Both are energies in transit

Heat transfer is the energy interaction due to temperature di#erence

only. All other energy interactions can be termed as work transfer


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upsc ese thermodynamics

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