Himanshu Vasistha

Thermodynamics-1

for UPSC IES and GATE 2014 Mechanical Engineering

By

Thermodynamics

The branch of science that deals with the relations between heat and other forms of energy, and of

the relationships between all forms of energy.

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.

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 the boundary 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.

Adiabatic System vs Isolated System

• An adiabatic system is one which is thermally insulated from its surroundings.

• This means no heat exchange across system boundary.

• Work transfer can take place across the system boundary.

• If work transfer does not occur across the boundary, it becomes an ISOLATED SYSTEM.

Macroscopic and Microscopic Points of View

Thermodynamic studies are undertaken by the two different approaches:-

• Macroscopic approach - means Big or Total.

• Microscopic approach - means small

Although both approaches are different, but when applied to a system, both provide same results.

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 a process.

• A cycle is defined as a series of state changes in which the initial and final states are identical.

• Two types of properties:- Intensive properties, which are independent of the mass of the system; e.g. Pressure, Temperature etc. Extensive properties, are related to mass; e.g. Volume, Energy etc.

• If mass is increased, the values of extensive properties also increase.

• Specific extensive properties, i.e. Extensive properties per unit mass, are intensive properties, e.g. Specific volume etc.

Process and Cycle

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 of thermodynamic 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.

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.

• Zeroth law is the basis of temperature measurement.

• There are five kinds of thermometer, each with its own thermometric property.

Thermometer Thermometric property

Constant vol. gas thermometer Pressure

Constant pressure gas thermometer Volume

Electrical Resistance Thermometer Resistance

Thermocouple Thermal e.m.f

Mercury in glass thermometer Length

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.

Displacement work

• When the piston moves, the volume changes, and the work done can be calculated as described in the figure.

• The magnitude of the work done is 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 only if the process is quasi static.

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 a path function.

• Thermodynamic properties are point 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.

Work in various Quasi Static Processes

PVk = constant

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.

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.

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 high temperature 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.

Important points for a Heat and Work Transfer

Heat and work transfer are energy interactions.

The same effect 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 work are 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 difference only. All other energy interactions can be termed as work transfer

Next topic:- First Law of Thermodynamics

For any doubts regarding this lecture feel free to drop me an email at, himanshu.vasistha@gmail.com.