BASICS OF THERMODYNAMICS

VIDEO LECTURE ON BASICS OF THERMODYNAMIC

Definition of thermodynamics
Basic terms used
Laws of thermodynamics
System, Boundary and Surroundings
Equilibrium
Intensive and Extensive properties
State, Process, Cycle
Point and path functions

Definition

Thermodynamics is the branch of science deals with relationship between heat and work.
It deals with transfer of energy within the system and across the boundary of the system.

Basics Terms Used

Mass
Weight
Volume
Density
Specific weight
Specific gravity
Specific volume
Pressure
Temperature
Heat
Work
Power
Energy

Mass

Mass is the amount of matter contained in an object, and does not depends on gravity.
Unit of mass is gram, kilogram.

W=mg
It is denoted as 'm'

Weight

The amount of mass depending upon gravity is called weight.
It is the product of mass and acceleration gravity.
W=mg.
Unit is Kgm/sec² (or) Newton.
It is denoted as'W'

Volume

Space occupied by the mass.
Ratio of mass to density
It is denoted as a term 'V'

Density

It is defined by amount of mass per unit volume
Unit of density is Kg/m³.
It is denoted as a term 'ρ'

Specific weight

Also called as weight density
It is defined by ratio of weight to volume
Unit is N/m³.
It is denoted as 'w'

Specific gravity

Also called as relative density
It is defined by ratio of density of given substance to density of standard substance.
It has no unit.
Density of standard liquid (ρ_w) = 1000 kg/m³.
It is denoted as a 'S'

Specific volume

Volume occupied by unit mass is called as specific volume.
It is the reciprocal of density.
Unit is m³/Kg.
It is denoted as 'v'

Pressure

It is defined as the force over area.
Units of pressure are N/m² ,bar, pascal
Absolute zero pressure is called as perfect vacuum
Below the atmospheric pressure is called as vacuum pressure.
If the pressure is measured by means of mechanical gauges that is called as gauge pressure.
It is denoted as 'P'

Relationship between various pressures

Pressure measuring devices

Barometer
Piezo meter
Manometer

U tube manometer
Differtial U tube manometer
Inverted U tube manometer
Micromanometers

Pressure gauges

Bourdon tube pressure gauge
Dead weight pressure gauge
Diaphragm type pressure gauge

Temperature

It is defined as degree of hotness or coldness.
It is denoted as 'T'
This is the amount of heat present in a system.
Unit is degree Celsius (or) degree Fahrenheit (or) Kelvin.
Four temperature scales available

Celsius or centigrade temperature scale
Absolute temperature scale (or) Kelvin temp. scale
Rankine temperature scale
Fahrenheit temperature scale

Relationship between kelvin and degree Celsius

T = t + 273
t-Temperature reading in Celsius scale
T-Temperature in kelvin scale
Freezing point of water is 0⁰C (or) 273K
Boiling point of water is 100⁰C (or) 373 K

STP and NTP conditions

Standard Temperature and Pressure

T= 15°C , P=P_atm

Normal temperature and pressure

T=0°C , P= P_atm

Heat

Heat is a form of energy. It can be transferred from one body to another due to the difference of temperature.
Heat is defined as the energy transferred, without transfer of mass, across the boundary of a system because of a temperature difference between the system and the surroundings.
It is expressed in Joule (J). It is usually represented by ‘Q’.

Sign convention in heat

Heat received by the system = +Q
Heat lost or heat rejection by the system = -Q

How much heat is transferred ?
Q = m C (T₂-T₁)

Specific heat capacity

It is defined as the amount of heat required to raise or lower the temperature of a unit mass of any substance through one degree. Its unit is kJ/kgK. It is denoted by ‘C’.

Specific heat capacity at constant volume process

It is defined as the amount of heat required to raise or lower the temperature of a unit mass of any substance through one degree when the volume remains constant. Its unit is kJ/kgK. It is denoted by ‘C_v’.

Q = m C_v dT

Where

Q = Heat transferred (kJ)

m = Mass of the gas (kg)

C_v= Specific heat capacity at constant volume (kJ/kgK)

dT =(T2-T1) = Temperature difference (K)

Specific heat capacity at constant pressure process

It is defined as the amount of heat required to raise or lower the temperature of a unit mass of any substance through one degree when the pressure remains constant. Its unit is kJ/kgK. It is denoted by ‘C_p’.

Q = m C_p dT

Where,

Q = Heat transferred (kJ)

m = Mass of the gas (kg)

C_p= Specific heat capacity at constant pressure (kJ/kgK)

dT =(T₂-T₁) = Temperature difference (K)

Why C_p always greater than C_v?

Work done = P dV

At V=C; Work done = 0

At P=C; Work done = P(V₂-V₁)

“When the gas is heated up at constant pressure volume will be increased so there is a work done, but incase of constant volume process there is not work done. So for all gases C_palways higher than that of C_v”

Constant values

C_p– C_v = R
C_p value of air = 1.005 kJ/kgK
C_v value of air = 0.718 kJ/kgK
R (Gas Constant) = 0.287 kJ/kgK
Ƴ = 1.4

Work

According to mechanics, Work will be done when the point of application of force moves in the direction of force. Its unit is Nm or Joules. It is denoted by ‘W’.

Work done (WD) = Force (F) x Distance moved (d)

According to thermodynamics work done is defined as the energy transferred across the boundary of a system because of an intensive property difference other than temperature exists between system and surroundings.

Work done (WD) = p dV (dV= change in volume)

Unit is Nm or Joule

1Nm = 1 Joule

Sign convention in work

Work done by the system = +W (Output)

Example : Expansion process - Turbine

Work done on the system = -W (Input)

Example : Compression process - Compressor

Power

Rate of doing work is called as power.
Unit is W and KW.
1Ws=3600J
It is usually represented as 'P'

Energy

Capacity to do the work is called as power.
Unit is J and KJ.
Stored energy and transit energy are the types of energy.
Potential energy, Kinetic energy, Internal energy, Flow energy are stored energy.
Heat energy and work done are transit energy
Usually denoted as 'E'

Energy formulae

Potential energy = mgh

Kinetic energy = 1/2 mv²

Internal energy = mC_vt

Flow energy = PV

Heat energy = mCdT

Work done = PdV

Law of conservation of energy

It states that, “The energy can neither be created nor destroyed, though it can be transformed from one form to any other form, in which the energy can exist”.

Laws of thermodynamics

Zeroth law of thermodynamics

This law states that “when two bodies are separately in thermal equilibrium with a third body, then they are in thermal equilibrium with each other.

Thermometer works under this law.

First law of thermodynamics

When a system undergoes a cyclic process, the algebraic sum of the work transfer is proportional to the algebraic sum of the heat transfer.

For isolated system dQ=0 and W=0

Δ U = Q – W

Second law of thermodynamics

Second law of thermodynamics --- Kelvin plank statement

It is impossible to construct a heat engine working on cyclic process, whose purpose is to convert all the heat energy supplied to it into work.

Second law of thermodynamics --- Clausius statement

It is impossible to construct a system working on a cyclic process whose purpose is to transfer heat from a colder body to a hotter body without the aid of external work

Thermodynamic systems

The thermodynamic system (simply known as system) may be defined as a definite area or a space where some thermodynamic process is taking place.

Types of thermodynamic systems

Open system (or) flow system (or) control volume system

In an open system or flow system or control volume system, both energy and mass (heat and work) cross the boundary of the system

Examples: Air compressor, flow through nozzles and turbines

Closed system

In a closed system or non-flow system, Energy crosses the system boundary in the form of heat and work, but there is no mass transfer.

Example: Gas contained in the cylinder

Isolated system

In an isolated system no mass and no energy crosses the boundary of the system.

Example: Thermal flask and insulated gas container

Thermodynamic boundary and surrounding

Boundary

The system and surroundings are separated by the system boundary. It may be real or imaginary.

Surrounding

Everything outside of the system which affects the behavior of the system is called surroundings.

Equilibrium

It is defined as state of balance when two forces cancel each other.

Types of equilibrium

Thermal equilibrium

A system is said to be in thermal equilibrium when the temperature at all points in the system is the same.

Example : Isothermal process

Mechanical equilibrium

A system is said to be in mechanical equilibrium when the pressure at all points in the system is the same.

Example : Constant pressure process – Boiling of water

Chemical equilibrium

A system is said to be in chemical equilibrium, if it does not undergo any chemical reaction.

Example : Cool drinks

Thermodynamic equilibrium

A system is said to be in thermodynamic equilibrium when the system available in thermal, chemical and mechanical equilibrium.

Example : Bursting of ballon

Property

The state of the system may be identified or described by certain observable quantities such as volume, temperature, pressure and density etc.

All the quantities which identify the state of the system are called properties.

There are two sorts of property:

Intensive properties

Extensive properties

Intensive and Extensive properties

Intensive (or) Intrinsic properties

These are the properties which are independent of the mass of the system.

Examples: Temperature, pressure, velocity, entropy, density

Extensive (or) Extrinsic properties

These are the properties which are dependent on the mass of the system

Examples: volume, energy, specific volume = volume / mass

Specific enthalpy = Enthalpy / mass

State of the system

It is the condition of the system at any particular moment which can be identified by the statement of its properties, such as pressure, volume, temperature etc.,

Process of the system

It is the change of system state at any particular moment from one equilibrium state to another equilibrium state.

Change of pressure
Change of temperature
Change of energy

Cycle of the system

When a process are performed on a system in such a way that the final state is identical with the initial state. This is known as cycle or cyclic process.

Otto cycle
Diesel cycle
Carnot cycle
Rankine cycle

Point and path fuction

Point (or) state function

The quantities which are independent on the process of path followed by the system are known as point functions.

Example : Pressure, volume, temperature, entropy, enthalpy

Path (or) process function

The quantities which are dependent on the process or path followed by the system are known as path functions.

Example : Work done, Heat transfer

CONCEPT PEN

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