Property:
The quantities which
characteristic the given state of the system are called properties or parameters.
A property can be measured directly or in directly while the system is in
equilibrium. The valve of the property depends only on the state and not on the
process or path by which the state is achieved. Temperature, pressure,
viscosity, velocity, thermal connectivity, volume, mass, enthalpy and entropy
are some of the properties. Heat and work are not properties as they are
functions of path.
Thermodynamic property are
classified into two groups intensive and extensive properties. An intensive
property is independent of the mass and an extensive property varies directly
with the mass. (E.X) If a matter in a particular state is divided into two equal
parts will have the same properties. Temperature, pressure and density are
example of intensive properties and mass and volume are examples of extensive
properties.
Force:
Thermodynamic property can
be measured and counted either directly or indirectly. Properties such as
force, mass, length time have the basic units and are related. NEWTON'S SECOND
LAW OF MOTION states that the force acting on a body is the product of the mass
and the acceleration of the body in the direction of the force.
F = ma
The popular system of
units in use presently throughout the world is the international system and
referred ad SI units. Matter, second and kilogram are the basic units for
length, time and mass respectively.
The unit of force is Newton (N) and
1 N = 1kg m/s²
Density:
Density is defined as the
ratio of mass per unit volume. The density varies significantly for a gas with
pressure and temperature but the variation is negligible for liquids.
Specific Weight:
Specific weight (w) is
defined as the ratio of weight per unit volume.
Specific weight, w
= weight/ volume ( N/m³)
Specific Gravity:
Specific gravity of a liquid is defined as
the ratio of specific weight of that liquid to the specific weight of water.
Specific Volume:
Specific volume of a
substance is defined as the volume per unit mads. The density of a substance,
defined as the mass per unit volume, is the reciprocal of the specific volume.
Specific volume and density are intensive properties.
The SI unit for volume is
cubic metre. The other volume unit is the litre and is equal to 0.001cubic
metres.
Specific Heat:
Heat required to raise the
temperature of 1kg of a gas by 1ºC while the volume of the gas remains constant
is known as the specific heat at a constant volume
( cᵥ' ) and if the gas expands at a constant
pressure then it is called the specific heat at a constant pressure ( Cp' ).
The specific heat of a gas at a constant pressure is always greater than the
specific heat at a constant volume.
Viscosity:
The property which
controls the rate of a liquid is known as viscosity. Viscosity is due to the
cohesive force between the liquid particles and it is exhibited when the liquid
is in motion. Newton's law of viscosity states that the shear stress on a layer
of a fluid is directly proportional to the rate of shear strain.
shear stress, τ α dv/dy → τ =
µ dv/dy
Where dv/du is the rate of
shear strain or velocity gradient and µ is the proportionality constant known
as coefficient of viscosity. µ is also known as coefficient of absolute
viscosity or dynamic viscosity. The unit of µ is Ns/m2 also
1 poise = 0.1 Ns/m2 and
1 centipoise = (1/100)poise
τ = µ dv/dy = (Ns/m²) (m/s/m) =
N/m²
Kinematic viscosity of a
liquid is the ratio of its absolute viscosity to its density.
Kinematic viscosity γ = µ/ρ,
m²/s
1
Stroke = 1/10000 m²/s = 10-4 m²/s
1 Centistoke = 1/100 stoke
Pressure:
Pressure is defined as the
normal component of force per unit area. Pressure at a point in a fluid in a
equilibrium is the same in all directions.
A fluid filled in a
container exerts a force at all points on the sides and bottom of the
container. This force per unit area is called pressure. The direction of this
pressure is always normal to the surface with which the fluid comes in contact.
Autospheric pressure:
The pressure exerted by
the weight of air column on the surface of the earth is called as atmospheric
pressure.
Atmospheric pressure =
weight of air column / area
=mass of air column X
acceleration due to gravity/area
= (Volume of air column X
Density of air column X Acceleration due to gravity) / Area
= Height of air column X
Density of air column X Acceleration due to gravity
= hρg
{m(kg/m³)(m/s²) = (N/m²)}
Since the acceleration due
to gravity g is constant (g = 9.81 m/s2 ), the atmospheric pressure
at any location depends on the height of air column and density of air at that
place. The value of atmospheric pressure is considered as standard at the
surface of the sea as it is horizontal everywhere. The maximum air column
height exists at the sea level and value of standard atmospheric pressure is
1.03*105 N/m2 or 1.03 kg/cm2 . The value of
atmospheric pressure will be less than this at any other places on earth due to
temperature zones due to pressure difference. This is how wind is formed.
Gauge pressure:
Pressure measuring
instruments are usually calibrated with respect to atmospheric pressure. They
Will read zero at atmospheric pressure. Pressure measured with these gauge is
known as gauge pressure. Pressure gauge are used to measure pressure above
atmospheric level and vacuum gauges are used to measure pressure below
atmospheric level.
Absolute pressure:
Absolute pressure is the
sum of gauge pressure and atmospheric pressure. Absolute pressure is zero at
absolute vacuum.
Absolute pressure = gauge
pressure+ absolute pressure
The unit for pressure in the international
system is ( force in newton acting on a square meter area) called the pascal
(Pa).
1 Pa = 1N/m2
The other units are:
1bar = 105 Pa
= 0.1 MPa and
1atm = 101325 Pa = 14.696 lbf/in2 (or)
Psi.
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