Specific
weight is defined as the weight of a unit volume of the substance. Weight
is the force, by virtue of mass of the body, with which earth attracts the body
towards itself.
Whenever a
body is placed in air at a higher or a lower temperature than that of the body
then, as a result of the temperature difference, heat flows between the air
and the body and causes a change in the density of the air in the vicinity of
the surface. For same volume, the hotter air has lower value of density
while the colder air has higher density.
For
instance, let us see the variation of density of air at 20 oC and 40
oC at 101.3205 kPa and 22.4 m3, assume MW = 29 kg/kg-mole.
PV = nRT
PV = (m / MW). RT
P. MW = (m / V). RT
P. MW / RT = ρ
ρ = P. MW / RT
ρ = (P. MW/ R) / T
ρ = k / T----------------------(1)
Now evaluating constant k = P. MW / R for air
k = 101.3205
kN/m2 . 29 kg/kg-mole . 1 kg-mole. 273 K / [(101.3205 kN/m2)
. 22.4 m3]
k =
101.3205 kN/m2 . 29 kg/ kg-mole / 8.3134 [(kN/m2.m3)/(kg-mole.
K)]
k = 353.44
kg / (m3. K)
From
equation (1), density of air at 20 oC = 293 K is calculated as
ρ1
= 353.44 / 293 = 1.21 kg/m3
From
equation (1), density of air at 40 oC = 313 K is calculated as
ρ1
= 353.44 / 313 = 1.129 kg/m3
For the
same volume, the lower density gas has smaller quantity of contained
mass and the higher density gas has the larger quantity of contained
mass. Therefore, the lower density air has lower specific weight and the
higher density air has more specific weight. As per universal
gravitational law, the weighty air is more attracted by the earth as
compared to lighter air. This means that weighty air will rest closer to
floor while the lighter air will rest farther from the floor.
If you gotsome queries, let me ask in comments.
Comments