TITLE.PM5

IDEAL AND REAL GASES 377

dharm

\M-therm\Th8-1.pm5

If p is the absolute pressure of the gas and V is the volume occupied by the gas, then

V ∝

1
p
or pV = Constant, so long as the temperature is constant ...(8.1)
Fig. 8.1 shows the graphical representation of Boyle’s law. The curves are rectangular
hyperbolas asymptotic to the p-v axis. Each curve corresponds to a different temperature. For any
two points on the curve,
p
p

1

2

=

V

V

2

1

...(8.2)

p (Pressure)

V (Volume)

T 1

T 2

T 3

T<T<T1 2 3

Fig. 8.1. p-V relation of a perfect gas at constant temperature.
Charle’s law. It states that if any gas is heated at constant pressure, its volume changes
directly as its absolute temperature.
In other words, V ∝ T

or
V
T = Constant, so long as pressure is constant ...(8.3)
If a gas changes its volume from V 1 to V 2 and absolute temperature from T 1 to T 2 without
any change of pressure, then
V
T

V

T

1

1

2

2

= ...(8.4)

Fig. 8.2 gives the graphical representation of Charle’s law.

T (Abs. temperature)

V (Volume)

p 1

p 2

p 3

p < p < p 123

• 273.15 Cº

Fig. 8.2. T-v relation of a perfect gas constant pressure.