Grade 11 - Physics - Kinetic Theory - Introduction - Basics Fundamentals
Laws of expansion
All gases expand on heating. The expansion of its container can be neglected as the expansion of gas is so large.
The gases can be compressed easily, even they expand in volume at a constant temperature, provided the pressure is allowed to fall.
The volume of gas depends on temperature and pressure as follows
1. Change in volume due to change in pressure when temperature remains constant. This is given by Boyle’s law.
2. Change in volume due to change in temperature when pressure remains constant. This is given by Charle’s Law.
3. Change in pressure due to change in temperature when volume remains constant. This is given by Pressure law.
At constant pressure, let be the volume of a given mass of gas respectively at .The coefficient of volume expansion for the gas at constant pressure is given by
(i) The coefficient of volume expansion of all non –conducting gases at a constant pressure (yp
) is equal to 0.00367 or 1/273.
(ii) The coefficient of volume expansion (yp
) is same for all gases.
(c) The plot of the volume as function of temperature of any gas, as shown in fig. intercepts the temperature axis at -273°C , regardless of
i. The original volume of the gas taken,
ii. The gas chosen, and
iii. The pressure
On the basis of absolute temperature scale, the equation can be written as,
Thus, the volume is proportional to absolute temperature. It follows that at constant pressure the volume of a given mass of gas is directly proportional to its temperature in absolute (or Kelvin) scale.
This statement is known as Charles’s Law of gases.
Absolute Scale of Temperature
If a given mass of gas at constant pressure is slowly cooled, its volume decreases uniformly with the temperature, as shown by the straight line in fig. 1.02. As the gas is cooled below 0°C, its volume shrinks further, and if extrapolated, there is a definite temperature at which the volume of the gas would become zero.
Therefore, it follows that when Note :
Thus, 0°C is 273 degrees above the absolute zero, and degrees above the absolute zero. When the temperatures are measured from the measured from the absolute zero, they are said to be absolute temperatures, and are expressed in “degree Kelvin” (K).
Though theoretically, the volume of gas becomes zero at -273°C (= 0 degree Kelvin ) but the fact cannot be experimentally checked because all gases become liquids or solids much above -273°C .