Question 13.
State briefly, how and why the atmospheric pressure of a place varies with the altitude. Draw an approximate graph to illustrate this variation.

1. We know atmospheric pressure = height of air column x density of air x acceleration due to gravity; P = hρg So, as we go up i.e. at higher altitudes, height of air column and hence atmospheric pressure decreases.
2. Also with the increase in altitude, density of air decreases and hence atmospheric pressure decreases.
If we take average density of air as 1.29 kgm-3 and the density of mercury as 13 600 kgm-3
we can find the height
Column which will exert as much pressure as*’ is exerted by 1 cm or (0.01 m) column of mercury as
Height of air column x density of air = height of mercury column x density of mercury height of air column x 1.29 kg/m3
= 0.01 m x 13600 kg/m3 Thus, 105 m of air column, on the average, will exert as much pressure as 1 cm column of mercury. Further,1 cm of mercury column exerts pressure = 105 m of air column 76 cm of mercury column exerts pressure = 105 x 76 m = 7980 m = 8 km (approx).
Thus, 8 km of air column will exert as much pressure as 76 cm of mercury column. However, it does not mean that atmosphere extends to only 8 km. As it is pointed out earlier, the density of atmosphere also changes with height. Thus, a fall of one cm in pressure does not mean that we have covered a vertical height of 105 m. On higher altitudes the vertical height of air is far in excess of 105 m, because of low density of air. A graph showing fall in pressure with height is shown in the figure. Question 14.
The blood pressure reading of a patient is recorded 160/ 100. Express the lower pressure in SI units.
[Take density of mercury as 13.6 x 103 kgm-3 and the value of ‘g’ as 10 ms-2] Answer:
Lower pressure of the patient = 100 mm of Hg column
Height of mercury column = h = 100 mm = 10 cm h = 0.1 m
Density of mercury = ρ=13.6x 103 kgm-3
Acceleration due to gravity = g = 10 ms-2
So lower pressure of patient = P = hρg
P = 0.1 x 13.6 x 103 x 10
P = 13600 Pa

Question 15.
State two advantages of aneroid barometer.

#### Advantages of aneroid barometer :

1. It is compact, portable and hence can be carried anywhere.
2. It does not contain any liquid and there is no chance of spilling over of liquid as in mercury barometer.

Question 16.
Explain, why a gas bubble released at the bottom of a lake grows in size as it rises to the surface of the lake.
Bubble released at the bottom of a lake grows in size as it rises to the surface of the lake because the pressure exerted on it by water of the lake DECREASES hence by BOYLE’S LAW PV = constant the VOLUME of bubble INCREASES and the bubble grows in size.

-: End of Goyal Brothers : Pressure in Fluids Class-9 ICSE Physics Ch-4:-