Goyal Brothers Pressure in Fluids Class-9 ICSE Physics Ch-4
State briefly, how and why the atmospheric pressure of a place varies with the altitude. Draw an approximate graph to illustrate this variation.
- 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.
- 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.
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
State two advantages of aneroid barometer.
Advantages of aneroid barometer :
- It is compact, portable and hence can be carried anywhere.
- It does not contain any liquid and there is no chance of spilling over of liquid as in mercury barometer.
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.
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