Question

When an air bubble rises from the bottom of a deep lake to a point just below the water surface, the pressure of air inside the bubble

• A. is greater than the pressure outside it
• B. is less than the pressure outside it
• C. increases as the bubble moves up
• D. decreases as the bubble moves up

Answer 1

Answer: (A), (D)

is greater than the pressure outside it
decreases as the bubble moves up

When bubble is deep down it has atmosphere pressure and water pressure i.e. $P_o+h\rho g$ .
As it rises, water pressure which is proportional to the depth, reduces and becomes $P_o$ just below the water surface Hence option 'D' is correct and 'C' is incorrect.
Pressure inside a bubble is always greater than the outside pressure by the amount of excess pressure. Outside pressure and pressure due to surface tension has to be compensated by the inside pressure for stable shape of the bubble. Hence option 'A' is correct and 'B' is incorrect.