Question: 14: If a soap bubble expands, the pressure inside the bubble
(1) Remains the same
(2) Is equal to the atmospheric pressure
(3) Decreases
(4) Increases
Answer: Option (3)
Explanation:
For a soap bubble, the excess pressure inside the bubble over the atmospheric pressure is given by
\Delta P = \frac{4T}{r},
where T is the surface tension of the soap solution and r is the radius of the bubble.
This relation shows that the excess pressure inside the soap bubble is inversely proportional to its radius.
When the soap bubble expands, its radius r increases.
As r increases, the value of \frac{4T}{r} decreases.
Therefore, the excess pressure inside the bubble decreases,
and hence the pressure inside the bubble decreases as it expands.
Thus, the correct answer is that the pressure inside the bubble decreases.