Physics · Mechanical Properties Of Fluids · NEET
The extra (excess) pressure inside a soap bubble is P_excess = 4T/r. The radius r sits in the denominator, so a smaller r makes the fraction larger. A tiny bubble needs a strong inward pull from its curved surface, so it must hold a higher pressure inside to stay in balance. Bigger bubble, less curved, less squeeze, lower excess pressure.
A soap bubble has TWO surfaces (an inner and an outer liquid-air surface), so its surface tension acts twice, giving 4T/r. A liquid drop or an air bubble inside a liquid has only ONE surface, so it gives 2T/r. Same idea, just count the surfaces. Do not mix them up in NEET.
As the bubble expands, r increases. In P_excess = 4T/r, a larger r makes the excess pressure smaller. So the pressure inside a soap bubble DECREASES as it expands. This is the exact idea tested in NEET 2022.
The small bubble has higher inside pressure (4T/r is larger for small r). Air always flows from high pressure to low pressure, so air moves from the small bubble into the big bubble. The small bubble shrinks and the big bubble grows. This surprises students who expect them to become equal.
No. Excess pressure is only the EXTRA pressure above the outside. The total inside pressure is P_inside = P_outside + P_excess = P_0 + 4T/r. The 4T/r part is what changes with radius; P_0 is the surrounding pressure.
If a soap bubble expands, the pressure inside the bubble:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
P_excess = 4T/r, where T is the surface tension of the soap solution and r is the radius of the bubble. The total pressure inside is P_0 + 4T/r.
A soap bubble has two liquid-air surfaces, so 4T/r. An air bubble inside a liquid (or a liquid drop) has only one surface, so 2T/r. Count the number of surfaces.
Yes. For a drop, excess pressure = 2T/r. A smaller r still gives a larger excess pressure, so a smaller drop has higher inside pressure too. Bubbles just have twice the effect because of two surfaces.
At the start the bubble radius is very small, so 4T/r is large and you need extra pressure. As the bubble grows, r increases and the needed excess pressure drops, so it gets easier.
From the smaller bubble (higher pressure) to the larger bubble (lower pressure). The small one shrinks and the big one grows.
