Excess Pressure – Complete Notes, Revision, Important Questions & Downloads
This topic is built from four TOC blocks: Definition and General Concept, Excess Pressure Formulas, Double Bubble Formation, and Single Bubble Formation. NEET tests this topic through direct formula questions on drops, soap bubbles, bubbles at depth, and communicating bubbles, and it also uses the coalescence relation as a fast numerical check. The OCR emphasis is simple: surface tension tries to contract the surface, so the pressure inside is greater than outside, and the radius decides how large that difference becomes. Once you separate one free surface, two free surfaces, and the special double-bubble interface, most numerical questions reduce to choosing the correct geometry before substitution.
NEET Weightage - Excess Pressure
Surface Tension| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| NEET 2024 | 0 | 0 | |
| NEET 2023 | 0 | 0 | |
| NEET 2022 | 0 | 0 | |
| NEET 2021 | 0 | 0 | |
| NEET 2020 | 0 | 0 | |
| NEET 2019 | 0 | 0 | |
| Total (2019-2024) | 0 | 0 |
A liquid drop or a single bubble in liquid has one liquid-air surface contribution of 2T/R, while a soap bubble in air has two surfaces and therefore 4T/R.
Smaller bubbles have higher internal pressure, so when two unequal bubbles communicate, air moves from the smaller bubble to the larger one.
Double-bubble and coalescence questions are still surface-tension questions; the only new step is writing the correct pressure relation before conserving volume.
Excess Pressure Strategy for NEET
Identify the interface first Before using any formula, decide whether the question is about a drop, soap bubble, bubble in liquid, cylindrical surface, or common film between two bubbles. The number of effective free surfaces fixes the coefficient of T/R.
Check which side is concave Pressure is higher on the concave side of the curved liquid surface. In double-bubble questions this tells you why the common film bends toward the smaller bubble.
Use radius comparison immediately If two bubbles have different radii, write the smaller radius first because 1/r is larger. That single comparison predicts the direction of air flow without any arithmetic.
Keep coalescence separate from simple excess-pressure formulae Single Bubble Formation is not solved by 2T/R or 4T/R alone. Write pressure-times-volume conservation under isothermal conditions, then insert the surface-tension correction terms.
Include outside liquid pressure when depth is given For a bubble at depth h, the outside liquid is already under atmospheric plus hydrostatic pressure, so the final pressure difference becomes 2T/R + hdg in the OCR convention.
Excess Pressure Study Materials
PDF · Cheat Sheet · MCQ Set · PYQExcess Pressure Subtopics
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Rapid Revision - Excess Pressure
Concept → Trap → Example1) Definition and General Concept
Why inside pressure is largerDue to the property of surface tension a drop or bubble tends to contract and so compresses the matter enclosed. Hence in equilibrium the pressure inside a bubble or drop is greater than outside, and that difference is called excess pressure.
- In a liquid drop, the inward pull is balanced by hydrostatic pressure of the enclosed liquid.
- In a gas bubble, the balancing term is the gauge pressure of the confined gas.
- Do not call absolute internal pressure itself the excess pressure; only the inside-minus-outside difference counts.
2) Excess Pressure Formulas
Choose the right interfacePlane surface: ΔP = 0; drop or single curved liquid surface: ΔP = 2T/R; soap bubble in air: ΔP = 4T/R; cylindrical liquid surface: ΔP = T/R; unequal radii surface: ΔP = T[1/R1 + 1/R2].
- A soap bubble has two liquid surfaces, so its excess pressure is twice that of a drop of the same radius.
- A bubble at depth h carries the curved-surface term plus the hydrostatic contribution of the surrounding liquid.
- The common trap is to use 4T/R for every bubble even when the bubble is inside liquid and only one liquid surface matters.
3) Double Bubble Formation
Common film between unequal bubblesIf two soap bubbles of radii r1 and r2 are in contact, the pressure difference is ΔP = 4T[1/r1 - 1/r2], and the radius of the common interface is r = r1r2/(r2 - r1).
- Take r1 as the smaller radius so that 1/r1 - 1/r2 is positive.
- The common interface is concave toward the smaller bubble because the smaller bubble has larger internal pressure.
- Students often reverse the direction of curvature by looking only at size, not at which side carries higher pressure.
4) Single Bubble Formation
Isothermal coalescenceFor two isothermal soap bubbles of radii a and b coalescing to radius c, 4T(a^2 + b^2 - c^2) = P0(c^3 - a^3 - b^3). In vacuum this reduces to c^2 = a^2 + b^2.
- This relation comes from conservation of gas mass using PV proportional to mT under isothermal conditions.
- Do not conserve only volume without correcting the internal pressure terms of the original and final bubbles.
- If P0 = 0, the vacuum result is a direct square-law relation, not c^3 = a^3 + b^3.
US Curriculum Gaps - Excess Pressure
What U.S. Students Usually MissAP Physics 2 often treats surface tension qualitatively, not as a curved-interface pressure law
Students may know that bubbles try to shrink, but NEET expects immediate use of 2T/R, 4T/R, and the unequal-radii expression without re-deriving them each time.
- Memorise which geometry gives one surface and which gives two.
- Use radius comparison to predict the sign of pressure difference before calculating.
Introductory university fluids courses rarely include double-bubble interface numericals
The chapter goes beyond a single bubble and asks for the radius and curvature of the common film between two touching soap bubbles. That specific interface logic is a NEET-style differentiator.
- Write P1 - P2 first, then match it with 4T/r for the common film.
- Keep the film concave toward the smaller bubble because its internal pressure is greater.
Concept IQ Check - Excess Pressure
4 NEET-style MCQs with AnswersPractice Questions - Excess Pressure
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NEET Physics Revision Checklist
Use this section for quick chapter tracking before mocks, part tests, and final NEET revision.
Excess Pressure FAQs
Notes · Downloads · Revision · Important QuestionsWhy is pressure inside a bubble greater than outside?
Why does a soap bubble have 4T/R while a liquid drop has 2T/R?
Why do smaller bubbles have higher excess pressure?
What is the difference between a bubble in liquid and a soap bubble in air?
Why is the common film concave toward the smaller bubble in a double bubble?
When should I use the coalescence equation instead of ordinary excess-pressure formulas?
Why does the vacuum case give c^2 = a^2 + b^2?
Can excess pressure ever be zero?
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