THE PRACTICE STUDIO
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PRACTICE / Excess pressure in drops and bubbles
Count curvature before multiplying
A long cylindrical liquid interface has radius 2.0 mm. A spherical drop of the same liquid has radius 4.0 mm. Each has one liquid–gas interface with the same surface tension. Neglect gravity-induced distortion and cylinder end effects.
What is the spherical pressure jump divided by the cylindrical pressure jump?
Organize the givens & assumptions
- Cylinder radius = 2.0 mm
- Sphere radius = 4.0 mm
- One interface each
- Same surface tension
Static or initially equilibrated interfaces, constant uniform surface tension, negligible gravity-induced shape distortion where spherical or cylindrical curvature is specified, and negligible evaporation. Capillary heights use the narrow-tube model with equal gas pressures and a large reservoir.
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Problem: Count curvature before multiplying A long cylindrical liquid interface has radius 2.0 mm. A spherical drop of the same liquid has radius 4.0 mm. Each has one liquid–gas interface with the same surface tension. Neglect gravity-induced distortion and cylinder end effects. Question: What is the spherical pressure jump divided by the cylindrical pressure jump? Givens: - Cylinder radius = 2.0 mm - Sphere radius = 4.0 mm - One interface each - Same surface tension Assumptions: Static or initially equilibrated interfaces, constant uniform surface tension, negligible gravity-induced shape distortion where spherical or cylindrical curvature is specified, and negligible evaporation. Capillary heights use the narrow-tube model with equal gas pressures and a large reservoir. Choices (LaTeX): A. 1 B. 2 C. 1/2 D. 4 Ask for my approach first. Help me distinguish a physical misconception from an algebra error without revealing which choice is correct. Tutoring approach: Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.
Start with the smallest hint.
Work through it with me.
One decision at a time. Pause, predict, then unpack the algebra.
A factor of two needs a physical explanation: curvature and interface count are distinct.
Write each jump
The long cylinder is flat along its axis; the sphere curves in both directions.
Why is this step valid?
The factor of two here counts curvature directions, not physical liquid surfaces.
Cancel common tension
Compare jumps without needing a numerical surface tension.
Why is this step valid?
The common material property cancels because both interfaces have the same tension.
Insert the radius ratio
The sphere’s radius is twice the cylindrical radius.
Why is this step valid?
Use the same length unit in both radii so the ratio is dimensionless.
For S = 0.048 N/m, both jumps evaluate to 24 Pa.
TRY THE IDEA AGAIN
If the two radii instead matched, the spherical-to-cylindrical pressure ratio would be…
YOUR LEARNING NOTE
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