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Physics · Class 11

Compare a drop and soap bubble

Mechanical Properties of Fluids

Surface tension makes the interior pressure exceed the exterior pressure. Smaller spheres need a larger pressure difference. A thin soap bubble has two interfaces and twice the excess pressure of a drop at the same radius and tension.

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Compare a drop and soap bubble

சிறிய துளியில் கூடுதல் அழுத்தம் அதிகம். சோப்புக் குமிழில் இரண்டு பரப்புகள் இருப்பதால் கூடுதல் அழுத்தம் 4γ/r.

Key notes, relationships and model limits

Key relationship

Δpdrop=2γ/r; Δpsoap bubble=4γ/r.

Remember

Surface tension makes the interior pressure exceed the exterior pressure. Smaller spheres need a larger pressure difference. A thin soap bubble has two interfaces and twice the excess pressure of a drop at the same radius and tension.

Common trap

A gas bubble immersed in a liquid does not have the soap bubble’s two interfaces.

Model limits

Spherical liquid drop with one surface or thin soap bubble with two interfaces. Surface tension is constant; gas bubble inside liquid would have one interface.

What you will learn

Use interface count and curvature to predict Laplace pressure.

Try the experiment

Try both scenarios, change one control, and predict the result before checking the graph and readouts.

Think of it this way

Change one input while keeping the others fixed, as you would in a controlled experiment.

Worked example

The matching single-surface drop has excess pressure 60 Pa. Halving either sphere’s radius doubles its excess pressure.

Why this matters

The model connects compare a drop and soap bubble with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

For γ=0.03 N/m and r=1 mm, what is soap-bubble excess pressure?

Read the explanation

4γ/r=4×0.03/0.001=120 Pa.

Course connection: Class XI Chapter 9. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.