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Physics · Class 11

Change tube radius and wetting

Mechanical Properties of Fluids

Surface tension around the tube balances the weight of the raised liquid column. Smaller radius produces a larger rise. A contact angle above 90° reverses the sign, producing capillary depression.

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Change tube radius and wetting

குழாய் ஆரம் குறைந்தால் நுண்புழை ஏற்றம் அதிகரிக்கும். தொடுகோணம் 90°க்கு மேல் இருந்தால் திரவ மட்டம் தாழும்.

Key notes, relationships and model limits

Key relationship

h=2γcosθ/(ρgr); h>0 is rise and h<0 is depression.

Remember

Surface tension around the tube balances the weight of the raised liquid column. Smaller radius produces a larger rise. A contact angle above 90° reverses the sign, producing capillary depression.

Common trap

Capillary rise is inversely proportional to radius, not tube cross-sectional area.

Model limits

Narrow circular tube; static equilibrium, negligible meniscus volume and reservoir level change. Negative h means depression; contact angle measured through the liquid.

What you will learn

Predict capillary magnitude and sign from geometry and wetting.

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

At 1 mm radius the rise halves to 0.01468 m. At contact angle 120°, cosθ=−0.5 gives half that magnitude downward.

Why this matters

The model connects change tube radius and wetting with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

How high does water rise for r=0.5 mm, γ=0.072 N/m and θ=0°?

Read the explanation

h=0.144/(1000×9.81×0.0005)=0.02936 m.

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