Write the practical applications of capillarity.

Question 76 of 84

Q. Write the practical applications of capillarity.

Tamil Nadu DgeTamil Nadu HSC First Year (DGE) Board 2023Subjective· 3mImportance★★★★★

90% · 76/84 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Capillary Action

Capillary Action: The Physics of Water Defying Gravity

You've seen it happen a hundred times. Dip the corner of a paper towel into a spill, and watch the water climb upward into the towel — against gravity. Or take a thin glass tube (a capillary), put it in water, and the water rises inside it, higher than the outside surface. That's capillary action.

The name comes from capillus, Latin for "hair" — because the effect is strongest in tubes as thin as a hair.

The Intuition: Two Forces at War

Think of water molecules as tiny magnets. They attract each other strongly (cohesion), and they also stick to the walls of a container (adhesion). In a narrow tube, adhesion to the glass pulls the water upward along the walls. The water molecules, clinging to each other, drag the entire column up with them.

But gravity pulls down. The water rises only until the upward pull from adhesion is exactly balanced by the weight of the water column. That's the equilibrium height.

Note

If adhesion is stronger than cohesion (water-glass), the liquid rises. If cohesion is stronger (mercury-glass), the liquid is depressed — it falls below the outside level. Mercury doesn't wet glass.

The Precise Physics: What Determines the Height?

For a liquid that wets the tube (contact angle $\theta < 90^\circ$ ), the rise height $h$ is given by the Jurin's law:

h = \frac{2\gamma \cos\theta}{\rho g r}

Where:

$\gamma$ = surface tension of the liquid (N/m)
$\theta$ = contact angle between liquid and tube wall
$\rho$ = density of the liquid (kg/m³)
$g$ = acceleration due to gravity (9.8 m/s²)
$r$ = radius of the tube (m)

h = \frac{2\gamma \cos\theta}{\rho g r}

The key insight: $h$ is inversely proportional to $r$ . Halve the tube radius, and the water rises twice as high. That's why the effect is only noticeable in very narrow tubes — in a wide pipe, $h$ is negligible.

Why Does Surface Tension Pull Upward?

The surface tension $\gamma$ acts along the circumference of the water-air interface inside the tube. The total upward force is:

F_{\text{up}} = (2\pi r) \times \gamma \cos\theta

The weight of the water column (height $h$ , density $\rho$ ) is:

F_{\text{down}} = (\pi r^2 h) \times \rho g

Set them equal, cancel $\pi r$ , and you get Jurin's law.

Tip

For water in clean glass, $\theta \approx 0^\circ$ (perfect wetting), so $\cos\theta = 1$ . Then $h \approx \frac{2\gamma}{\rho g r}$ . For water at room temperature, $\gamma \approx 0.073$ N/m, so $h \approx \frac{0.015}{r}$ (with $r$ in metres). A tube of radius 0.1 mm gives a rise of about 15 cm.

Real-World Examples

Paper towels and sponges: The fibres form millions of tiny capillary channels. Water rises through them, soaking the towel. …

Start your 14-day free trial to unlock the full solution →

Capillary action explains many everyday phenomena where liquid moves through narrow spaces -- wicks, blotting paper, plant sap rise, and soil moisture movement.

Capillarity (capillary action) is the phenomenon by which a liquid rises or is drawn up a narrow tube or a porous/fibrous material, due to the combined effect of surface tension and the adhesive forces between the liquid and the material's surface (adhesive forces exceeding cohesive forces, as with wetting liquids like water).

Practical applications of capillarity:

Wicks: oil rises up the narrow fibres of a lamp or lantern wick by capillary action, feeding the flame continuously.
Fountain pens: ink rises through the narrow capillary channel of the pen's nib to keep the tip supplied with ink.
Plants and trees: water (with dissolved minerals) rises from the soil through the roots and up through the narrow xylem vessels to reach the leaves, aided by capillary action, along with root pressure and transpiration pull.
Blotting paper and towels: these are made of fine fibres with narrow capillary spaces that absorb ink or water quickly by capillary action. …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ13 — Define Poisson's ratio.→Next questionQ15 — (a) Derive the expression for the terminal velocity of a sphere moving…

Q.Write the practical applications of capillarity.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days