What is the primary cause of capillarity?

The primary cause of capillarity is the interplay between surface tension, cohesive forces (attraction between liquid molecules), and adhesive forces (attraction between liquid and solid molecules). When adhesive forces are stronger, the liquid 'wets' the surface and rises. When cohesive forces are stronger, the liquid doesn't wet the surface and falls. Surface tension provides the upward or downward pull along the meniscus.

Does capillarity always result in a liquid rising in a tube?

No, capillarity does not always result in a liquid rising. If the adhesive forces between the liquid and the tube material are weaker than the cohesive forces within the liquid (e.g., mercury in a glass tube), the liquid will be depressed, meaning its level inside the capillary tube will fall below the external liquid level. This is known as capillary fall or depression.

How does the radius of the capillary tube affect the capillary rise or fall?

The radius of the capillary tube has a significant inverse relationship with the capillary rise or fall. According to Jurin's Law, $h \propto 1/r$. This means that the narrower the tube (smaller radius), the greater the height to which the liquid will rise or fall. This is why the effect is most pronounced in very fine, hair-like tubes, hence the term 'capillary'.

What is the angle of contact, and why is it important for capillarity?

The angle of contact ($\theta$) is the angle formed by the tangent to the liquid surface at the point of contact with the solid surface, measured inside the liquid. It's crucial because it determines whether the liquid will rise or fall. An acute angle ($\theta 90^\circ$) indicates non-wetting and capillary fall. A $90^\circ$ angle means no capillary action.

What happens if a capillary tube is of insufficient length for the calculated rise?

If a capillary tube is shorter than the height to which the liquid would normally rise, the liquid will rise to the top of the tube but will not overflow. Instead, the radius of curvature of the meniscus will increase (become flatter) to adjust the upward surface tension force, ensuring it precisely balances the weight of the liquid column that has risen. The angle of contact effectively adjusts to a new value to maintain equilibrium.

How does temperature affect capillary action?

Temperature significantly influences capillary action primarily by affecting the surface tension of the liquid. Generally, as the temperature of a liquid increases, its surface tension decreases. A decrease in surface tension, according to Jurin's Law ($h \propto T$), leads to a decrease in capillary rise (or a decrease in the magnitude of capillary fall). Therefore, liquids tend to exhibit less pronounced capillary effects at higher temperatures.

Capillarity

Physics

NEET UG

Version 1Updated 23 Mar 2026

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Capillarity, often referred to as capillary action, is a phenomenon where a liquid spontaneously rises or falls in a narrow tube, known as a capillary tube, or porous material. This behavior is a direct consequence of the interplay between the cohesive forces within the liquid (attraction between liquid molecules) and the adhesive forces between the liquid and the solid surface (attraction between…

Quick Summary

Capillarity is the phenomenon of a liquid rising or falling in a narrow tube, driven by the interplay of surface tension, cohesive forces (liquid-liquid attraction), and adhesive forces (liquid-solid attraction).

When adhesive forces dominate, the liquid wets the surface, forms a concave meniscus, and rises (e.g., water in glass, $\theta < 90^\circ$ ). When cohesive forces dominate, the liquid doesn't wet, forms a convex meniscus, and falls (e.

g., mercury in glass, $\theta > 90^\circ$ ). The height of rise or fall ( $h$ ) is given by Jurin's Law: $h = \frac{2T\cos\theta}{r\rho g}$ , where $T$ is surface tension, $\theta$ is the angle of contact, $r$ is the tube radius, $\rho$ is liquid density, and $g$ is acceleration due to gravity.

Key factors influencing capillarity are the tube's radius (inversely proportional), liquid's surface tension (directly proportional), and angle of contact. This principle is crucial in plant physiology, ink absorption, and various industrial processes.

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Key Concepts

Angle of Contact and Wetting

The angle of contact ( $\theta$ ) is a critical parameter that dictates the nature of capillary action. It's…

Jurin's Law and its Dependencies

Jurin's Law, $h = \frac{2T\cos\theta}{r\rho g}$ , is the mathematical core of capillarity. It highlights that…

Effect of Insufficient Tube Length

A common misconception is that if a capillary tube is too short for the calculated rise, the liquid will…

Capillarity: — Rise/fall of liquid in narrow tubes due to surface tension, cohesive, and adhesive forces.

Jurin's Law: —

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

$T$ — Surface Tension (N/m)

$\theta$ — Angle of Contact

- Acute ( $\theta < 90^\circ$ ): Rise, concave meniscus, wetting (e.g., water/glass) - Obtuse ( $\theta > 90^\circ$ ): Fall, convex meniscus, non-wetting (e.g., mercury/glass) - $90^\circ$ : No action, flat meniscus

$r$ — Radius of tube (m) (

h \propto 1/r

)

$\rho$ — Density of liquid (kg/m

^3

) (

h \propto 1/\rho

)

$g$ — Acceleration due to gravity (m/s

^2

) (

h \propto 1/g

)

Inclined Tube: — Vertical height

h

remains same, length along tube

L = h/\cos\alpha

(where

\alpha

is angle with vertical).

Insufficient Length: — Liquid rises to top, meniscus flattens (radius of curvature increases), no overflow.

How Tall Can Radius Drop Gravity? (H = 2TCos $\theta$ / RDG)

Height (

h

) is proportional to:

Tension (

T

)

Cosine of angle of contact (

\cos\theta

)

Inversely proportional to:

Radius (

r

)

Density (

\rho

)

Gravity (

g

)