When a capillary is dipped into a beaker full of water then water rises in it because the pressure just below the meniscus is-

Options

A. less than the atmospheric pressure
B. more than the atmospheric pressure
C. equal to the atmospheric pressure
D. 13.6 N/m

Answer

To solve the question of why water rises in a capillary when dipped into a beaker full of water, we can break down the explanation into a series of logical steps:

### Step-by-Step Solution:

1. **Understanding Capillarity**: When a capillary tube is dipped into water, the water inside the tube forms a meniscus. The shape of the meniscus depends on the adhesive forces between the water molecules and the molecules of the tube material.

2. **Meniscus Shape**: In the case of water in a glass capillary, the meniscus is concave. This means that the water level inside the capillary is higher than the water level in the beaker.

3. **Pressure Consideration**: The pressure at different points in the capillary tube is crucial to understanding why the water rises. Just below the meniscus, the pressure is lower than the pressure just above the meniscus due to the effects of surface tension.

4. **Surface Tension Effect**: The pressure difference created by surface tension can be quantified using the formula:
\[ \Delta P = \frac{2t}{r} \]
where \( t \) is the surface tension of the liquid and \( r \) is the radius of the capillary tube.

5. **Pressure Below the Meniscus**: The pressure just below the meniscus can be expressed as:
\[ P_{\text{below}} = P_{\text{above}} - \frac{2t}{r} \]
This indicates that the pressure below the meniscus is less than the pressure above it.

6. **Comparison with Atmospheric Pressure**: Since the pressure above the meniscus is equal to the atmospheric pressure, the pressure below the meniscus is:
\[ P_{\text{below}} < P_{\text{atmospheric}} \]
This means that the pressure below the meniscus is less than the atmospheric pressure.

7. **Resulting Water Rise**: Due to the pressure difference, water rises in the capillary tube until the pressure at the same level inside the tube equals the atmospheric pressure outside the tube. This balance allows the water to rise to a certain height, known as the capillary rise.

### Conclusion:
The water rises in the capillary tube because the pressure just below the meniscus is less than the atmospheric pressure, which allows the liquid to be drawn up into the tube.

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