A capillary tube of radius r is immersed in water, as shown in the...

The pressure on the concave side of a liquid meniscus is greater than the pressure on the convex side by an amount $\dfrac{2S}{R}$, where $S$ is the surface tension and $R$ is the radius of curvature of the meniscus.

For water in a capillary tube, the meniscus is concave upwards. The air is on the concave side and the liquid is on the convex side.

The pressure just above the meniscus is the atmospheric pressure $P_0$. Point A is located just below the meniscus, on the convex side.

Assuming the angle of contact for water and glass to be zero, the radius of curvature $R$ is equal to the radius of the capillary tube $r$.

Therefore, the pressure difference across the meniscus is given by:

$P_{\text{above}} - P_A = \dfrac{2S}{r}$

Substituting $P_{\text{above}} = P_0$, we get:

$P_0 - P_A = \dfrac{2S}{r}$

$P_A = P_0 - \dfrac{2S}{r}$