Question

Explain the rise of liquid in the capillary on the basis of pressure difference.

Answer 1

When a capillary tube is dipped in water, the water meniscus inside the tube is concave. The pressure just below the meniscus is less than the pressure just above it by $2T/R$, where $T$ is the surface tension of water and $R$ is the radius of curvature of meniscus. The pressure on the surface of water is atmospheric pressure $P$. The pressure just below the plane surface of water outside the tube is also $P$, but that just below the meniscus inside the tube is $P-\left(2T/R\right)$. We know that the pressure at all points in the same level of water must be the same. Therefore, to make up the deficiency of pressure, $2T/R$, below meniscus, water begins to flow from outside to inside the tube. The rising of water in the capillary takes place.