The rise or fall of liquid in a small-diameter capillary tube is affected by what forces?

The rise or fall of liquid in a small-diameter capillary tube is fundamentally influenced by cohesive forces, adhesive forces, and surface tension. Cohesive forces are the intermolecular forces that hold the liquid molecules together, while adhesive forces are those between the liquid molecules and the walls of the capillary tube.

When a liquid is introduced into a capillary tube, if the adhesive forces between the liquid and the tube's walls are stronger than the cohesive forces within the liquid, the liquid will climb up the tube (capillary rise). This phenomenon is enhanced by surface tension, which acts to minimize the surface area of the liquid. Surface tension is a result of cohesive forces at the surface of a liquid, where molecules experience a net inward force due to their interactions with other molecules.

In contrast, gravity plays a role in opposing the capillary action, but it does not determine the initial rise or fall; rather, it opposes the action initiated by surface tension and adhesive forces. Therefore, while gravity and viscosity have their significance in fluid dynamics, they do not directly control the extent of liquid movement in a capillary tube to the same degree as the mentioned forces.