• id: JME-09-N13
• title: Surface Tension — Concept and Formula
• tags: surface-tension, meniscus, capillary

Surface tension ($S$ or $T$) is the force per unit length acting along the surface of a liquid, perpendicular to any line drawn on the surface: $S = F/l$. SI unit: N m$^{-1}$. Dimensional formula: [M T$^{-2}$]. It arises because surface molecules have unbalanced intermolecular forces (net inward pull). Excess pressure inside a spherical drop: $\Delta P = 2S/R$. Inside a soap bubble (two surfaces): $\Delta P = 4S/R$. Surface tension decreases with temperature (molecules have more kinetic energy to overcome cohesive forces). Surface tension causes capillary rise, meniscus formation, and spherical shape of small drops.