Physics · Mechanical Properties Of Fluids · NEET
It is both, seen two ways. As force per unit length, S = F/L (unit N/m). As surface energy per unit area, S = Work/Area (unit J/m2). Both give the same number, because N/m and J/m2 are the same unit. NCERT says exactly this: surface tension is force per unit length OR surface energy per unit area.
It is caused by cohesion — the attraction between molecules of the same liquid. A molecule deep inside is pulled equally in all directions, so the net pull is zero. But a molecule at the surface has liquid only below it, so it feels a net inward pull. This inward pull makes the surface act like a stretched skin that tries to shrink. That shrinking tendency is surface tension.
The surface tension force acts ALONG (in the plane of) the surface, not inward. It is the net molecular attraction that is inward, and that is the cause. The measured surface tension force is tangential to the surface, pulling the surface edges together so the area becomes minimum.
The water surface behaves like a stretched membrane. When the needle rests on it, the surface bends slightly and the surface tension forces along the contact line push up. This upward component balances the weight. It is not buoyancy — it is the surface tension of the skin holding the object.
It decreases as temperature rises. Higher temperature means faster molecules and weaker net cohesion, so the skin becomes weaker. At the critical temperature the surface tension becomes zero. This is why hot water cleans better than cold water.
S = F/L uses one straight line of length L on the surface (force per unit length). S = W/A uses the work done to create new area A (energy per unit area). They are the same quantity. Use F/L for a wire/ring/disc touching the surface, and W/A when the question gives work done to stretch a film.
A rectangular film of liquid is extended from (4 cm x 2 cm) to (5 cm x 4 cm). If the work done is 3 x 10^-4 J, the value of the surface tension of the liquid is:
A thin flat circular disc of radius 4.5 cm is placed gently over the surface of water. If the surface tension of water is 0.07 N/m, then the excess force required to take it away from the surface is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the force per unit length acting along a liquid surface, making the surface behave like a stretched skin that tries to shrink to minimum area.
S = F/L (force per unit length). SI unit is N/m (newton per metre), which is the same as J/m2 (joule per square metre).
Cohesion — the attraction between like molecules. Surface molecules are pulled inward because they have no liquid above them, which makes the surface act like a tight membrane.
Surface tension tries to make the surface area as small as possible. For a fixed volume, a sphere has the least surface area, so small drops (where gravity is weak) become spherical.
It decreases when temperature rises and usually decreases when a soluble impurity like soap or detergent is added. Some impurities like salt can slightly increase it.
Surface tension itself (the property S) is treated as a scalar. The surface tension force it produces is a vector acting in the plane of the surface.
