Physics · Mechanical Properties Of Fluids · NEET
They describe the same effect in two ways. Surface tension T is the force acting per unit length along the edge of a surface (unit N/m). Surface energy is the extra potential energy stored in the surface because surface molecules have fewer neighbours (unit J). The link is simple: surface energy = T x area, so T is also the surface energy per unit area. That is why N/m and J/m2 are the same unit.
A thin liquid film, like a soap film held in a wire frame, has TWO free surfaces: a front face and a back face. Both faces get bigger when you pull the film. So if the film area (of one face) increases by delta A, the total new surface area created is 2 x delta A. Work done W = T x (total new area) = T x 2 x delta A. Forgetting the factor 2 is the most common mistake.
Use 2 only for a FILM (two surfaces), such as a soap film in a frame or a soap bubble. A single liquid drop or a flat pool of water has only ONE free surface, so for those W = T x delta A (no factor 2). A soap bubble is special: it has an inner and outer surface, so its area is counted as 2 x 4(pi)r2.
When you slowly pull a movable wire to enlarge a soap film, you do work against the surface tension pulling the wire back. That work is not lost; it is stored as extra surface energy in the newly created surface. So work done by you = increase in surface energy = T x (increase in total area). This is an energy-conservation statement, not friction or heat.
Rearrange W = T x 2 x delta A to get T = W / (2 x delta A). Convert areas to m2 (1 cm2 = 10^-4 m2) before dividing. Example: if W = 3 x 10^-4 J and the film area increases by delta A = 12 cm2 = 12 x 10^-4 m2, then T = 3 x 10^-4 / (2 x 12 x 10^-4) = 0.125 N/m.
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:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Surface tension is measured in N/m (newton per metre). Surface energy per unit area has the unit J/m2. Since 1 J = 1 N x 1 m, J/m2 = N/m, so both quantities share the same unit. Total surface energy (not per unit area) is measured in joules (J).
It is stored, not lost. The work you do against surface tension becomes extra surface (potential) energy in the newly formed surface. If you let the film relax, the surface can do that energy back, which is why films and bubbles shrink on their own to reduce area.
Molecules inside a liquid are pulled equally in all directions by neighbours, so their net force is zero. Molecules at the surface have neighbours only below, so they are pulled inward. This inward pull makes the surface behave like a stretched elastic sheet and gives it extra energy, which we call surface tension or surface energy.
Yes. Surface tension of a liquid decreases as temperature rises, because faster molecular motion weakens the net inward pull at the surface. At the critical temperature the surface tension becomes zero. NEET rarely needs the exact value, but you should know the trend: higher temperature means lower surface tension.
Surface tension appears as force per length: T = F / L. Surface energy appears as energy: E = T x A (for one surface) or E = T x 2A (for a film with two surfaces). Numerically T equals surface energy per unit area, so the same number describes both.
