The potential energy per unit area of the surface film is called surface energy. Consider a metal frame ABCD in which AB is movable. The frame is dipped in a soap solution. A film is formed which pulls AB inwards due to surface tension. If T is the surface tension of the film and l is the length of the wire AB. this inward force is given by 2 x Tl. The number 2 indicates the two free surfaces of the film.
Fig: Surface energy
If AB is moved through a small distance x as shown in Figure, to the position AB’, then work done is, W = 2Tlx
Work done per unit area: W/Tlx
So, Surface energy = T2lx/2lx
Surface energy = T
Surface energy is numerically equal to surface tension.