Physics · Mechanical Properties Of Fluids · NEET
A thin film is very thin but it still has TWO free surfaces in contact with air: one on the front and one on the back. Each surface has surface energy T per unit area. When you stretch the film, you create new area on both surfaces at the same time. So the total new area is 2 x (change in one-side area), and W = T x 2 x change in area.
No. A single free surface (like the flat top of water in a beaker, or a plain sheet of water) has only ONE surface. For that, W = T x change in area, with no factor of 2. The factor of 2 is ONLY for a thin film with two sides.
A liquid drop has only ONE surface (outer surface), so its surface energy uses one 4(pi)r^2. A soap bubble or a soap film has TWO surfaces. So a drop: E = T x 4(pi)r^2. A bubble: E = T x 2 x 4(pi)r^2. A flat film: W = T x 2 x change in area.
Rearrange the formula. If a film's area changes by (change in area) and work W is done, then T = W / (2 x change in area). Always convert area to square metres (1 cm^2 = 10^-4 m^2) before dividing. The answer comes in N/m.
You use only the CHANGE in area (final area minus initial area), not the initial area alone. Work is done only for the EXTRA surface you newly create. change in area = (final area) - (initial area), then multiply by 2 for the two surfaces.
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.
W = T x 2 x (change in area), where T is surface tension in N/m and change in area is the increase in one-side area in m^2. The 2 is because the film has two surfaces.
A thin film is a thin layer of liquid with air on both sides. The front face touches air and the back face touches air, so there are two free liquid surfaces, each carrying surface energy.
Surface tension has units of N/m (newton per metre), which is the same as J/m^2 (joule per square metre). It can be seen as force per unit length or energy per unit area.
For a single free surface, such as the top of water in a beaker or a plain liquid sheet, use W = T x change in area with no factor of 2. Use 2 only for a two-sided film or a soap bubble.
Yes. The extra work you do to create new surface is stored as extra surface energy. So work done = increase in surface energy = T x 2 x change in area for a film.
