Physics · Mechanical Properties Of Fluids · NEET
A soap bubble is a thin film of soap solution with air inside AND air outside. So it has TWO liquid surfaces: one inner surface and one outer surface. Each surface has area 4 pi r^2, so total new area created = 2 x 4 pi r^2. A liquid drop (like a raindrop) is solid liquid inside with only ONE outer surface, so you use only 4 pi r^2. Energy = surface tension x new area, so bubble energy = T x 2 x 4 pi r^2 = 8 pi r^2 T, while drop energy = T x 4 pi r^2 = 4 pi r^2 T.
No. These are two different questions that students mix up. Energy to FORM the bubble = work to create the surface = 8 pi r^2 T (for a soap bubble). Excess PRESSURE inside a soap bubble = 4T/r. One is measured in joules (energy), the other in pascals (pressure). If a question says 'energy' or 'work done', use 8 pi r^2 T. If it says 'pressure difference' or 'excess pressure', use 4T/r.
The work equals the change in surface energy. Energy at any radius r is 8 pi r^2 T. So work W = 8 pi T (r2^2 - r1^2). You only need the CHANGE in area, so subtract the squares of the two radii. If the bubble grows from zero (freshly formed), r1 = 0 and W = 8 pi r2^2 T.
Always use RADIUS in W = 8 pi r^2 T. If the question gives you the diameter d, first divide by 2 to get r = d/2. A common NTA mistake is to plug in the diameter, which makes your area 4 times too big and your answer 4 times too large.
No. For these NEET numericals the energy to form a bubble is purely surface energy = surface tension x new area. The mass of the film is tiny and is ignored. You do NOT add any mgh term. Only the two surfaces and the surface tension matter.
The amount of energy required to form a soap bubble of radius 2 cm from a soap solution is nearly (surface tension of soap solution = 0.03 N/m):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
W = T x 2 x (4 pi r^2) = 8 pi r^2 T, where T is surface tension and r is the bubble radius. The factor 2 is because a soap bubble has two surfaces.
A soap bubble is a thin film with air on both sides (inside and outside), so both faces of the film are liquid surfaces. A liquid drop is filled with liquid and touches air only on the outside, so it has one surface.
W = 4 pi r^2 T, using only one surface. For n identical small drops of radius r joining into one big drop, use surface energy conservation with the same one-surface formula.
Energy to form the bubble = 8 pi r^2 T (units: joules). Excess pressure inside a soap bubble = 4T/r (units: pascals). They answer different questions; do not swap them.
A smaller bubble has higher excess pressure (4T/r increases as r decreases), so more pressure is needed at the start. This connects to the next concept: why a smaller bubble has higher inside pressure.
