A U-shaped wire is dipped in a soap solution, and removed. A thin soap film formed between the wire and a light slider supports a weight of `1.5xx10^(-2)N` (which includes the small weigh of the slider). The length of the slider is 30cm. What is the surface tension of the film?

Options

A. `5.6 xx 10^(-4) Nm^(-1)`
B. `3.8 xx 10^(-6) Nm^(-1)`
C. `2.5 xx 10^(-2) Nm^(-1)`
D. `7.3 xx 10^(-5) Nm^(-1)`

Answer

C

Explanation

Length of the slider (l) `= 30 cm`
As a soap film has two free surfaces, therefore total length of the film to be supported
`l' = 2l = 2 xx 30 = 60 cm = 0.60 m`
Let S be the surface tension of the soap solution.
Total force on the slider due to surface tension,
`F = S xx 2l`
`F = S xx 0.60 N` ....(i)
Weight (w) supported by the slider `= 1.5 xx 10^(-2)N` (Given)
In equilibrium,
Force on the slider due to surface tension = Weight supported by the slider
`:. F = W`
`S xx 0.60 = 1.5 xx 10^(-2)`
`rArr S = (1.5 xx 10^(-2))/(0.60) = 2.5 xx 10^(-2) Nm^(-1)`