A rectangular film of liquid is extended from `5 cm xx 3 cm to 6 cm xx 5 cm`. If the work done is `3.0 xx 10^(-4) J`. The surface tension of liquid is

Options

A. `0.5 Nm^(-1)`
B. `0.1 Nm^(-1)`
C. `0.2 Nm^(-1)`
D. `2 Nm^(-1)`

Answer

B

Explanation

Given, `A_(1) = 5 cm xx 3 cm = 15 cm^(2)`
`A_(2) = 6 cm xx 5 cm = 30 cm^(2)`
`W = 3.0 xx 10^(-4) J`
`S = ?`
Increase in area,
`Delta A = 2(6 xx 5-5 xx 3) = 2 xx 15 cm^(2)` [film has 2 free surfaces]
`= 30 xx 10^(-4) m^(2)`
As work done , W = surface tension `xx` increase in area
`3.0 xx 10^(-4) = S xx 30 xx 10^(-4)`
`rArr S = 0.1 Nm^(-1)`