A water film is formed between two parallel wires of 10 cm length. The distance of 0.5 cm between the wires is increased by 1mm. What will be the work done ? (Given, surface tension of water of `72xx10^(-3) Nm^(-1)`)

Options

A. 36 erg
B. 288 erg
C. 144 erg
D. 72 erg

Answer

B

Explanation

Work done = Surface tension `xx` increase in area of the film
`W =S xx Delta A `
Increae in area = Final area - initial area
` = 10 xx (0.5 + 0.1) - 10 xx 0.5 = 1 cm ^(2)`
`therefore W = 72 xx 2 xx 1 = 144` erg
`[because` There are 2 free surfaces,` therefore Delta A = 2 xx 1 ].`