1.

A film of water is formed between two straight paralel wires each `10 cm` long and at seperation `0.5 cm`. Calculate the work requied to increase `1mm` distance between the wires. Surface tension of water `=72xx10^(-3)N//m`.

Answer» Initial surface area `=2xx"length"xx"separation"`
`=2xx10cmxx0.5cm=10cm^(2)=10xx10^(-4)m^(2)`
Final surface area `=2xx10cmxx(0.5+0.1)cm`
`=2xx10xx0.6cm^(2)=12xx10^(-4)m^(2)`
The required work
`WT/_A=72xx10^(-3)xx(12xx10^(-4)-10xx10^(-4))J`
`=72xx10^(-3)xx2xx10^(-4)=144xx10^(-7)J`


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