Water film of thickness 0.5 mm exists between two flat circular glass plates of diameter 10 cm each. If the surface tension of water is `7 xx 10 N m^-1`, the force in Newton required to pull the plates apart is

Options

A. `1.75pi xx 10^-6`
B. `7pixx10^-3`
C. `7/pi xx 10^(-5)`
D. `7/10 xx 10^(-4)`

Answer

B

Explanation

Area of film, `A=pi r^(2)`
Diameter=10cm
`r = d/2 = (10xx10^(-2))/2 = 5xx10^(-2)m`
`A=pixx(5xx10^(-2))^(2)=25pi xx 10^(-4) m^(2)`
Workdone, w=2TA
`=2xx7xx10^(-4)xx25pixx10^(-4)`
`35pi xx 10^(-7)J`
`(35 pi xx10^(-7))/(0.5xx10^(-3))`
`7pixx10^(-3)N`