A wire ring of 4 cm radius on the surface of a liquid and then raised. If surface tension of the tension of the liquid is `78.8 dyn e//cm`, find the pull (in g wt) required to raise the ring more before the film breaks than it is afterwards.

Explanation

Here, `S=78.8 dyn e//cm , r=4cm`
As the liquid touches the ring both along the inner and outer circumference, so force on the ring due to surface tension is
`F= 2 xx 2 pi r xx S = 4 pi r S`
`=4 xx 22/7 xx 4 xx 78.8 dyn e`
`= 4 xx 22/7 xx (4 xx 78.8)/(980) g wt = 4.04 g wt`.