A ring is cut from a platinum tube `8.5 cm` internal and `8.7 cm` external diameter. It is supported horizontally from the pan of a balance, so that it comes in contact with the water in a glass vessel. If an extra `3.103 g.f.` is required to pull it away from water, the surface tension of water is

Options

A. 72 dyne `cm ^(-1)`
B. `70.80 ` dyne `cm ^(-1)`
C. `63.35` dyne `cm ^(-1)`
D. 60 dyne `cm ^(-1)`

Answer

A

Explanation

`(2 pir _(1) + 2pi r _(2)) sigma = mg`
`[ 2pixx (8.7)/( 2) + 2 pi xx (8.5)/( 2) ] sigma = 3. 97 xx 980 implies sigma = 72 ` dyne `cm^(-1)`