A very light, rectangular wire-frame of dimensions `7 cm xx 5 cm` hangs just above the free surface of a liquid of surface tension T, with its plane parallel to the free surface. The wire -frame is just brought in contact with the liquid surface and then, lifted up. If the force required to lift the wire-frame is 3.36 N, then what is the value of T in `(N m^(-1))`?

Answer

`7`

Explanation

The downward force of surface tension acting on the wire frame is
`F_("surface tension")=Txxl` where l is the total lenth of the wire which is in contact with the liquid.
Now since it is a wire frame, the free surface of the liquid will be in contact with the inner as well as the outer perimeter of the wire.
Hence `l=2xx3xx(7+5)=48cm`
`Txx48/100=3.36N`
`impliesT=7Nm^(-1)`