Question

A wire ring of diameter $14cm$ is gently lowered on to a liquid surface and then pulled up. When the film just breaks, the force required is $0.0616N$. The surface tension of the liquid is

• A. $70N/m$
• B. $70dyne/cm$
• C. None of these
• D. $70N/m$

Answer 1

The correct option is B $70dyne/cm$

To break the film the force required is
$0.0616N=6160dyne$
$F=T\times l$
$6160=2\pi rT\times 2$
$6160=2\times 3.14\times 7\times T\times 2$
$T=\frac{6160}{2\times 2\times 3.14\times 7}dyne/cm$
$T=70dyne/cm$

Final answer: (c)