In air, a charged soap bubble of radius ' $R$ ' breaks into 27 small soap bubbles of equal radius ' $r$ '. Then the ratio of mechanical force acting per unit area of big soap bubble to that of a small soap bubble is

Two parallel conductors carrying unequal currents in the same direction ............

A layer of atmosphere that reflects medium frequency radio waves which is ineffective during night, is

A transverse wave is propagating on the string. The linear density of a vibrating string is $10^{-3} \mathrm{~kg} / \mathrm{m}$. The equation of the wave is $Y=0.05 \sin (x+15 t)$ where $x$ and $Y$ are measured in metre and time in second. The tension force in the string is