Considering interference between two sources of intensities ' I ' and ' 4 I ', the intensity at a point where the phase difference is $\pi$ is $(\cos \pi=-1)$

If number of turns per unit length in a solenoid is tripled, the self inductance of solenoid will

$A$ sphere ' $A$ ' of radius ' $R$ ' has a charge ' $Q$ ' on it. The field at point B outside the sphere is ' $E$ '. Now another sphere of radius ' $2 R$ ' having a charge ' $-2 Q$ ' is placed at B. The total field at the point midway between A and B due to both the spheres is

Two rain drops of same radius are falling through air each with a steady speed of $5 \mathrm{~cm} / \mathrm{s}$. If the drops coalesce, the new steady velocity of big drop will be