Magnetic induction produced at the centre of a circular loop of radius ' $R$ ' carrying a current is ' B '. The magnetic moment of the loop is ( $\mu_0=$ permeability of free space)

The pulleys and strings shown in figure are smooth and of negligible mass. For the system to remain in equilibrium, angle $\theta$ should be

In biprism experiment, the fringe width is 0.6 mm . The distance between $6^{\text {th }}$ dark fringe and $8^{\text {th }}$ bright fringe on the same side of central bright fringe is

A radioactive substance has half-life of 60 minute. During 3 hour, the amount of substance decayed would be