An elasticized conducting band r is around a spherical balloon T (figure). Its plane passes through the center of the balloon. A uniform magnetic field of magnitude 0.04 T  is directed perpendicular to the plane of the band. Air is let out of the balloon at 100 cm3s-1 at an instant when the radius of the balloon is 10 cm. The induced emf in the band is

Volume of the balloon at any instant, when radius is r,V=43πr3Time rate of change of volume,dVdt=4πr2drdtTime rate of change of radius of balloon,drdt=14πr2dVdtFlux through rubber band at the given instant,ϕ=Bπr2Induced emf =-dϕdt=-ddtBπr2=-2πrBdrdt=-2πrB14πr2dVdt=-B2rdVdtAs volume of the balloon is decreasing, dVdt is negativeEinduced=-0.042×10×10-2×-100×10-6=20μV