The current in the primary coil of a pair of coils changes from 7 A to 3 A in 0.04 s. The mutual inductance between the two coils is 0.5 H. The induced emf in the secondary coil is -

A square sheet of side 'a' is lying parallel to XY plate at z = a. The electric field in the region is $$\overrightarrow E = c{z^2}\widehat k$$. The electric flux through the sheet is

Three charges q, $$-$$q and q₀ are placed as shown in figure. The magnitude of the net force on the charge q₀ at point O is $$\left[ {k = {1 \over {(4\pi { \in _0})}}} \right]$$

A + 3.0 nC charge Q is initially at rest at a distance of r₁ = 10 cm from a + 5.0 nC charge q fixed at the origin. The charge Q is moved away from q to a new position at r₂ = 15 cm. In this process work done by the field is