A solenoid has 2000 turns wound over a length of 0.30 m. The area of its cross-section is 1 .2 x 10-3 m2. Around its central section of a coil of 300 turns is wound. If an initial current of 2 A in the solenoid is reversed in 0.25 sec., the e.m.f. induced in the coil is equal to

Magnetic flux at the centre of solenoidB1=μ0Npi1/l   ......(1)Magnetic flux linked with the coil of Ns turns wound at the centre of solenoidΦ2=NSB1A2(A2 = area of cross-section of the coil)=Nsμ0NpilA2    .......(2)According to definition of mutual inductanceΦ2=Mi1∴Mi1=NSμ0NpilA2 or M=μ0NpNsA2lIn the given problemM=4π×10−7×2000×300×1⋅2×10−30⋅30e=Mdidt4π×10−7×2000×300×1⋅2×10−30⋅30×40⋅25=4⋅8×10−2 volt