A pair of adjacent coils has a mutual inductance of 1.5 H. If the current in one coil changes from 0 to 20 A in 0.5 s, what is the change of flux linkage with the other coil?

1. 35 Wb

2. 25 Wb

3. 30 Wb

4. 20 Wb

An air-cored solenoid having a length of 30 cm whose area is $25 c{m}^2,$ and the number of turns is 500 carries a current of 2.5 A. Suddenly the current is turned off and the time taken for it is ${10}^{-3} s.$ What would be the average value of the induced back-emf across the ends of the open switch in the circuit? (Neglect the variation in the magnetic field near the ends of the solenoid.)

1. 5.5 V

2. 4.5 V

3. 6.5 V

4. 4.0 V