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