An air-cored solenoid with length 30 cm, area of cross-section `25 cm^2` and number of turns 500, carries a current of 2.5 A. The current is suddenly switched off in a brief time of `10^-3s`. How much is the average back emf induced across the ends of the open switch in the circuit? Ignore the variation in magnetic field near the ends of the solenoid.