The coercive force for a certain permanent magnet is 4.0 × 104 A m–1. This magnet is placed inside a long solenoid of 40 turns/cm and a current is passed in the solenoid to demagnetize it completely. Find the current.


Given:


Coercive force for magnet =4.0× 104Am-1


No. of turns per cm inside solenoid=40


No. of turns per m inside the solenoid =4000 turns/m


We know that,


Coercive force is defined by


Coercive force: the value of magnetic intensity (H) needed to make magnetization(I) zero is known as coercive force .


Therefore, magnetic intensity H=4.0× 104Am-1


Magnetic field produced by solenoid at the centre (B) is given by


…(i)


Where,


n=no. of turns per unit length


I=current in the solenoid


μ0=absolute permeability of vacuum


also magnetizing field intensity (H) in the absence of any material is given by



Where


B=net magnetic field


Putting the value of eqn.(i)



Putting the values of n and H




Therefore, current is given by 10A


1