Q. A closely wound solenoid80cm long has 5 layers of winding of400 turns each. The diameter ofthe solenoidis 1.8cm. If the current carried is 8A, the magnitude of B inside the solenoid near its centre will be- 1. 8pie*10^-3T 2. 6pie*10^-3T 3. 4pie*10^-3T 4. 3pie*10^-3T

magnetic field due to a solenoid is = B=    …...here n is the winding density I is the current is the permiability of free space

 

=>B = 5 x 4x 10^-7 x (400/0.8) x 8    here 5 is multiplied as 5 layers are present

=>B = 8x10^-3 T

 

dosent this answer seem satisfying but actually logically this is wrong the maker of this question must be an idiot who dosent know physics just think logically no..  let current thru  the 1^st layer be towards the right hand side then the current in the 2^nd layer should be in the left hand direction hence the field by the two layers should cancel out  then only one layer will contribute to  the magnetic field then the answer would be 16x10^-4   T... hence the idiot who made this question should have mentioned the way of winding

cause the direction of winding changes the answer totally ..

 

P.S so understand the concepts completly to shine in physics and parctise a lot hope this is helpful ;)