Question

The magnetic induction at a point P which is at a distance 4cm from a long current carrying wireis 10-8 tesla. The field of induction at a distance 12cm from the same current would be(A) 3.33 *10^9 tesla (B) 1.11*10^4 tesla(C) 3 *10^3 tesla (D) 9 *10^2 tesla

Answer 1

the value of magnetic field given is not clear... 10-8 ??  is that  10^{-8}

The magnetic field B of induction at a point P at distance d from the long wire is given by :  B =  Mu_0  i / (2 π d)

   It is inversely proportional to the distance.

   So the field at 12 cm will be = 1/3 of field at 4 cm.. which is 10^{-9} Tesla.
 
  So   (A)  3.33 * 10^{-8} Tesla