Question

if we compare the magnetic field of a current carrying solenoid with that of pipe carrying current parallel to its axis assuming both to be infinitesimally long then
a)the field inside the solenoid is zero
b)the field outside the solenoid is zero
c) the field inside the pipe is zero
d) the field outside the pipe is zero

Answer 1

The magnetic field inside a solenoid =  μ₀ n i ,   if  radius << length.
             n = number of turns per unit length
this is directed parallel to the axis of the solenoid.

the magnetic field outside a solenoid = 0  (nearly)

the magnetic field inside a pipe =  0 as  integral of  B.dS = μ i   and  the current enclosed with in a circuital path (closed)is zero inside the pipe...  We know that the current (electric charges) reside on top of the outer cylindrical surface (of the conductor).   This is according to the circuital law.. Is that Lenz's ?

the magnetic field outside the pipe at a distance  d  from the axis of the pipe is :
       B . 2 π d l =  μ i
         so  B  =  μ i / (2 π d l )            and it is radially pointing outwards...