Answer:
(a) 
(b) 
Explanation:
(a) The surface current density of a conductor is the current flowing per unit length of the conductor.

Considering a wire, the current is uniformly distributed over the circumferenece of the wire.

The radius of the wire = a

The surface current density 
(b) The current density is inversely proportional
......(1)
k is the constant of proportionality

........(2)
substituting (1) into (2)





substitute 

I’m not too sure but I think it’s 8,91 m/s2
Answer:
Gravity.” or “Gravity causes the moon to orbit the earth.” “Acceleration due to gravity” is much more precise. It is the acceleration that an object experiences when it is dropped with no other external forces on it. ... Near the surface of the earth the acceleration due to gravity is about 9.8