Question

The electric field at 4cm from a long-straight wire is 40N/C, what is the electric field at 16cm from the wire (In SI units)?

Answer 1

Electric field at a distance R from a long straight wire is λ/2πϵR  

It means for a long straight wire charge distribution E is inversely proportional to R. So

$E_4/E_{16}=16/4\\Or\\40/E_{16}=4\\E_{16}=40/4\\E_{16}=10N/C$