The electric field strength is inversely related to the square of the distance.so the strength of the electric field is given by
[tex]E=\frac{1}{4\pi \epsilon _{0} } \frac{q}{r^{2} } = \frac{k q}{r^{2} }[/tex]
Here, [tex]\frac{1}{4\pi \epsilon _{0} } = k[/tex] is constant depend upon medium and its value is [tex]9.0 \times10^{9} \ N m^2/C^2[/tex] and q is charge and r is the distance.
Given [tex]r = 0.90 mm = 9.0 \times 10^{-4} m[/tex] and we know the charge of proton, [tex]q = 1.6\times 10^{-19} \ C[/tex].
Therefore,
[tex]E=\frac{9.0 \times10^{9} \ N m^2/C^2 \times1.6\times 10^{-19} \ C }{(9.0 \times 10^{-4} m)^2} = 0.177 \times 10^{-2} \ N/C \\\\ E= 1.77 \times 10^{-3} N/C[/tex]
