Electric field for a disk? The total electric field at a point on the axis of a

Question

Electric field for a disk?

The total electric field at a point on the axis of a uniformly charged disk, which has a radius R and a uniform charge density of ?, is given by the following expression, where x is the distance of the point from the disk.

(a) Using the expression above, compute the electric field at a point on the axis and 2.97 mm from the center.
Consider a disk of radius R = 3.15 cm having a uniformly distributed charge of +5.08 ?C.

(b) Explain how the answer to part (a) compares with the field computed from the near-field approximation

E = ?/2?0.

(c) Using the first expression above, compute the electric field at a point on the axis and 29.7 cm from the center of the disk.
N/C

(d) Explain how the answer to part (c) compares with the electric field obtained by treating the disk as a +5.08-?C charged particle at a distance of 29.7 cm.

For part a I have been getting the same answer of 260301 N/C and I dont know what I am typing in wrong....so if someone could help out that would be great.

Explanation / Answer

Area, A = pi*r^2
        = pi*0.0315^2
        = 3.1172 * 10^-3 m^2
Q = 5.08 * 10^-6 C
so,
? = Q/A = 1.6297 * 10^-3 C/m^2

so,
E = 2*pi* (8.99 * 10^9) * (1.6297 * 10^-3)* [ 1 - {(2.97 * 10^-3)/((3.15*10^-2)^2 + (2.97*10^-3)^2)^0.5 ]
E = 87.7115 * 10^6 N/C

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