A charge of -7.00 nC is spread uniformly over the surface of one face of a nonco

Question

A charge of -7.00 nC is spread uniformly over the surface of one face of a nonconducting disk of radius 1.20 cm

Find the magnitude of the electric field this disk produces at a point P on the axis of the disk a distance of 3.00 cm from its center.

Suppose that the charge were all pushed away from the center and distributed uniformly on the outer rim of the disk. Find the magnitude of the electric field at point P.

If the charge is all brought to the center of the disk, find the magnitude of the electric field at point P.

Explanation / Answer

(1) Electric field due to a uniformly charged disk is
E = (2KQ/R2)(1 - x/(R2 + X2)1/2)
where K is constant , Q is total charge , R is radius of disk and x is the distance from the centre.
Therefore on putting the values
we get,
E = (2*8.99*109*7*10-9 / (0.012)2 )(1 - (0.03/(.0122 + 0.032)1/2)
E = 8930.47 N/C
(2)Electric field due to charged ring is
E = kxQ/ (x2+R2)3/2
E = (8.99*109*(0.03*7*10-9) ) / (0.032 +0.0122)3/2
E = 55966.511 N/C
(3) For point charge we know that
E = kQ/x2
E = 8.99*109*7*10-9 / (0.032) = 69922.22 N/C

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