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ID: 2274141 • Letter: #

Question

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Explanation / Answer

to calcualte the EF at point p a distance of z units from disk along the central axis.

our paln is to diide the disk into a very large no. of disks and then integrate them.

with r being radius and radial width being dr and let sigma be the charge per unit area

the chargeh on the ring is given by dq = sigma dA = sigma *(2pi r dr)

where dA is the differential area of the ring

EF due to ring og charge dq is given by dE = Z sigma ( 2pir) dr/( 4pie0 ( Z^2 +r^2)^3/2

dE = (sigma Z/4e0)*8( 2rdr/(Z^2+r^2)^3/2

now by integarting

E = int dE = (sigma Z/4e0) int ( Z^2+r^2)^-3/2 ffrom 0 to R

E = (sigma/4eo)* {(z^2+r^2)^-1/2} from 0 to R

E = (sigma/2e0){1-Z/sqrt(z^2+R^2)