85. (a) We denote the surface charge density of the disk as σ1for 0 < r < R/2, and as 2for R/2 < r < R. Thus the total charge on the disk is given by ()2212disk0222627292232.20 10 m1.50 10 C m3 8.00 10 C m1.48 10 C .RRRqdqrdrrdrRσσσ−−−−π==π+π=+4πªº=××+×¬¼4³³³(b) We use Eq. 24-36: VzdV kRdRzRRzRzzRzRRRRb gbgbg′′+′++′
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