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Asked in GSEB Board August 2020 · Shell and uniform solid sphere
Given: R=0.10 m; at r=0.20 m from the centre, E=1.5×10³ N/C, pointing inward.
Idea: outside a charged sphere the field is the same as if all its charge sat at the centre, E=1/(4πε₀)(|q|)/(r²). The point is outside (r>R), so this applies.
|q|=4πε₀Er²=(1.113×10⁻¹⁰)(1.5×10³)(0.20)²
|q|=6.68×10⁻⁹ C=6.68 nC
The field points inward, towards the sphere, so the charge is negative.
q=-6.68 nC
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