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littlecat
The surface charge density of an object is σ = dq/dA. Why is the total charge on the surface, Q = double integral σ dA ?
total charge on surface = surface charge density* area basically, surface charge density is the charge per unit area ...which means how much charge does an unit area has. so here \[q=\int\limits_{0}^{A}dA*\sigma \]
if charge density would have varied then we might have used \[q= \int\limits_{0}^{A}dA* \sigma + \int\limits_{0}^{\sigma} d \sigma*A\]