In one my physics classes I was finding the charge of a half sphere with constant radius $R$ (and I got this expression (just for context purposes): $$Q=\int_S \rho dS$$ and now my teacher did this $dS=2\pi rRd\theta=2\pi rR^2\sin\theta d\theta$. How did he got that differential?
Second question. Why is there only one differential $d\theta$ in this case for a surface, shouldn't there be 2 at least? Like I thought for curves or lines it was one differential and surfaces 2 differentials and volumen 3 differentials?
