If a centrifugal pump has an impeller speed of $N$ (in rpm), discharge $Q$ (in $m^3/s$) and the total head $H$ (in $m$), the expression for the  specific speed $N_s$ of the pump is given by
1. $N_s = \frac{NQ^{0.5}}{H^{0.5}}$
2. $N_s = \frac{NQ^{0.5}}{H}$
3. $N_s = \frac{NQ^{0.5}}{H^{0.75}}$
4. $N_s = \frac{NQ}{H^{0.5}}$