An X-ray tube operating at a P.D. of 50 kV has a tube current 20 mA. Calculate the electric input and the number of electrons hitting the target per second.

$$P = VI$$ $$P = (50 \times 10^3 \text{ V}) \times (20 \times 10^{-3} \text{ A})$$ $$P = 1000 \text{ W}$$

The electric input is $\boxed{\text{1000 W}}$.

$$I = \frac{Q}{t} = \frac{ne}{t}$$

Where $n$ is the number of electrons and $t$ is time. We need to find $\frac{n}{t}$.

$$\frac{n}{t} = \frac{I}{e}$$ $$\frac{n}{t} = \frac{20 \times 10^{-3} \text{ A}}{1.6 \times 10^{-19} \text{ C}}$$ $$\frac{n}{t} = 1.25 \times 10^{17} \text{ electrons/second}$$

The number of electrons hitting the target per second is $\boxed{1.25 \times 10^{17} \text{ electrons/second}}$.

$$KE = eV$$ $$KE = (1.6 \times 10^{-19} \text{ C}) \times (50 \times 10^3 \text{ V})$$ $$KE = 8.0 \times 10^{-15} \text{ J}$$

$$KE = \frac{1}{2} m_e v^2$$ $$v = \sqrt{\frac{2 \times KE}{m_e}}$$ $$v = \sqrt{\frac{2 \times (8.0 \times 10^{-15} \text{ J})}{9.1 \times 10^{-31} \text{ kg}}}$$ $$v = \sqrt{1.75824 \times 10^{16}}$$ $$v \approx 1.326 \times 10^8 \text{ m/s}$$

The velocity of electrons when they hit the target is $\boxed{1.33 \times 10^8 \text{ m/s}}$.