A power supply of 20 V is connected across a copper wire having a...

Given $V = 20 \text{ V}$ and $R = 1 \text{ k}\Omega = 1000 \Omega$.

The current flowing through the wire is given by Ohm's law:

$I = \dfrac{V}{R} = \dfrac{20}{1000} = 0.02 \text{ A}$

The current represents the rate of flow of charge, so the charge transferred per second is $q = 0.02 \text{ C}$.

The number of electrons transferred per second is:

$n = \dfrac{q}{e} = \dfrac{0.02}{1.6 \times 10^{-19}}$

$n = 1.25 \times 10^{17}$

The radius of the wire is extraneous information not required for this calculation.