#### Answer

To create a spark, $2.7\times 10^{11}$ electrons must be transferred from one disk to the other.

#### Work Step by Step

We can use the expression for the electric field strength in a plate capacitor to find the required charge on each disk.
$E = \frac{\sigma}{\epsilon_0}$
$E = \frac{Q}{A~\epsilon_0}$
$Q = E~A~\epsilon_0$
$Q = (3.0\times 10^6~N/C)(0.040~m)(0.040~m)(8.85\times 10^{-12}~C^2/N~m^2)$
$Q = 4.25\times 10^{-8}~C$
We can find the number of electrons which must be transferred.
$N = \frac{4.25\times 10^{-8}~C}{1.6\times 10^{-19}~C}$
$N = 2.7\times 10^{11}~electrons$
To create a spark, $2.7\times 10^{11}$ electrons must be transferred from one disk to the other.