Question

An electric field E can pull an electron out of a Helium atom in the same time that the electron takes
to go around the nucleus once. Then E is

Answer 1

We can find the required electric field magnitude from the equilibrium of the force of electrostatic attraction and the force caused by the external electric field:

$$\frac{1}{4\pi\epsilon_0}\frac{Ze^2}{r^2}=eE,\\\space\\ E=\frac{Ze}{4\pi\epsilon_0r^2}.$$

For helium:

$Z=2\\ r_{1s}=3.02 \cdot10^{-11}\text{ m}.$

$$E=3.16\cdot10^{12}\text{ V/m}.$$