Problem 4.205

Non-relativistic protons accelerated by a potential difference $U$ form a round beam with current $I .$ Find the magnitude and direction of the Poynting vector outside the beam at a distance $r$ from its axis.

S=I^{2} \sqrt{m / 2 e U} / 4 \pi^{2} \varepsilon_{0} r^{2}

Electromagnetic Waves. Radiation