A device for extracting electrons from an electron beam at a certain speed consists of a flat capacitor of length l, which is covered on both sides by screens with an input hole A and a long output channel B. An alternating voltage with a frequency ω and an amplitude V is applied to the capacitor plates. Distance between the plates d.
a. What is the speed of electrons released by the device from an electron beam flying parallel to the plates?
b. How much narrower must the hole A be than the channel B in order for the selected group of electrons to pass through the channel?