Hi, I find an interesting related to electric field when doing experiment. I put a chip which has the following circuit inside Scanning Electron Microscopy(SEM), an electron beam is irradiating on R1. The AC source is isolated from earth by using a capacitor C1 and resistor R3 in the middle. I found that the electron beam is oscillating with a frequency the same with the AC source. So I think there may be electric field resulted which deflects the electron beam periodically. Then I did the following experiment. The only change I made is cutting off the connection between point A and B. I found this time electron beam is no longer oscillating. My question is that 1. how does the oscillating electric field produced? 2. why in the second situation, there is no electric field? I know that electric field is produced due to potential gradient in space. I think in the second situation Point B should have a potential of 0V since it is connected to earth, Point A has a potential not equal to 0V, there is potential difference between them, why there is no electric field be produced between point A and B?