Capacitor

What to observe

1. Start in Manual and push the slider up slowly. Electrons are dragged off the top plate, travel the long way round through the wire, and land on the bottom one. Nothing crosses the gap. The top plate is positive only because it is missing exactly as many electrons as the bottom plate gained.
2. Watch the whole line of electrons in the wire, not just one of them. They all creep forward together, and a ball lands on the plate at one end while another leaves at the far end in the same instant. Real drift is millimetres per second: charge appears at the plate immediately not because an electron raced there, but because the entire queue shifted.
3. Now hold the slider still at any value. The balls stop dead. Current flows only while the charge is changing, which is the whole of I = C dV/dt in one picture. Jerk the slider and watch the current spike, move it gently and barely anything happens.
4. Look at the two markers on the voltage ladder. The current is set by thegap between them, not by either one alone. As the capacitor catches up with the source that gap closes and the flow dies, which is exactly why charging tails off instead of stopping abruptly.
5. Count the field lines. Every one starts on a positive ball and ends on a negative ball, so the field between the plates is a picture of the charge, and the voltage is that field times the gap. Squeeze the gap and the same charge now means less voltage: that is what more capacitance is.
6. Push the slider negative. The balls drain off, cross zero, and pile up on the other side with the field lines reversing with them. A capacitor has no preferred direction, it only ever holds a difference.
7. Switch to AC and start slow. The charge sloshes back and forth, the same electrons going nowhere on average, and yet a current flows all cycle. Now raise the frequency: the plates end up less charged each cycle, and the current gets bigger, because current is charge per unit time and the sloshing got faster.
8. In AC, watch when the current peaks. It is largest as the voltage crosses zero and vanishes at the peaks of the voltage, a quarter cycle out of step. The current leads, and that is why a capacitor blocks DC and passes high frequencies.