I'm trying to understand how the charging of a plate capacitor works which lead me to question my understanding of current flow. In many explanations, it is said that the voltage source pulls the electrons from the plate connected to the positive pole and pushes electrons to the plate connected to the negative pole at the same time. Since there is a positively charged plate and a negatively charged plate after some time, an electric field is created.
My understanding of current flow is, that electrons get attracted to the positive pole of the voltage source, as there is a lack of electrons at the positive pole (thus relatively more protons) and a surplus of electrons at the negative pole.
So how can electrons get pushed to one plate in the capacitor? Is my understanding of current flow as it was taught to me wrong?
Best Answer
Are you saying all this with the understanding that electrons do not jump from one capacitor plate to the other? The electrons and holes (or rather, their charges) take the long way around through the wires. They do not pass between the plates through the dielectric. In a vacuum tube, the electron does actually boil off one plate, travel through the vacuum and land on the other plate. But not in a capacitor.
Protons do not move. But when the electrons move around and leave an unpaired proton without a negative charge to cancel it out, the "positiveness" of a proton's charge appears to move around even though the proton doesn't. This moving positiveness is called a "hole". Think of an electron as a person, and an empty chair as a proton. 2 people might move around between 3 different chairs. The physical chairs themselves never move, yet the chair that is empty does change location in the opposite direction of the people. The "emptiness" of the chair moves around.
Now that you know that, there are two ways to look at current flow: Electron flow and hole flow. They are in opposite directions of each other. Electron flows is what physicists use. But due to historical reasons (mistakes) early on, humans assigned the wrong charge to the thing that physically moved and it became widespread and engrained before we realized the mistake. It took time to figure out because as you can see from the above parapgraph, it mathematically works out until you do something that actually requires you to know the movement of electrons (something like vacuum tubes or semiconductor physics). Therefore, electronics almost always uses hole flow. Electron flow out of the negative terminal into positive terminals on a battery. Hole flow goes out of the positive terminal of a battery into the negative terminal of the battery.
The voltage applied actually sets up an electric field. It's this electric field that moves the electrons to accumulate on one capacitor plate (and the holes inherently "move" in the opposite direction to the other plate due to the electrons' absence). This imbalance of electrons (and holes) sets up an electric field that opposes the electric field produced by the applied voltage producing a net reduction in the total electric field. The electrons and holes continue to accumulate on different plates until their own electric field that cancels out the applied voltage (until the net field is zero). Removing the applied voltage removes the applied field leaving only the field produced by the imbalance of the electrons and holes. Since the field is no longer net-zero, this electric field pushes them all back to where they originally where (until the net field is again zero).
One more correction. Up until now, I always said things like "electrons move". This is not actually the case. The electrons do move, but individual electrons do not actually move from one plate to the other. It's the negative and positive charges that move. It's like an accounting game for charge imbalances, not actually electrons and holes. Similar to how debt can move around around much differently and much faster than each individual unit of currency. Individual electrons move at near walking pace through wires but the wave/energy moves near the speed of light. In the AC wall line, electrons are not moving back and forth between your light and the power plant 50 or 60 times per second. It is similar to how sound travels at Mach 1 but wind does not. A wind blowing at Mach 1 would strip your skin off and pulverize everything it hit.