Video: Understanding the Process of Charging a Capacitor

As a capacitor is charged, the amount of charge on it _, and the potential difference across it _.

02:55

Video Transcript

As a capacitor is charged, the amount of charge on it blank, and the potential difference across it blank.

Okay, so in this question, we’re talking about charging a capacitor. And one way to do this is to connect a DC cell to the capacitor in series. Now, the DC cell applies a potential difference across the circuit that sets up a current in the circuit. And so if we can set a conventional current, then what that means is that positive charges are flowing away from the positive terminal of the cell and being deposited onto this particular plate of the capacitor.

Similarly, negatively charged electrons are flowing this way around the circuit and being deposited onto this plate. This is what it means for the capacitor to be charged, because as the current is present in the circuit and charges cannot flow across the gap in between the plates of the capacitor, we see that there’s a buildup of positive charge on the left-hand side plate, as we’ve drawn it, and a buildup of the same amount of negative charge on the right-hand side plate, as we’ve drawn it.

And so what we’ve got here is a plate where there’s an increasing amount of positive charge being deposited onto it and another plate parallel to this, where an increasing amount of negative charge is being deposited. Now, if we were to zoom in slightly to the setup, we can see that there’s the positively charged plate and the negatively charged plate. Now, we can recall that, in between two oppositely charged parallel plates, an electric field will be set up. And that field will be going from the positively charged plate to the negatively charged plate. So we can draw in the electric field lines between these two parallel plates.

Now, what this electric field means in practice is that if we were to take an external electric charge so, for example, a positively charged particle from somewhere else and place it into this electric field, then that charged particle would experience a force. And that force would be in the direction of the plate with the opposite charge to that particle. In other words, a positive charge would flow in this direction. And a negative charge would flow in this direction.

Now, it’s important to note that the charges that were placing in the field are not the same as the charges on the plates of the capacitor. Those cannot flow across the gap between the plates. But anyway, so what we’ve got is an electric field between these plates. And external charges placed between these plates will be moving towards one plate or another. In other words, this electric field is causing a flow of external charge. Or another way to think about this is that the external charges are forming a current, even if there’s just one charged particle. The fact that there’s a charged particle moving means that there is momentarily a current because, remember, current is defined as the rate of flow of charge.

And we can see that if we build up the charge on these plates, so we increase the amount of charge on each plate, then the electric field gets stronger, which in other words means that the force on any of these charged particles will be larger. And so a positively charged particle with this increased electric field strength will experience an even larger force towards the negatively charged plate. And similarly, the negatively charged particle will experience a larger force towards the positively charged plate. In other words then, as the charge on these plates increases, the strength of the electric field increases. And therefore, the potential difference across the plates increases as well.

Therefore, coming back to our original statement, we can say that as a capacitor is charged, the amount of charge on it increases, and the potential difference across it increases as well.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.