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.
