Movement of charges in presence of electric potential difference This might appear to be a bad question to some but i can't figure out the answer.
While reading about an electric circuit, i found this on the web:


Over the course of time, one could think of positive charges moving from the positive plate through the charge pipe (wire) to the negative plate. That is, positive charge would naturally move in the direction of the electric field that had been created by the arrangement of the two oppositely charged plates.  As a positive charge leaves the upper plate, the plate would become less positively charged as illustrated in the animation at the right. As a positive charge reaches the negative plate, that plate would become less negatively charged. 

So my question is that why does only the positive charge flow? Since positive and negative charges attract each other, it seems to me that the negative charge can also move towards positive charge. Am, I wrong at some point?
If the question is unclear, ask me to edit it in the comment.
 A: Conventional current and flow of charge carriers in a circuit are two different things.
A charge carrier is defined as a particle carrying an electrical charge through a conductor or semiconductor. What these charge carriers are can differ as for metals, these charge carriers are usually electrons. Therefore, current in the circuit is determined by the flow of electrons(i.e flows from negative plate to positive plate as you suggested). On the other hand, in semiconductors, the charge carriers can be positive holes(and in that case the current direction would be from positive plate to negative plate). Charge carriers of a circuit can also be both positive and negative at the same time.
Due to the fact that flow of charge carriers can be in any direction, the direction of current is usually specified in the circuit with an arrow. Thus, $I$ has a positive value if its direction is the same as the arrow(see below) and a negative value if its direction is opposite.

However, the term conventional current can also be used. The direction of conventional current is assumed to be from positive terminal to negative terminal although the flow of the charge carriers may not be in the same direction. This is not so important for technicians since the direction of $I$ doesn't affect its value. If we are not working with circuits, however, that is a different story.
A: What flows are mobile charge carriers, i.e. charges which can move under the influence of an electric field. For metals this is usually the negatively charged electrons, for semiconductors the mobile charge carriers are positively charged holes and negatively charged electrons.
