The diagram shows two charged parallel plates. Red represents positive charge and blue represents negative charge. If a particle with a charge of three millicoulombs is placed at point 𝑃, which plate will it move toward?
Okay, we see this diagram with the two charged parallel plates and we know that red represents positive and blue represents negative. We’re also told that at point 𝑃, there is a three-millicoulomb charge. One important thing to notice about this charge is that it’s positive; it is a positive electrical charge. There are a couple of different ways to answer this question of which of the two plates this charge will move toward.
One is to consider how electric charges interact. For example, we know that a positive and a negative charge will attract one another, while on the other hand, charges of like type, for example, two negative charges or two positive charges, will push one another away. They’ll repel each other. Based on that since our charge at point 𝑃 is a positive electric charge, we know it’ll be pushed away from the red plate, the positive plate, and pulled toward the negative plate. So we would expect then that the motion of our charge will look like this.
So that’s one way to answer this question. But there’s another way based on electric fields. In between these two charged parallel plates, an electric field is established. That field points from the positive plate towards the negative plate. That’s the convention of electric field lines. These field lines always point from positive to negative. And additionally, electric field lines always point in the same direction that a positive charge would move in an electric field.
In our case, we have the positive charge, positive three millicoulombs. Therefore, the field itself tells us which way this charge will move. It will move downward towards the negative blue plate. Whichever of these two approaches we use, we’ll find the same result. We find that this charge located at point 𝑃 will move toward the negative plate.