[tex]F=qE[/tex]
This equation is a simplified version of the more complex model. It only works for uniform E-fields, and a common simplifying assumption is that a field between two conducting plates is uniform, meaning it should work fine here.
By the way, if you have a decent understanding of mechanics, often a prerequisite to electricity and magnetism, you can analogize this equation with
[tex]F=ma[/tex]
In both, force on some entity is proportional to the field where the entity exists. a is a gravity field and E is an electric field. For gravity, the proportionality constant is known as mass, and for electricity, the proportionality constant is known as charge. There is one major difference of concern when using this analogy: that electric properties of an entity can be negative or positive, and so can the fields produced by electrical entities be either negative or positive. Differently, mass and the fields entities with mass produce are always positive.