Question

Which of the following electric field configurations describes the lines of force in an electric field between two parallel plates of opposite charge?

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

An important point to remember about electric fields is that they emanate from positive charges and point towards negative charges. With this in mind, we know choices $A$ and $C$ are incorrect. The next question is to judge the straightness or curvature of the lines between the plates.
Imagine a point placed directly between two parallel plates (see the diagram above). It will have a net electric field determined by all the individual electric fields generated by every charge on the two plates. For clarity, only five electric field lines are shown in the diagram. If these field lines are added vectorially, all the horizontal components will cancel each other out; a net field oriented vertically downward will remain (the field lines shown in the choices are the possible net electric fields). The cancellation is due to the symmetry of the point in space with respect to the charges on the plates. However, this horizontal component cancellation continues only until one approaches the left and right edges of the plates. Here, the field lines bow out, much like those around a dipole (the parallel plates are like several dipoles positioned next to each other). Remember, between parallel plates of opposite charge, the field lines are straight. They are also constant in strength throughout the interplate region. Thus, a parallel plate arrangement is a good design for capacitors.
The correct choice is B.