INTRODUCTION

Flat, parallel metallic surfaces fulfill many useful functions in electrical and electronic components.  The electric field between them is used to steer and accelerate electrons in some cathode ray tubes. Capacitors, essential in electronic circuitry, are commonly formed from parallel conducting surfaces placed very close together. It is important to have a clear understanding of electric field and potential distribution. We will use the parallel plate configuration as a proving ground to test our understanding of these concepts.

ELECTRIC FIELD

The electric field is a vector quantity representing the influence of the charge plates on other charges in their vicinity. Specifically, it is the electrostatic force per coulomb that would be exerted on a small charge placed at some position . Electric field   has units of Newtons/Coulomb (), which is equivalent to volts/meter (). This second form is convenient when we view electrostatic forces as the tendency of (positive) charges to move towards lower potentials. Then we would write:

(1.1)  

where is the change in potential over a distance .  The direction of is the direction of fastest decrease of   and is perpendicular to the equipotential surfaces, since these surfaces represent the directions in which voltage is constant.

Electric field lines are used to show the field directions at various positions. They are constructed by joining up the arrows representing , so that the field direction anywhere along a line is parallel to the field line. When carefully drawn, an electric field line will always begin at the positive plate and continue unbroken to the negative plate, crossing each equipotential surface at right angles.