1.5 Voltage and the Electric Field

The force field that exists around any group of charges is called the E or electric field. It is a vector field, as it has intensity and direction at all points in space. The charges (or absence of charge) we will consider are usually distributed on the surfaces of circuit conductors. This force field in the space around charged objects can be sensed by placing a very small test charge in the field. The test charge has to be small enough that it does not contribute to the field being tested. Thus, a test charge is a small accumulation of charge on a small mass.

Work must be done in moving this test charge in an electric field. The work required to move a unit charge between two points is called the potential difference (voltage) between those two points. We usually measure potential difference between conductors. In a radiated field there are no conductors to consider, yet there are potential differences.

Definition: Voltage is the work required to move a unit charge over a distance in an electric force field.

If there are voltage differences, there are electric fields. The converse is also true. If there are electric fields there must be voltage differences. If there are charges on a surface there must be an electric field. Conversely, if there are nonradiated electric fields there must be charges on conductive surfaces.

Voltage differences can exist between points in space or between conducting surfaces. Electric fields exist at all frequencies including dc. Electric fields are represented by curves that follow the direction of the field forces. In a field representation, lines of force start on a fixed amount of positive charge and terminate on the same amount of opposite charge. When the lines are close together, the forces are the greatest. In a field representation it is only necessary to use a limited number of lines to outline the shape of the field. When there is voltage, electric field lines terminate on the surface charges of a conductor. As we will see a very small electric field inside, a conductor is required for current flow. When there is no current flow there is no field inside of a conductor.

The words ground or ground plane will be used frequently. A ground is a conducting surface that is larger than the surrounding circuitry. In a facility, the earth may be called ground. In an integrated circuit die, a conducting surface can be called ground. On a circuit board, a conducting plane can be called ground. A ground has the quality that it allows charges to move freely on the surface and collect where the field line terminates. An ideal ground plane has no potential differences from point to point. In most circuits this is very nearly true. A superconductor can have current flow with zero internal electric field.