In this experiment, you will use an electric field mapping apparatus to explore the electric field pattern created by a dipole. The apparatus consists of two metallic electrodes painted on a thin insulating sheet. A power supply connects to the two electrodes, which charges them with opposite charges. The electric potential difference between the electrodes is measured using a probe connected to a digital multimeter, a device that measures voltages. The probe may be moved to different points on the insulating sheet, so you can measure the voltage at any point. You will find the shape of equipotential lines. You will use measurements of voltage at different positions to calculate the electric field at a few points.

General Relation between E and V In a region where E is not uniform, the connection between E and V takes on a different form than Eqs. 17-4. In general, it is possible to show that the electric field in a given direction at any point in space is equal to the rate at which the electric potential decreases over distance in that direction. For example, the x component of the electric field is given by Ex = - AV / Ax, where AV is the change in potential over a very short distance AI. Breakdown Voltage When very high voltages are present, air can become ionized due to the high electric fields. Any odd free electron can be accelerated to sufficient kinetic energy to knock electrons out of O2 and No molecules of the air. This breakdown of air occurs when the electric field exceeds about 3 x 10" V/m. When electrons recombine with their molecules, light is emitted. Such breakdown of air is the source of lightning, the spark of a car's spark plug, and even short sparks between your fingers and a doorknob after you walk across a synthetic rug or slide across a car seat (which can result in a significant transfer of charge to you).The electric potential can be represented by drawing equipotential lines or, in three dimensions, equipotential surfaces. An equipotential surface is one on which all points are at the same potential. That is, the potential difference between any two points on the surface is zero, so no work is required to move a charge from one point on the surface to the other. An equipotential surface must be perpendicular to the electric field at any point. If this were not so-that is, if there were a component of E parallel to the surface-it would require work to move the charge along the surface against this component of E; and this would contradict the idea that it is an equipotential surface. The fact that the electric field lines and equipotential surfaces are mutually perpendicular helps us locate the equipotentials when the electric field lines are known. In a normal two-dimensional drawing, we show equipotential lines, which are the intersections of equipotential surfaces with the plane of the drawing. In Fig. 17-6, a few of the equipotential lines are drawn (dashed green lines) for the electric field (red lines) between two parallel plates at a potential difference of 20 V. The negative plate is arbitrarily chosen to be zero volts and the potential of each equipotential line is indicated. Note that E points toward lower values of V. The equipotential lines for the case of two equal but oppositely charged particles are shown in Fig. 17-7 as green dashed lines. (This combination of equal + and - charges is called an "electric dipole," as we saw in Section 16-8; see Fig. 16-32a.) Figure 17-6 15 V. I. 1045VOV Equipotential lines (the green dashed lines) between two charged parallel plates are always perpendicular to the electric field (solid red lines)