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Allan Hancock College Physics 163 lab Mapping the Electric Field Purpose: To explore the relationship between the electric potential, the electric field, and various charge configurations. The electric field, which is so important in electric phenomena, can not be measured directly. What we can measure however is the electric potential, and then, using the relationship Er = - V/r ==> Ex = - V/x and Ey = -V/y, we can calculate the electric field. The negative sign indicates that the electric field points in the direction of decreasing potential. In this lab we will measure the potential at different points to find regions where the potential is the same. By connecting these points we have lines that are called “equipotential contours”. These are much like the contour lines on a topographical map that show areas where the height of the land is the same, or isobars on a weather map which show regions of constant pressure. Having marked the equipotential contours, we can now map the electric field lines, which will cross the equipotentials at right angles to them, and point from the high potential to the low potential. Apparatus: Masonite boards covered with weakly conducting paper with an electrode pattern (silver paint), board holders which create a conducting path to the electrodes, voltage probe, power supply, digital volt-meter, and various connective wires. Experiments: We will use the Pasco power supplies. Turn on the Science Workshop interface and power supply, then turn on the computer. Open Science Workshop, connect the power supply to the interface box, then make the same connection on the interface screen on your monitor. Now tell the computer that it is a power supply. Set it to 10 volts DC. The electrode boards attach on the bottom of the board holders, with the silver paint pattern facing downward, so that the screws holding the board in place make contact with the silver paint. Place a piece of paper on the top of the board holder, fix it so it remains stationary, and mark the position of the electrodes on it. With the power supply connected to the electrodes, set the voltage of the power supply to 10 volts. Connect the positive lead from the voltmeter to the probe, and the ground lead to the ground connection of the board. Now with the bottom half of the probe in contact with the conducting paper board, the top half is on the top of the board holder and can be used to plot equipotential points. 1) Using the board with 2 parallel strips, plot 4 or 5 equipotential contours. Is the voltage near the positively charged electrodes close to 10 volts? Is the voltage near the grounded electrode close to 0 volts? Now map the Electric field lines perpendicular to the equipotential contours. Which way do the E-field lines point? The electric field between 2 parallel plates is constant. To calculate the magnitude of the electric field, measure the change in potential at several points and the distance between the points and then find the average value of the electric field between the plates. E = V/x 2) Repeat the procedure for the 2 dot pattern. This pattern represents positively and negatively charged point charges, an electric dipole. The E field will not be constant between the point charges, so its magnitude is not so easily measured. However it will be constant in a circular pattern around the electrodes. Try calculating it at different points for the same radial distance r for one of the electrodes. Note: the pattern will not make a perfect circle near the outside edge of the board due to edge effects, so limit your measurements to the inside of the board. 3) Repeat the above procedures for 2 more boards of different patterns. You will not be able to calculate the magnitude of the magnetic field for these asymmetric patterns, but try to deduce the pattern and direction of the electric field lines.