2 Electric Field Mapping Department of Physics （2电场映射物理系）.pdfVIP

Electric Field Mapping Rev1/08 Physics 246 Theory: An electric field is a vector field that is produced by electric charges. The source of the field may be a single charge or many charges. To visualize an electric field, we use lines of force. The arrows on the lines point in the direction of a force felt by a unit positive charge placed into such a field, so that lines of force leave positive charges and enter negative ones for simple one charge configurations such as seen in (Figure 1) Figure 1: Electric fields of positive and negative charges. . Figure 2: Electric Field near two equal charges of opposite sign If the charge configuration has more than one member as in the configuration in Figure 2, then the vector properties of the electric field must be considered. Here we see two charges of equal magnitude but opposite sign and want to see what would happen to a positive test charge introduced into the system. Let’s imagine the test charge is introduced at point B. Generally, we must use vector analysis to predict the electric field on the charge. Let E1 be the electric field felt by the test charge because of the presence of the positive charge. Let E2 be the electric field felt by the test charge due to the negative charge. Here, using vector addition, the electric field vector is calculated for this system of charges. The resultant, E, 1 is tangent to the electric field lines, thus the lines can be drawn in by calculating the resultant at each point in the diagram and sketching the electric field lines from these observed resultant vectors. To maintain the charge at a specific position in the electric field or move it against the field requires an expenditure of energy. The work done to bring a charge from infinity to a particular