3.4 The Electric Field Intensity
That there is a force acting on charges due to the presence of other charges is by now well understood. We also know how to calculate these forces for point charges. Now, we change our point of view slightly. Consider, for example, a point charge which we tie down so that it cannot be moved. Although nothing has changed in terms of the forces between the charges (remember: charges are assumed to be stationary), it is now more convenient to view the fixed charge as the source of the force acting on the second charge. Using Figure 3.5, the force on Q2 due to Q1 is