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Electromagnetic Field Theory: A Problem Solving Approach Part 9. Electromagnetic field theory is often the least popular course in the electrical engineering curriculum. Heavy reliance on vector and integral calculus can obscure physical phenomena so that the student becomes bogged down in the mathematics and loses sight of the applications. This book instills problem solving confidence by teaching through the use of a large number of worked problems. To keep the subject exciting, many of these problems are based on physical processes, devices, and models. This text is an introductory treatment on the junior level for a two-semester electrical engineering. | The Coulomb Force Law Between Stationary Charges 55 two stationary charged balls as a function of their distance apart. He discovered that the force between two small charges qi and q2 idealized as point charges of zero size is proportional to their magnitudes and inversely proportional to the square of the distance ri2 between them as illustrated in Figure 2-6. The force acts along the line joining the charges in the same or opposite direction of the unit vector ii2 and is attractive if the charges are of opposite sign and repulsive if like charged. The force F2 on charge q2 due to charge qi is equal in magnitude but opposite in direction to the force Fi on qi the net force on the pair of charges being zero. F2 -Fi - ii2nt kg-m-s 2 1 4tt o r12 2-2-2 Units The value of the proportionality constant 1 4tf 0 depends on the system of units used. Throughout this book we use SI units Systeme International d Unites for which the base units are taken from the rationalized MKSA system of units where distances are measured in meters m mass in kilograms kg time in seconds s and electric current in amperes A . The unit of charge is a coulomb where 1 coulomb 1 ampere-second. The adjective rationalized is used because the factor of 4tt is arbitrarily introduced into the proportionality factor in Coulomb s law of 1 . It is done this way so as to cancel a 4ir that will arise from other more often used laws we will introduce shortly. Other derived units are formed by combining base units. Figure 2-6 The Coulomb force between two point charges is proportional to the magnitude of the charges and inversely proportional to the square of the distance between them. The force on each charge is equal in magnitude but opposite in direction. The force vectors are drawn as if q and q2 are of the same sign so that the charges repel. If qt and q2 are of opposite sign both force vectors would point in the opposite directions as opposite charges attract. 56 The Electric Field The parameter e0 is
