Charles Coulomb carried out a series of experiments in the 1700's to measure electrical attractions. From these experiments, he deduced the law that governs electrical attractions. Lets say we have two positive charges, q1 and q2, separated by a distance r. It can be shown that the electrical force of one charge on the other is proportional to the product of the two charges and inversely proportional to the square of the distance between them. The proportionality constant has been determined to be equal to 1/4peo The factor eo is known as the permittivity constant and has a value of 8.85 ´ 10-12 C2/Nm2. This expression for the electrical force is known as Coulomb's Law, and like any force in the Metric system, it has units of Newtons. Like all forces, it is a vector, and for positive charge, its direction is radially away from the charge. For two positive charges, the force of one charge on the other is in the direction away from the charge. Thus if released, the two charges move away from each other. This shows that like charges repel one another, as was discussed previously. If one charge is positive and the other is negative, the force on the negative charge is towards the positive charge, as indicated by the negative sign in Coulomb's Law. In this case, the force causes the charges to move towards each other, supporting the statement that opposite charges attract. If more than two charges are present, the total force acting on one of the particles is simply the sum of the forces exerted on that particle by the other particles. For the example here, the force on charge one is equal to the force of charge two acting on charge one plus the force of charge three acting on charge one plus the force of charge four acting on charge one. Note that the total force acting on charge one is the vector sum of all the individual forces. Let's calculate the force acting on a charge that is situated between two other charges. The charges are positioned along a straight line and are separated by one meter and two meters, respectively. The outer two charges are positive and the center charge is negative, with the amount of charge on each as shown. What is the force acting on the charge in the center? We will call the central charge, charge number two. What is the total force acting on charge two? What geometrical simplification of this problem can be made for the force? ... Assuming charge 1 is at the origin, what is the force of charge 1 acting on charge 2? What is the force of charge 3 acting on charge 2? Using the given parameter values, what is the total force acting on charge 2? What is the significance of the negative sign in the answer for the force?

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