18.4 Electric Field: Concept of a Field Revisited Contact forces, such as between a baseball and a bat, are explained on the small scale by the interaction of the charges in atoms and molecules in close proximity. They interact through forces that include the Coulomb force. Action at a distance is a force between objects that are not close enough for their atoms to "touch." That is, they are separated by more than a few atomic diameters. For example, a charged rubber comb attracts neutral bits of paper from a distance via the Coulomb force. It is very useful to think of an object being surrounded in space by a force field. The force field carries the force to another object (called a test object) some distance away. Concept of a Field A field is a way of conceptualizing and mapping the force that surrounds any object and acts on another object at a distance without apparent physical connection. For example, the gravitational field surrounding the earth (and all other masses) represents the gravitational force that would be experienced if another mass were placed at a given point within the field. In the same way, the Coulomb force field surrounding any charge extends throughout space. Using Coulomb's law, F = klq192|/r?, its magnitude is given by the equation F = klqQI /r. for a point charge (a particle having a charge Q) acting on a test charge q at a distance r (see Figure 18.20). Both the magnitude and direction of the Coulomb force field depend on Q and the test charge q. F, (a) F2 (b) Figure 18.20 The Coulomb force feld due to a positive charge Q is shown acting on two different charges. Both charges are the same distance from Q. (a) Since q1 is positive, the force F1 acting on it is repulsive. (b) The charge 92 is negative and greater in magnitude than 91. and so the force F2 acting on it is attractive and stronger than F1- The Coulomb force field is thus not unique at any point in space, because it depends on the test charges qI and 92 as well as the charge 2.