The Electric Field The electric field E at a point in space is defined as an electric force F, acting on a positive test charge q divided by the magnitude of the test charge F E q 1 Q E 2 4 0 r Units of Electric Field 1 N/C Superposition Principle for Electric Fields Field Lines 1. At every point of electric field line, electric field E is tangent to this line. No two field lines can cross! 2. The line must begin at positive charge and terminate on the negative one unless go to infinity. 3. The number of line per unit area is proportional to the magnitude of electric field. Electric Fields and Conductors Important: electric field inside of good conductor is zero!!!! Important: any net charge on a good conductor distributes itself on the surface!!!! Electric field is always perpendicular to the surface of conductor: Conceptual Example A hollow metal box is placed between two parallel charged plates. What’s the field like inside the box? P d q d Q Is it possible to place a charge at point Q such that the electric field produced at point P by the two charges will be add to zero? Two positive charges are a fixed distance apart. The sum of their charges is QT. What charge must each have in order: (a) Maximize the electric force; (b) Minimize it.