Physics 121: Electricity & Magnetism – Lecture 3 Carsten Denker NJIT Physics Department Center for Solar–Terrestrial Research

January 31, 2007Center for Solar-Terrestrial Research Coulomb’s Law and Flux Gauss’ law relates the electric fields at a point on a (closed) Gaussian surface and the net charge enclosed by that surface. Gauss’ law uses symmetry and is fully equivalent to Coulomb’s law. Which one you have to use depends on the application!

January 31, 2007Center for Solar-Terrestrial Research Flux of an Electrical Field Electric flux through a Gaussian surface discrete form integral form The electric flux  through a Gaussian surface is proportional to the number of field lines passing through that surface.

January 31, 2007Center for Solar-Terrestrial Research Gauss’ Law Gauss’ law Gauss’ law and Coulomb’s law

January 31, 2007Center for Solar-Terrestrial Research A Charged Isolated Conductor If an excess charge is placed on an isolated conductor, the amount of charge will move entirely to the surface of the conductor. None of the excess charge will be found within the body of the conductor. Conducting surface

January 31, 2007Center for Solar-Terrestrial Research Cylindrical and Planar Symmetry Planar symmetryCylindrical symmetry

January 31, 2007Center for Solar-Terrestrial Research Spherical Symmetry Spherical shell Uniform charge (field at r  R) A shell of uniform charge attracts or repels a charged particle that is outside the shell as if the shell’s charge were concentrated at the center of the shell. A shell of uniform charge exerts no electrostatic force on a charged particle that is located inside the shell.