ΔΩ at P and an element of area r ΔΩ at P , respectively. The number of lines (say n ) cutting these area elements are the same. The number of field lines, cutting unit area element is therefore n /( r ΔΩ ) at P and n /( r ΔΩ ) at P , respectively. Since n and ΔΩ are common, the strength of the field clearly has a / r dependence.
The picture of field lines was invented by Faraday to develop an intuitive non- mathematical way of visualizing electric fields around charged configurations. Faraday called them lines of force. This term is somewhat misleading, especially in case of magnetic fields. The more appropriate term is field lines (electric or magnetic) that we have adopted in this book.
Electric field lines are thus a way of pictorially mapping the electric field around a configuration of charges. An electric field line is, in general, FIGURE . Dependence of electric field strength on the distance and its relation to the number of field lines. * Solid angle is a measure of a cone.
Consider the intersection of the given cone with a sphere of radius R . The solid angle ΔΩ of the cone is defined to be equal to Δ S/R , where Δ S is the area on the sphere cut out by the cone.