A small conducting spherical shell with inner radius `a` and outer radius `b` is concentric with a larger conducting spherical shell with inner radius `c` and outer radius `d` ( as shown in Fig . 2.121). The inner shell has a total charge ` + 2 q`, and the outer shell has a total charge ` + 4 q`. Calculate the electric field in terms of `q` and the distance `r` from the common center of the two shells for

`c lt r ltd`