If a potential difference of \(9 \mathrm{~V}\) is applied across the conductor in Figure 31.21, what is the magnitude of the potential difference 

(a) across the wide part 

(b) across the narrow part?

Figure 31.21 (a) Conductor of circular cross section that decreases in radius by a factor of 2 at the midpoint. (b) Electric field lines in the con- ductor when it is connected between two charged objects (not shown). (a) (b) -E-