Dear sir,Why should the magnitude of electric field change in non uniform cross section ? It is equal to potential difference divided by lenght. Why should that vary? Does the potential difference change in a non uniform criss section?

Dear student,

Current is a quantity that must not vary along a conductor, be it of non uniform cross section since the number of electrons flowing thorught a cross section must be same. But due to varying cross section, the regions of smaller cross section will have higher current density which means that there will be higher electric in the region.

It can also be seen in another way- As the cross section decreases, the resistance increases in a region, and due to same current throughout the wire, the potential difference starts decreasing more.