Sketch the electric field lines in the vicinity of the charged insulator in Figure 18.51 noting its nonuniform charge distribution.
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Insulator does not allow free movement of charges inside them. If a insulator is carrying charge it can be thought of as the charge is embedded in it.
Moreover, insulators have a very large value of resistivity and very low value of conductivity. Also they are very bad conductor of electricity.
So if charges are embedded in an insulator, electric field due to them will be same of as if they are placed at that point except field due to them cannot exist inside a conductor.
Insulator does not allow field lines to pass through it.