How does a dielectric develop a net dipole moment in an external field when it has
1) polar molecule
2) non polar molecule
 

Dear student

when it has a polar molecule, the charges get separated within the molecule in presence of electric field. This charge distribution is uneven since individual molecule is already polar. Hence, their is an uneven  charge separation implying the presence of dipole moment

when it has a non polar molecule, the charge separation is even and hence here also there is charge separation. so, even dipole moment is set up.

Regards