When the polarity of the source emf reduces from a maximum value to zero, the number of positive charges in the left plate reduces, that means positive charges are flowing from this plate to the negative plate externally. The arrangement in the dielectric also is modified accordingly. When there are no charges on the plate, the charges in the dielectric returns to the initial position, i.e. to the position before charging the plates .
Now you can guess what will happen next .Because of a.c, now the negative plate becomes charged with positive charges and the left side plate becomes negatively charged .
-│ + - + - + - + - + -│+
-│ + - + - + - + - + -│+
-│ + - + - + - + - + -│+
-│ + - + - + - + - + -│+
Accordingly the charges in the dielectric arrange them selves .
There is completer rotation of the positive and negative charges of the dipoles present in the dielectric.
Thus event though there is no direct flow of charges between the plates, there are some changes are made in the dielectric which is equal to a current flow and this current is called displacement current.
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