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yeah, but why does the voltage difference decrease?
Voltage is itself already a potential difference so there is a potential difference not voltage difference. Who told you that the potential decrease ? The aim of the dielectric is to create a potential difference and to store the electric energy in the electrice field which occur between that two pole.
If there was not a dielectric which disable the current to flow, the potential difference wouldnt be exist. So it creates the potential difference, not decrease the voltaj.
You might ask the potential decrease because of internal electric field of the dielectric. If you take a good look at the picture above which UnkleRhaukus uploaded, you can see how the charges distribute. At the edge of the dielectric you can see the negative and positive charges. They create an internal electric field and this electric field is opposite to the electric field which plates create. In this stuation there are two vector field in opposite direction. The internal electric field of dielectric lower the electric field which paltes create.
the internal charges in the dielectric move as to reduce the potential,
The dielectric starts getting induction.. and hence they form DIPOLES.. as shown in the figure.. these dipoles now start producing an internal field which are in the opposite direction of the external field.. so the field goes down, and so does the potential!