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People say that dielectric should be an insulator so that it can decrease net electric field increasing capacitance..
My question is if the object is a insulator how does the object get polarized and how does it get a electric field inside? How do you know that electrical field applied by the dielectric is always lesser than the ones made by the parallel plates of the capacitor?
Dielectric is an insulator. Electric charges does not flow through it (as in a conductor) but cause polarization (not all insulators can be equally polarized using the same electric field) [1].
... dielectric is used to indicate the energy storing capacity of the material [1]
The capacitors store energy electrostatically in an electric field [2]. This electric field interacts with atoms:
Each atom consists of a cloud of negative charge (electrons) bound to and surrounding a positive point charge at its center. In the presence of an electric field the charge cloud is distorted [1].
A better understanding about how a capacitor works can be done using a hydraulic model:
"CapacitorHydraulicAnalogyAnimation" by Sbyrnes321 - Own work. Licensed under CC0 via Wikimedia Commons.
Charge can flow "through" a capacitor even though no individual electron can get from one side to the other [2].
References:
All electrical insulators are dielectrics "without exception". Some are highly polarized like water (εr >80), or conjugated polymers (εr < 100k) , some are low like epoxy (4), transformer mineral oil (2), air, (1.0006) and even a vacuum (1.0000...) is still a dielectric.
Generally conductors radiate Electric Fields, not insulators, but if you have a sharp variation in the dielectric constant in a constant E Field, the voltages drops will be uneven like a small cap in series with a big one will have most of the charge or dynamic voltage on the smallest cap in series.
Charge build can be tribo-electric from friction of polarized insulators or bombarding accelerated particles in a high E field or simply by conduction from an external source. Double charge cloud layers form on the electrodes which is enhanced in "super-caps" with alternating layer of +/- charges.
You don't know the charge potential inside a series of insulators unless you can measure it or know how it is generated.
For example magnetic iron molecules on insulating silicate dust can generate higher potentials than the E field because they are not only partially conductive clouds but excited by high current at very low voltage, such as >1 or 2 Tesla inside laminated steel cores (if contaminated) and create partial discharge (PD) arcs in distribution transformers which breaks down oil into hydrogen like a slow time bomb.
