The charge on the capacitor remains the same but the capacitance increases in proportion to the dielectric constant. The product C*V (= Q) remains constant.
In this case, the drop in V by a factor of 3.40 means that C increases by the same factor and that the dielectric constant is 3.40.
That is a typical value for many plastics such as epoxy and polyimide.
The voltage across an air-filled parallel-plate capacitor is measured to be 219.0 V. When a dielectric is inserted and completely fills the space between the plates as in the figure below, the voltage drops to 64.4 V. What is the dielectric constant of the inserted material? Can you identify the dielectric? (b) If the dielectric doesn't completely fill the space between the plates, what could you conclude about the voltage across the plates?
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