suppose you charge two capacitor plates until the voltage be

suppose you charge two capacitor plates until the voltage between the plates is the same as the voltage of the charging source(battery). If you then disconnect the capacitor from the battery and insert a dielectric between the two plates, how does this change the electric field between the plates? What if we keep the capacitor connected to the battery?

Solution

When dielectric is placed between the plates after disconnecting the battery,

the charge on the capacitor remains constant.

But the capacitance increases. So, to keep the charge constant, the voltage across the plates decreases <----

--- (As Q = C*V )

Thus the electric field also decreases due to the decrease in voltage across the plates <-------answer

If we keep the capacitor connected to the battery,

the voltage remains constant.

Thus the electric field also remains constant.