Final answer:
A dielectric material increases a capacitor's capacitance by attracting more charge onto the plates and reducing the electric field strength inside the capacitor, which allows a greater voltage before reaching breakdown. This is because the dielectric material polarizes and the reduced electric field means less voltage for the same charge. The correct option is (a).
Step-by-step explanation:
The reason why a dielectric material increases capacitance as compared with air between the plates of a capacitor is primarily related to how a dielectric influences the electric field within the capacitor. When a dielectric is present between the plates, it becomes polarized by the charged plates, which in turn creates a layer of opposite charge on the surface of the dielectric. This polarization attracts more charge onto the plates, thereby allowing the capacitor to store more charge for a given voltage, which increases its capacitance.
Independently, the presence of a dielectric reduces the electric field strength inside the capacitor. This results in a reduced voltage V for the same amount of charge Q. Since capacitance C is defined as the ratio of charge Q to the voltage V (C = Q/V), when the voltage decreases but the charge remains the same, the capacitance increases. Therefore, a dielectric material permits a greater voltage to be applied to a capacitor before the dielectric reaches its breakdown point, compared to air. This is because the reduced electric field within the dielectric material means that a higher voltage can be applied before the electric field reaches the level that would cause the dielectric to fail.
Therefore, the correct option that explains both effects of a dielectric on a capacitor's performance is option (a): "Dielectric reduces electric field, increasing capacitance; allows greater voltage due to reduced charge."