What material separates the two conductors inside a capacitor?

• A. A magnetic core
• B. A vacuum
• C. An insulator (dielectric)
• D. A metal plate

Answer: (C)

Inside a capacitor, the two metal plates are kept apart by an insulating material called a dielectric. This insulator prevents current from flowing directly from one plate to the other, which would short the device, while still allowing an electric field to exist between the plates. The dielectric becomes polarized in that field, which reduces the effective field within the material and increases the amount of charge the plates can hold for a given voltage, hence increasing capacitance. The relationship is C = εA/d, where ε is the permittivity of the dielectric (ε = ε0ε_r). A vacuum can act as a dielectric, but most practical capacitors use a solid or liquid dielectric to achieve higher capacitance and appropriate voltage ratings. A magnetic core is used with inductors, not capacitors, and placing a metal plate between the plates would create a direct short. Thus, the material separating the conductors is an insulator known as a dielectric.