Question

The voltage across an air-filled parallel-plate capacitor is measured to be 85V. When a dielectric is inserted and completely fills the space between the plates, the voltage drops to 25V. (a) What is the dielectric constant of the inserted material? (b) If the dielectric does not completely fill the space between the plates, what could you conclude about the voltage across the plates?

Answer 1

(a) Voltage change is 85/25 = 3.4 (answer), and because voltage is proportional to C, that is the dielectric constant.

(b) The presence of a dielectric decreases the effective voltage across the plates. If the dielectric doesn’t completely fill the space, the voltage won’t drop all the way to the 25 volt point, which means voltage across the plates with the dielectric partially filling the space will be between 25 volts and 85 volts.