Capacitors with Dielectrics Types of Capacitors AP Physics

-Capacitors with Dielectrics -Types of Capacitors AP Physics C Mrs. Coyle

Dielectric: n A nonconducting material that, when placed between the plates of a capacitor, increases the capacitance q n Dielectrics include rubber, plastic, and waxed paper If the dielectric completely fills the space between the plates, the capacitance increases by a factor κ called the dielectric constant. C= κ Co

Effect of Dielectric on Voltage across a Capacitor • Potential difference across the plates with a dielectric is less than the potential difference across free space Does the C change with or without the dielectric?

n If the V is kept constant by a battery and then a dielectric slab is inserted between the plates, the charge between the plates increases by a factor of κ.

Effect of a Dielectric on E, C • Eo, Co are the values without a dielectric (free space or air between plates) • Electric field between the plates is weaker with dielectric • Capacitance is greater with a dielectric • More charge is stored in the capacitor

Example of Dielectric Constants for Various Materials “Dielectric strength” : maximum field in the dielectric before spark discharge

Dielectrics n n In theory, d could be made very small to create a very large capacitance In practice, there is a limit to d q n d is limited by the electric discharge that could occur though the dielectric medium separating the plates For a given d, the maximum voltage that can be applied to a capacitor without causing a discharge depends on the dielectric strength of the material

Dielectrics, final n Dielectrics provide the following advantages: q q q Increase in capacitance Increase the maximum operating voltage Possible mechanical support between the plates n n This allows the plates to be close together without touching This decreases d and increases C

Tubular Capacitors n Metallic foil - thin sheets of paper or Mylar, layers are rolled into a cylinder

Electrolytic Capacitors n n Used to store large amounts of charge at relatively low voltages The electrolyte is a solution that conducts electricity by virtue of motion of ions contained in the solution

Variable Capacitors n n Variable capacitors consist of two interwoven sets of metallic plates One plate is fixed and the other is movable These capacitors generally vary between 10 and 500 p. F Used in radio tuning circuits