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Copyright © 2009 Pearson Education, Inc. Dielectrics
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Copyright © 2009 Pearson Education, Inc.
Dielectrics
Copyright © 2009 Pearson Education, Inc.
•A dielectric is an insulator, & is characterized bya dielectric constant K.•The capacitance of a parallel-plate capacitor filled with adielectric is:
•Using the dielectric constant, the permittivity is defined as:
•Dielectrics in capacitors provide the following advantages:Increase in capacitanceIncrease the maximum operating voltagePossible mechanical support between the plates
•This allows the plates to be close together without touching.•This decreases d and increases C.
Copyright © 2009 Pearson Education, Inc.
The dielectric strength is the maximum electric field a dielectric can experience without breaking down.
Dielectric Constants & Dielectric Strengths
Copyright © 2009 Pearson Education, Inc.
More Dielectric Constants & Dielectric Strengths
Copyright © 2009 Pearson Education, Inc.
•Consider two experiments where a dielectric is inserted& removed a from a capacitor.
Experiment #1•The capacitor is connected to a battery, so the voltage V0
remains constant. See figures below. •When the dielectric is inserted, the capacitance C
increases.•So, the charge Q on the plates must also increase.
Copyright © 2009 Pearson Education, Inc.
Experiment #2:•A capacitor is charged & then disconnected from thebattery. Then, a dielectric is inserted. In this case, thecharge remains constant. See figures below. •Since the dielectric increases the capacitance, thepotential across the capacitor must drop.
Copyright © 2009 Pearson Education, Inc.
There are Many Types of CapacitorsExample: Tubular Capacitors
•Metallic foil may be interlacedwith thin sheets of paraffinimpregnated paper or Mylar.•The layers are rolled into acylinder to form a smallpackage for the capacitor.
Section 26.5
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Example: An Oil Filled Capacitor
•These are commonly used forhigh-voltage capacitors•They consist of a number ofinterwoven metallic platesimmersed in silicon oil.
Section 26.5
Copyright © 2009 Pearson Education, Inc.
•These are commonly used to storelarge amounts of charge atrelatively low voltages•The electrolyte is a solution thatconducts electricity by virtue ofmotion of ions contained in thesolution.•When a voltage is applied betweenthe foil & the electrolyte, a thinlayer of metal oxide is formed onthe foil.•This layer serves as a dielectric.•Large values of capacitance can beobtained because the dielectriclayer is very thin & the plateseparation is very small.
Section 26.5
Example: An Electrolytic Capacitor
Copyright © 2009 Pearson Education, Inc.
•Variable capacitors consistof two interwoven sets ofmetallic plates.•One plate is fixed and theother is movable.•Air is contained as thedielectric.•These capacitors generallyvary between 10 & 500 pF.•They are commonly usedin radio tuning circuits
Section 26.5
Example: Variable Capacitors
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Example: Dielectric removal.A parallel-plate capacitor, filled with a dielectric (K = 3.4) is connected to a 100-V battery. After it is fully charged, the battery is disconnected. Plate area A = 4.0 m2. Separation d = 4.0 mm. (a) Find the capacitance, the charge on the capacitor, the electric field strength, and the energy stored in the capacitor.
(b) The dielectric is carefully removed, without changing the plate separation nor does any charge leave the capacitor. Find the new values of capacitance, electric field strength, voltage between the plates, and the energy stored in the capacitor.
