Teflon Capacitors
2008
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Teflon Capacitors
What is the maximum energy that can be stored if the plates are separated by a 0.90 mm-thick Teflon sheet?
The dielectric in a capacitor serves two purposes. It increases the capacitance, compared to an otherwise identical capacitor with an air gap, and it increases the maximum potential difference the capacitor can support. Recall that air near the sphere of a Van de Graaff generator undergoes breakdown, creating a spark, when the electric field is sufficiently strong. The same is true for any material. That critical field strength, at which breakdown occurs, is 3.0 MV/m for air, but 60 MV/m for Teflon.
A parallel plate capacitor consists of two 5.0cm x 5.0 cm plates spaced 0.90mm apart with only air between them. What is the maximum energy that can be stored by the capacitor?
What is the maximum energy that can be stored if the plates are separated by a 0.90 mm-thick Teflon sheet?
Define Ecrit = critical field strength, d = plate separation, kd = dielectric constant
Vbreakdown = Ecrit*d
C = kd*e0*area/d
Energy = v^C/2
Max energy = Vbreakdown^2*C/2 = (Ecrit*d)^2*kd*e0*area/(2d) = Ecrit^2*d*kd*e0*area/2
Air: kd = 1, Ecrit = 3E6 V/m
Max energy = 3E6^2*0.0009*1*8.8541878176E-12*0.0025/2 = 8.964865E-5 J (answer)
Teflon: kd = 2, Ecrit = 6E7 V/m
Max energy = 6E7^2*0.0009*2*8.8541878176E-12*0.0025/2 = 7.171892E-2 J (answer)
The teflon/air energy ratio is 800, consistent with a 2-fold kd increase and a 20-fold (which is squared) Ecrit increase.
Alan Stivell - Jenovefa 1974
