Question

You are asked to construct a capacitor having a capacitance near 1 nF and a breakdown potential in excess of 11000 V. You think of using the sides of a tall plastic drinking glass as a dielectric (with a dielectric constant 6.0 and dielectric strength 15 kV/mm), lining the inside and outside curved surfaces with aluminum foil to act as the plates. The glass is 16 cm tall with an inner radius of 3.67 cm and an outer radius of 3.99 cm. (a) What are the capacitance and (b) breakdown potential of this capacitor

Answer 1

Capacitance of Cylindrical Capacitor is Givne by

C = (2pieo k L)/(ln b/a)

where eo is constant = 8.85 e-12

a is inner radius, b is outer radius

so here

C = (2*3.14* 8.85 *10^-12 * 6* 0.16)/(ln (3.99/3.67)

C = 0.638 nF

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Electric Potential Difference V = Ed

V = 15000*1000*(0.0399 - 0.0367)

V = 48 kV