You are asked to construct a capacitor having a capacitance near 1 nF and a brea

Question

You are asked to construct a capacitor having a capacitance near 1 nF and a breakdown potential in excess of 12000 V. You think of using the sides of a tall plastic drinking glass as a dlelectric (with a dlelectric constant 6.7 and dlelectric strength 14 kV/mm), lining the inside and outside curved surfaces with aluminum foll to act as the plates. The glass is 12 cm tall with an inner radius of 4.52 cm and an outer radius of 4.73 cm. (a) What are the capacitance and (b) breakdown potential of this capacitor? (a) Number Units (b) Number Units

Explanation / Answer

a) This glass will make a cylindrical capacitor, and for cylindrical capacitor the formula for capacitance is

C= 20L/(ln(b/a))

where, b and a are the inner and outer radius of the cylinder respectively

C=2*3.14*8.85*10-12*0.12 / (ln(4.73/4.52))

C=1.4685*10-10 F =

b) breakdown voltage, V= dielectric strength * distance between plates

V=(14*103*103) *(0.0473-0.0452)= 29400 Volts

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