3.An isolated charged conducting sphere of radius 14.0 cm creates an electric fi

Question

3.An isolated charged conducting sphere of radius 14.0 cm creates an electric field of 4.90 104 N/C at a distance 25.0 cm from its center.

(a) What is its surface charge density? _________µC/m2

(b) What is its capacitance? _________pF

7.(a) Determine the capacitance of a Teflon-filled parallel-plate capacitor having a plate area of 1.90 cm2 and a plate separation of 0.070 0 mm.
_________pF

(b) Determine the maximum potential difference that can be applied to a Teflon-filled parallel-plate capacitor having a plate area of 1.90 cm2 and a plate separation of 0.070 0 mm.
_________ kV

Explanation / Answer

3.)

E = Kq/r^2

4.9*10^4 = 8.99*10^9 *q / 0.25^2

q = 3.40*10^-7 C

Surface charge density = Q/ A = 3.40*10^-7 / (4*pi*0.14^2)

Surface charge density = 1.38 µC/m2

b.) C = 4 * pi * epsilon * r

C = 4*Pi*8.85*10^-12 *0.14 = 15.5 pF

7.)C =K epsilon * A / d

C = 2.1 *8.85*10^-12 * 1.90*10^-4 / 0.07*10^-3

C =2.1* 2.40*10^-11 F =50.4 pF

b.). it is known that Teflon has dielectric strength of 60 MV/m. Since the parallel plates are 0.07 mm apart, the voltage difference between the plates at this critical voltage is:
V = 60 * 10^6 V/m * 0.07*10^-3 m = 4.2 kV

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