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Andzus
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Homework Statement
A capacitor is formed by two concentric spheres r1= 30 cm and r2=15 cm, with a liquid insulant between them with relative permittivity Er=4. The potential difference between the spheres is 1 kV.
Find:
1. capacitance
2. maximum field strength in the gap between the two spheres
Homework Equations
C=4*pi*epsilon/[1/r1-1/r2]
Q=C*potential_difference
Emax=Q/4*pi*epsilon*r^2
The Attempt at a Solution
1. To get the capacitance I added Er=4 to the formulae
C=4*pi*Er*epsilon/[1/r1-1/r2]
and got C=133.4 pF, is this right? I added Er=4 because we usually use Er=1[for air]
2. to get the max field strength I substited Q=C*potential_difference into Emax and used r=r1-r2. So now the equation is:
Emax=C*potential_difference/4*pi*Er*epsilon*(r1-r2)^2
and got Emax=2.14