Q1- A capacitor is made from two hollow, coaxial iron cylinders, one inside the other. The inner cylinder is negatively charged and the outer is positively charged; the magnitude of the charge on each is 10.0 pC. The inner cylinder has a radius of 0.200 mm , the outer one has a radius of 5.40 mm, and the length of each cylinder is 25.0cm.
a) What is the capacitance?
b) What applied potential difference is necessary to produce these charges on the cylinders?

Solution

Q2- A spherical capacitor has two different layers of dielectrics between its spheres. Their permittivities are ϵ₁ for a < r < r₀ and  ϵ₂ for r₀ < r < b. Find the capacitance of this system by finding the total energy of the fields between the spheres.  

Solution