Question

An infinitely long, solid non-conducting rod (cylinder) with circular cross section of radius a has its axis along the z-axis. It has a non-uniform volume charge density given in cylindrical coordinates by ρ(s) = C (s/a)^2 ,where C is a positive constant. In addition, there is a uniform volume charge density −σ on the outer cylindrical shell of radius b, where σ is a positive constant. Region 2 is a vacuum.

For parts (a) through (c), use Gauss’ Law and determine the electric fields (both magnitude and direction of the electric field) in

(a) Region 1: inside the inner cylinder (s < a)

(b) Region 2: between the inner and outer cylinders (a < s < b)

(c) Region 3: outside the outer cylinder (s > b)

(d) Is the electric field continuous at each surface? (s = a and s = b surfaces)

(e) What is the electric potential difference ∆Vab between the surface of the inner cylinder (s = a) and the surface of the outer cylinder (s = b)? Which surface has a higher potential? 1.

2) We can model the earth as a solid spherical conductor of radius RE surrounded by a concentric spherical conducting shell with inner radius Ri , and outer radius Ro, which is the ionosphere. The earth has charge +Q, while the ionosphere has zero net charge.

Write all answers in terms of Q, RE, Ri , Ro, and 0.

For parts (a) through (c), what is the surface charge density on

(a) the outer surface of the inner sphere of radius RE?

(b) the inner surface on the spherical shell at radius Ri?

(c) the outer surface of the spherical shell at radius Ro?

For parts (d) through (g), determine the electric field E(r) everywhere in space:

(d) r < RE

(e) RE < r < Ri

(f) Ri < r < Ro

(g) r > Ro

(h) Calculate the energy of the system.

(i) Determine the potential at the center given that the potential is zero at r = ∞.

(j) Find the capacitance of the earth-ionosphere system assuming that the ionosphere has net charge −Q instead of zero.

3) Two charges are on the z-axis, charge +q at z = +a and −q at z = −a. (Hint: This is NOT a continuous charge distribution but two discrete point charges.)

(a) Find the electric potential V (x, y, z) at a field point r = (x, y, z).

(b) Find the potential V (x, y, 0) at a point (x, y, 0) on the xy-plane.

(c) What is the total electrostatic energy of this system?

(d) Using the result of part (c), find out how much work it takes to move the charges closer so that their separation is a rather than 2a.

Answer 1

Using Gauss's law draw first Gaussian surface and find net charge enclosed by the Gaussian surface.

direction of electric field be as like as a dipole ,from positive charge to negative charge.so , electric field us continuous.