4. A solid, nonconducting (insulating) sphere of radius is centered at the origin. The sphere has a uniform volume charge density . The region is filled with a dielectric material of permittivity , and the region is vacuum with permittivity . A small test particle of mass and charge is placed at rest at point on the +x-axis at . Gravitational effects are not negligible. Figure 1 shows the experimental setup.
Figure 1. Uniformly charged insulating sphere with dielectric interior; test particle at point P inside the sphere; concentric Gaussian surface of radius r.
At point , the magnitude of the electric force on the test particle is and the magnitude of the gravitational force on the test particle is .
Indicate whether is greater than, less than, or equal to by writing one of the following.
Justify your answer.
Derive an expression for the magnitude of the electric field at point (where ) in terms of , , , and physical constants, as appropriate. Begin your derivation by writing a fundamental physics principle or an equation from the reference information. Your final expression should be valid for any point inside the sphere (any ).
Figure 2. Modified interior permittivity; same geometry and point P; Gaussian surface at r = 0.030 m.
Indicate whether is greater than, less than, or equal to by writing one of the following. Later, the dielectric material inside the sphere is replaced with a different dielectric with permittivity , while the charge density remains uniform at and the radius remains . The test particle is again placed at rest at the same point (still at ). Let be the new magnitude of the electric force on the test particle at .
Briefly justify your answer by referencing your derivation in part B. Figure 2 shows the modified setup.
00:00