2 Electric field above a uniformly charged squre sheet Find the electric field at a large distance z above the center of a square sheet of side length a, carrying a uniform surface charge density σ. Check your result in the following limiting cases: 1. When a → ∞, compare with the result for an infinite plane. 2. When >> a, compare the result with that of a point charge.

Problem #1 A hollow metal sphere of radius 4 cm has a charge of 20 nC distributed evenly on the entirety of the surface. Find the surface charge density (o).

Problem 4: who is right? Two parallel, infinite planes of charge have charge densities +20 and -o as shown in Fig. 1 They are at a distance L, from each other. Three students are asked to determine the electric field at points A and B, at a distance L above the top plane, and at a distance L below the bottom plane, respectively, as shown in the figure. Each student comes up with a different answer and a different explanation. Read carefully their statements below. The students are reminded that the electric field magnitude of an infinite plane of charge with charge density a is E = . +26 3+26 Figure 1: Two cquivalent views of the two infinite parallel planes • Student 1 draws the sketch in Fig. 2 [top] and says: "I think from the outside we can think of the two planes as a single infinite plane with charge density +20 - o =0, so then we know the electric field must be a/(2) pointing upwards at point A and also o/(2c) pointing doumwards at B". • Student 2 draws the sketch in Fig. 2…

A charge of 140 μC is at the center of a cube of edge 45.0 cm. No other charges are nearby. (a) Find the flux through each face of the cube. Nm²/c (b) Find the flux through the whole surface of the cube. N . m²/c (c) Would your answers to parts (a) or (b) change if the charge were not at the center? (a) would change. O (b) would change. O Both (a) and (b) would change. O Neither would change.

Finura 2 Positive charge Q is distributed uniformly throughout an insulating sphere of raduis R. What is the magnitude of the electric field at any point a. Outside the sphere? b. Inside the sphere? +Gai naface R nde **