A hollow metal sphere has 6 cm and 9 cm inner and outer radii, respectively. The surface charge density on the inside surface is −100 nC/m². The surface charge density on the exterior surface is +100 nC/m².
What are the strength and direction of the electric field at points 4, 8, and 12 cm from the center?

Asheet of steel 1.5 mm thick has nitrogen atmospheres on both sides at 1200oc and is permitted to achieve a steady-state diffusion condition. the diffusion coefficient for nitrogen in steel at this temperature is 6 x 10- 11 m2 /s, and the diffusion flux is found to be 1.2 x 10-7 kg/m2 -s. also, it is known that the concentration of nitrogen in the steel at the high-pressure surface is 4 kg/m3 . how far into the sheet from this high-pressure side will the concentration be 2.0 kg/m3 ? assume a linear concentration profile.