2a, -6Q a The figure shows a metal spherical shell with inner radius a and outer radius 2a. There is a stationary negative charge, -6Q, at the center of the shell. If the charge of the shell is +4Q, then what is the surface charge density on the outer surface of the shell? O + Q / ( ria 2 ) O + Q / (2 Tia2) O + Q / (4 nia2 ) O - Q / (2nia2 ) O - Q/ (Ta 2 ) O - Q / (4 Ta 2) O - Q/ (8na2) O + Q / (8Tia? )