thank you
i forgot that V in a shell/sphere is the same as if it's on the surface
I thought the p.d. by the larger shell were different on the surface of the larger shell and on the surface on the smaller shell...
that's why i got trouble
anyway, thanks~
for the part of Capacitance of concentric shells
why Va equals that?
why the voltage on the surface of a is affected by -Q/(4pi*epsilon*b),
which represents the voltage on the surface of b
these two voltage represents something at different position!