physics
posted by joy .
A brass lid screws tightly onto a glass jar at 20 oC. To help open the jar, it can be placed into a bath of hot water. After this treatment, the temperature of the lid and the jar are both 60 oC. The inside diameter of the lid is 8.0 cm at 20 oC. Find the size of the gap (difference in radius) that develops by this procedure.
so radius  4.0cm
is this the right equation for it?
delta L = alpha (initial Length) (delta Temp)

No. You need to work with area expansion, find the new diameter from area, then the change in area.

:O our teacher never mentioned area expansion..

Dear Bobpursely
You're a terribly teacher.