1. The problem statement, all variables and given/known data The windings of high-current electromagnets are often made of copper pipe. The current flows in the walls of the pipe, and the cooling water flows in the interior of the pipe. Suppose the copper pipe has an outside diameter of 0.8 cm and inside diameter of 0.5 cm 2. Relevant equations ∏r2 3. The attempt at a solution I need to find the area between the inside pipe where the cooling water has and the outside wall. To do this, should I simply subtract the area of the 0.8 cm diameter pipe by the area of the 0.5 cm diameter pipe? Or it's something more complicated than that?