The problem statement, all variables and given/known data A steel bar 10cm long is welded end to end to a copper bar 20cm long. Both bars are insulated perfectly on their sides. Each bar has a square cross section, 2cm on a side. The free end of the steel bar is maintained at 100 celsius and the free end of the copper bar is maintained at 0 celsius. Find the temperature at the junction of the two bars. The attempt at a solution The problem is a worked example so I can see the method for solving it. But I don't understand why they take the heat currents through the two bars to be equal? Would someone be able to explain it to me? :) Thanks!