A copper rod has an internal heater that maintains its surface temperature at 50C while it is

Question:

A copper rod has an internal heater that maintains its surface temperature at 50ºC while it is buried vertically in a semi-infinite medium. The rod is 2 cm in diameter and 40 cm long and the isothermal surface of the medium is at 20ºC. Calculate the heat lost by the rod if the thermal conductivity of the medium is 3.4 W/m · ºC.