When internal heat sources are present, the continuity equation (5) must be modified to read where gu is the heat generation rate per unit volume. Examples include Joule heat generated in a wire; heat from the radio3ctive decay of trace elements inside the Earth or the Moon. Give an expression for the temperature rise at the center of
(a) A cylindrical wire and
(b) The spherical Earth, on the assumption that gu is independent of position and is constant with time.