Two different metal rods of the same length have their ends kept at the same temperatures θ1 and θ2 with θ2 > θ1. If A1 and A2 are their cross-sectional areas and k1 and k2 their thermal conductivities, the rate of heat flow in the two rods will be same if
Rate of heat flow is Qt=kA(θ2−θ1)l
Since (θ2−θ1) and l are the same for the two rods, the rate of flow Qt will be the same if the product kA is the same for the two rods, i.e., if
Hence, the correct choice is (b).