Which action increases thermal resistance and therefore decreases heat transfer in conduction?

• A. increase L
• B. decrease A
• C. decrease k
• D. all of the above

Answer: (D)

In conduction, heat transfer resistance grows with thickness and shrinks with area and material conductivity. The thermal resistance is R_th = L/(kA). So increasing the thickness L raises R_th, decreasing the cross-sectional area A raises R_th, and decreasing the thermal conductivity k also raises R_th. Since the heat transfer rate is related to these quantities by Q ∝ kAΔT/L (or Q = ΔT/R_th), raising any of these factors that boost resistance reduces heat transfer. Therefore, each of these actions increases thermal resistance, and all of the above is correct. For a quick check: doubling L doubles R_th and cuts Q in half for the same ΔT and material; halving A or halving k likewise increases R_th and reduces Q correspondingly.