H is the“ rate of heat transfer, r的 Experiment shows that H(g) ∠1T △t h=kA (23-1 k is the thermal conductivity" of the material a is the area of the slab Ax is the thickness of the slab Considering the direction of h and infinitesimal thickness of the slab. we have H=-kA (23-4)Experiment shows that is (23-1) H is the “rate of heat transfer” k is the “thermal conductivity” of the material A is the area of the slab is the thickness of the slab ( ) t Q H  = Δx ΔT H = kA x Considering the direction of H and infinitesimal thickness of the slab, we have: dx dT H = −kA (23-4)