(a) State Fourier’s law for heat conduction.
(b) Explain the physical meaning of the negative sign that appears in its differential form.
(c) Consider a uniform cylindrical rod whose ends are maintained at different fixed temperatures. Qualitatively describe how the rate of heat flow through the rod will change if (i) its cross-sectional area is doubled while length is kept the same, and (ii) its length is doubled while area is kept the same. Justify your answers without performing any numerical calculation.