If the inner and outer surfaces of a hollow cylinder (having radii \(r_{1}\) and \(r_{2}\) and length \(\mathrm{L}\) ) are at temperature \(t_{1}\) and \(t_{2}\), then rate of radial heat flow will be:

(a) \(\frac{k}{2 \pi L} \cdot \frac{t_{1}-t_{2}}{\log _{e} \frac{r_{2}}{r_{1}}}\)

(b) \(\frac{1}{2 \pi k L} \cdot \frac{t_{1}-t_{2}}{\log _{e} \frac{r_{2}}{n}}\)

(c) \(\frac{2 \pi L}{k} \cdot \frac{t_{1}-t_{2}}{\log _{e} \frac{r_{2}}{\eta}}\)

(d) \(2 \pi k L \cdot \frac{t_{1}-t_{2}}{\log _{e} \frac{2}{1}}\)