d
\(Q = \frac{{kA({T_1} - {T_2})t}}{l}\)

અહીં, Q અને \(\Delta T\)બને ગોળાઓ માટે સમાન છે.  

\(\therefore k \propto \frac{1}{{At}}\,\,\therefore k \propto \frac{1}{{{r^2}t}}\,\,\,\therefore \frac{{{k_{l\arg er}}}}{{{k_{smaller}}}}\, = \,\frac{{{l_1}}}{{{l_s}}} \times {\left( {\frac{{{r_s}}}{{{r_1}}}} \right)^2} \times \frac{{{t_s}}}{{{t_1}}}\)

અહીં  \({\text{ }}{r_l} = 2{r_s},\,\,{l_l} = \frac{{{l_s}}}{4}\,\,\) અને \(\,{t_l} = 25\,\min ,\,\,{t_s} = 16\,\min \,\,\,\therefore \,\,\frac{{{k_{l\arg er}}}}{{{k_{smaller}}}}\)

\( = \,\left( {\frac{1}{4}} \right){\left( {\frac{1}{2}} \right)^2} \times \frac{{16}}{{25}} = \frac{1}{{25}}\)