Conduction Law: Differential Form

Q.

Two different metal rods of the same length have their ends kept at the same temperatures ${\theta }_1$ and ${\theta }_2$ with ${\theta }_2$ > ${\theta }_1$. If $A_1$ and $A_2$ are their cross-sectional areas and $k_1$ and $k_2$ their thermal conductivities, the rate of heat flow in the two rods will be same if

1. $\frac{k_1}{k_2}=\frac{A_1}{A_2}$

2. $\frac{k_1}{k_2}=\frac{A_1{\theta }_2}{A_2{\theta }_1}$

3. $\frac{k_1}{k_2}=\frac{A_1{\theta }_1}{A_2{\theta }_2}$

4. $\frac{k_1}{k_2}=\frac{A_2}{A_1}$