Two plates of same thickness of coefficients of thermal conductivity $k_{1}$ and $k_{2}$ and areas of cross-section $A_{1}$ and $A_{2}$ are connected as shown in fig. The common coefficient of thermal conductivity $k$ will be:

K_{1} A_{1} + K_{2} A_{2}

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\frac{K_{1} A_{1} + K_{2} A_{2}}{2}

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\frac{K_{1} A_{1} + K_{2} A_{2}}{A_{1} + A_{2}}

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\frac{K_{1} A_{1}}{K_{2} A_{2}}

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Solution

The correct option is A $K_{1} A_{1} + K_{2} A_{2}$
Heat is in the direction from left to right so the left ends of both the plates will be at same temperature and their right ends are at same temperature that is the plates will be in parallel combination.
In parallel combination the common coefficient of thermal conductivity will be given as $K = \frac{K_{1} A_{1} + K_{2} A_{2}}{A_{1} + A_{2}}$
Option C is correct.

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