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Two metal rods 1 and 2 of same lengths have same temperature difference between their ends. Their thermal conductivities are K₁ and K₂ and cross-sectional areas A₁ and A₂, respectively. If the rate of heat conduction in rod 1 is four times that in rod 2, then:

1. K₁A₁ = K₂A₂
2. K₁A₁ = 2K₂A₂
3. K₁A₁ = 4K₂A₂
4. K₁A₁ = K₂4A₂

Correct answer: K₁A₁ = 4K₂A₂

Solution

The rate of heat conduction is proportional to the product of thermal conductivity and cross-sectional area. Since the heat conduction in rod 1 is four times that in rod 2, we have K₁A₁ = 4K₂A₂.

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