If two metallic plates of equal thickness and thermal conductivities $$K_1$$ and $$K_2$$ are put together face to face and a common plate is constructed, then the equivalent thermal conductivity of this plate will be

The zeroth law of thermodynamics for three systems $$A, B$$ and $$C$$ in contact demands that

The velocity of sound in a gas is $$1300 \mathrm{~m} / \mathrm{s}$$ at STP and specific heat at constant pressure is $$6.84 \mathrm{~cal} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$$. The rms velocity at STP is $$(R=1.98 \mathrm{~cal} \mathrm{~K}^{-1} \mathrm{~mol}^{-1})$$

The efficiency of a Carnot engine kept at the temperatures of $$27^{\circ} \mathrm{C}$$ and $$127^{\circ} \mathrm{C}$$ is