A Carnot engine $C_1$ operates between temperature $T_1$ and $T_2\left(T_1>T_2\right)$. A second Carnot engine $C_2$ uses all the heat rejected by the engine $C_1$ and operates between temperature $T_2$ and $T_3$ (where $T_2>T_3$ ). The efficiency of this combined ( $C_1$ and $C_2$ together) engine is

All gases deviate from gas laws at

Assertion A thermos bottle consists of a double walled glass vessel with the space between the two walls evacuated, so that the heat transfer between the contents of the bottle and outside is minimised.

Reason The vacuum between the two walls inhibits the heat transfer by radiation mechanism.

Which of the following is correct?

How much heat energy is supplied when 5 kg of water at $20^{\circ} \mathrm{C}$ is brought to its boiling point? (Assume, specific heat of water $=4.2 \mathrm{~J} / \mathrm{g}^{\circ} \mathrm{C}$ )