Practice free Thermodynamics (Physics) NEET multiple-choice questions online with instant answers and detailed explanations. No login required.
Which of the following is NOT a consequence of the Second Law of Thermodynamics?
Option 4 is a statement of the First Law of Thermodynamics (conservation of energy). Options 1, 2, and 3 are direct consequences or statements of the Second Law.
Consider a book lying on a table. If it were to spontaneously jump to a certain height by cooling the table, this would violate the:
As explained in the NCERT text, such a process, while conserving energy (First Law), is never observed and is forbidden by the Second Law of Thermodynamics because it implies a spontaneous transfer of internal energy from a colder object (table) to provide ordered mechanical work (book jumping).
In the context of the Second Law, a 'perfect heat engine' or 'perfect refrigerator' refers to a hypothetical device where:
The NCERT text states: 'Put simply, the Second Law implies that no heat engine can have efficiency $\eta$ equal to 1 or no refrigerator can have co-efficient of performance $\alpha$ equal to infinity.' A 'perfect' engine/refrigerator would achieve these impossible limits.
Which of the following processes would be considered 'irreversible' in nature?
The NCERT text mentions that 'Spontaneous processes of nature are irreversible.' Combustion is a spontaneous and irreversible chemical process. Ideal gas expansion (if quasi-static) and phase changes at equilibrium are often considered idealized reversible processes, and heat transfer in a vacuum itself doesn't make the entire system reversal without other actions.
A Carnot engine has an efficiency of 50% when its source is at a temperature $327^\circ\text{C}$. The temperature of the sink is:
$\eta = 1 - T_2/T_1\Rightarrow 0.5 = 1 - T_2/600\Rightarrow T_2 = 300\ \text{K} = 27^\circ\text{C}$.
A thermodynamic system is taken through the cycle $abcda$. The work done by the gas along the path $bc$ is:
$bc$ is isochoric ($V$ constant), so $W = \int P\,dV = 0$.
Two gases A and B are filled at the same pressure in separate cylinders with movable pistons of radius $r_A$ and $r_B$, respectively. On supplying an equal amount of heat to both the systems reversibly under constant pressure, the pistons of gas A and B are displaced by 16 cm and 9 cm, respectively. If the change in their internal energy is the same, then the ratio $r_A/r_B$ is equal to:
Same $Q$ and same $\Delta U\Rightarrow$ same $W = P\Delta V$. With equal $P$: $\pi r_A^2(16) = \pi r_B^2(9)\Rightarrow\dfrac{r_A}{r_B} = \sqrt{\dfrac{9}{16}} = \dfrac{3}{4}$.
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