# Important Questions from Thermodynamics Class 11 Physics

JEE Mains & AdvancedGet important questions from Thermodynamics and Heat for Class 11 exams. View all Physics important questions for all the chapters. These important questions from thermodynamics class 11 physics will play significant role in clearing concepts of Physics. This question bank is designed by expert faculties keeping NCERT in mind and the questions are updated with respect to upcoming Board exams. You will get here all the important questions from heat and thermodynamics chapter. Learn all the concepts of thermodynamics from these important questions of class 11 Physics. Click Here for Detailed Notes of Heat and Thermodynamics along with other chapters and subjects.

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**Q. How is the mean kinetic energy of a gas related to its temperature?**

**Q. Although the root mean square speed of gas molecules is of the order of the speed of sound in that gas, yet on opening a bottle of ammonia in one corner of a room its smell takes time in reaching the other corner. Explain why?**

**Q. There are n molecules of gas in a box. If the number of molecules is increased to 2n. What will be the effect on the pressure of the gas? On the kinetic energy of the gas? On the root mean square speed of the molecules?**

**Q. On**

**which factor does the average kinetic energy of gas molecules depend : nature of the gas, absolute temperature, volume.**

**Q. Equal masses of monoatomic gas and diatomic gas at the same temperature are given equal quantities of heat. Which gas will have greater temperature rise?**

**Q. The graph shows the variation of the product PV with respect to the pressure (P) of given masses of three gases A, B and C. The temperature is kept constant, state with proper arguments which of these gases is ideal.**

*C*is ideal, because

*PV*is constant for this gas. It means the gas

*C*obeys Boyle’s law at all pressures.

**Q. There is a temperature known as Boyle's temperature ( $\left.T_{\mathrm{B}}\right)$ for each real gas at which it behaves like an ideal gas. But if gas temperature T is less than $\mathrm{T}_{\mathrm{B}}$ then does it behaves like ideal gas.**

**Q. What does universal gas constant signifies.**

**Q. The molecules of a gas are 2n a state of continuous, rapid and random motion. They move in all directions with different speeds. What is the range of their speeds?**

**Q. Two different gases have exactly the same temperature. Does this mean that their molecules have same r.m.s. speed?**

*c*) of molecules of different gases shall be different.

**Q. Distinguish between average speed and r.m.s. speed. If three molecules have speed u1, u2, u3 what will be their average speed and r.m.s. speed.**

**Q. A gas is filled in a cylinder fitted with a piston at a definite temperature and pressure. Explain on the basis of kinetic theory : on pulling the piston out, the pressure of gas decreases.**

**Q. State law of equipartition of energy.**

*k*is Boltzmann constant and

*T*is temperature of the system).

**Q. Obtain the relation between degree of freedom of a gas and ratio of two principal specific heats of the gas.**

*n*degrees of freedom. Total energy associated with a gram molecule of the gas This is the relation between $\gamma$ and

*n*. Hence the value of for a polyatomic gas can be determined from degree of freedom

*n*.

**Q. Why the molecular motion of the molecules ceases at zero kelvin?**

**Q. Explain the concept of temperature on the basis of kinetic theory?**

*T*can never be negative. This is the reason due to which the lowest temperature on kelvin’s scale is assumed to be 0K.

**Q. At what temperature does all molecular motions cease?**

**Q. What is the nature of graph, taking $0^{\circ} \mathrm{C}$ along the $\mathrm{y}$ -axis and 'F along the x-axis. What is the slope of this graph?**

**Q. How is the molar gas constant related to the principal gas constant?**

*R = Mr*.

**Q. Why temperature beyond $1200^{\circ} \mathrm{C}$ cannot be measured accurately by a platinum resistance thermometer?**

**Q. Two bodies at different temperatures $\mathrm{T}_{1}$ and $\mathrm{T}_{2}$if brought in thermal contact do not necessarily settle to the mean temperature $\left(\frac{T_{1}+T_{2}}{2}\right)$**

**Q. Why platinum wire can be sealed into glass but not the copper wire?**

**Q. What is the ice-point on kelvin scale? If the temperature of body increases by $1^{\circ} \mathrm{C},$ what will be change in temperature on kelvin scale?**

**Q.**Which single point is used as reference point in thermometer ?**Q. An electric refrigerator transfers heat from the cold cooling coils to the warm surroundings. Is it against the second law thermodynamics ? Justify your answer ?**

**Q. Can the carnot engine be realised in practice ?**

**Q. Does the mass of body change when it is heated or cooled ?**

**Q. It is possible to convert internal energy into work ?**

**Q. An ideal gas is compressed at constant temperature, will its internal energy increase or decrease ?**

**Q. In summer, when the valve of a bicycle tube is opened, the escaping air becomes cold. Why?**

**Q. Why does a gas get heated on compression ?**

**Q. Why are the brake-drums of a car heated when the car moves down a hill at constant speed ?**

**Q. If an electric fan be switched in a closed room, will the air of the room be cooled ? If not, why have feel cold ?**

**Q. Is superheating of steam an isobaric process or isothermal process, and why ?**

**Q. What are the limitations of first law of thermodynamics ?**

**Q. A gas does work during isothermal expansion ? What is the source of mechanical energy so produced ?**

**Ans.**The energy required for doing work during isothermal expansion is acquired as heat by the gas from surroundings. Here $\quad \Delta Q=\Delta W i . e$. $\Delta U=0$

**Q. What is the valve of work-done in a cyclic process ?**

*P-V*-diagram.

**Q. Name the sink in case of steam engine ?**

**Q. Why can a ship not use the internal energy of sea water to operate its engine ?**

**Q. What do you understand by triple point of water ? Why it is unique ?**

**Q. Two cylinders A and B of equal capacity are connected to each other via a stop-cock.**

**A contains a gas at standard temperature and pressure, B is completely evacuated. The entire system is thermally insulated. The stop-cock is suddenly opened. Answer the following**

**(i) What is the final pressure of the gas in A and B?**

**(ii) What is the change in internal energy of the gas ?**

**(iii) What is the change in temperature of the gas?**

**(iv) Do the intermediate state the system (before setting to final equilibrium state) be on its P-V-T surface ?**

*i.e.*pressure will reduce to 0.5 atm. (ii) An explained above, there is no change in internal energy. (iii)As explained above, there is also no change in temperature. (iv)No, since the process (called free expansion) is rapid and cannot be controlled. The intermediate states are non-equilibrium states and do not satisfy the gas equation. After a short time interval, the gas returns to equilibrium states which lies on its P-V-T surface.

**Q. A Thermos bottle containing coffee is vigorously shaken. Considering coffee as a system**

**(a) has heat been added to it? (b) has work been done on it? (c) has its internal energy changed? (d) does its temperature rise ?**

*i.e.*$\Delta Q=0$ (b) Yes, in shaking work is done on the coffee against viscous force $i . e . \Delta W$ is negative. (c) According to first law, $\Delta Q=\Delta U+\Delta W,$ however as here and negative, so be positive i.e. internal energy of coffee will increase. (d) Now, as internal energy of system depends upon its temperature

*i.e.*, so temperature of the system (coffee) will also increase.

**Q. Why is the conversion of heat into work not possible without a sink at lower temperature ?**

**Q. What is meant by reversible engine ? Explain why the efficiency of reversible engine is maximum ?**

**Q.**

*Show that an adiabatic curve is always steeper than an isothermal curve.**PV*= constant (

*K*) Adiabatic process That thermal process in which pressure, volume and temperature of the system change but there is no exchange of heat between the system and the surroundings, is called an adiabatic process. This process is achieved (a) if the gas in enclosed in a vessel whose walls are made of a highly insulating material and (b) the gas is compressed or allowed to expand very quickly. The gas equation is, $P V^{\gamma}=$ constant, Slopes of isothermal and adiabatic curves The slope of an isothermal or adiabatic curve is given by $\frac{d P}{d V}$. (a) For an isothermal process, $P V=k$ Differentiating on both sides, we get, $P d V+V d P=0$ $\Rightarrow \frac{d P}{d V}=-\frac{P}{V}$ is slope of isothermal curve, $\left(\frac{d P}{d V}\right)=-\frac{P}{V}$ .............. (1) (b) For an adiabatic process, $P V^{\gamma}=K$ (constant) Differentiating on both sides, we get, $P_{\gamma} v^{\gamma-1} . d V+V^{\gamma} . d P=0$ $\Rightarrow V^{\gamma} d P=-\gamma P V^{\gamma-1} d V |$ $\therefore \frac{d P}{d V}=-\frac{\gamma P V^{\gamma-1}}{V^{\gamma}}=-\frac{\gamma P}{V}$ i.e. slope of adiabatic curve, $\left(\frac{d P}{d V}\right)_{A}=-\frac{\gamma P}{V}$ From equation ( 1) and $(2),$ we have, $\left(\frac{d P}{d V}\right)_{A}=\gamma\left(-\frac{P}{V}\right)=\gamma\left(\frac{d P}{d V}\right)_{I}$ As $\gamma$ is always greater than one, hence slope of adiabatic curve is greater than the slope of isothermal curve. In other words, an adiabatic curve is sleeper than an isothermal curve.

**Q. State and explain first law of thermodynamics. Establish the relation between two principal specific heats of a gas on the basis of this law.**

*dQ*) is supplied to system capable of doing external work, then the quantity of heat absorbed by the system (

*dQ*) is equal to the sum of the increase in internal energy of the system (

*dU*) and external work-done by the system (

*dW*) ie. $d Q=d U+d W$ Relation between two principal specific heat of a gas : A gas has two specific heats Relation between two principal specific heat of a gas: A gas has two specific heats (i) Specific heat at constant volume $\left(C_{V}\right):$ It is the amount of heat required to raise the temperature of unit mass of a gas by $1^{\circ} C$ at constant volume. It is denoted by $C_{V}$ (ii) Specific heat at constant pressure $\left(\mathrm{C}_{P}\right):$ It is the amount of heat required to raise the temperature of unit mass of gas by $1^{\circ} \mathrm{C}$ at constant pressure. It is denoted by $C_{P}$. Let 1 mole of a gas is supplied $\Delta Q$ amount of heat at constant volume so that the temperature of the gas rises by $\Delta T$. $\therefore \Delta Q=1 \times C_{V} \times \Delta T=C_{V} \Delta T$ ...............(1) $\begin{array}{ll}{\text { As volume remains constant, }} \\ {d V=0} & {\therefore \Delta W=P \Delta V=0} \\ {\text { According to first law of thermodynamics, }} \\ {\Delta Q=\Delta U+\Delta W} \\ {\Rightarrow \Delta U=C_{V} \Delta T}\end{array}$ Now, let 1 mole of same gas is given an amount of heat $\Delta Q$ at constant pressure so as to increase the temperature by $\Delta T,$ then, $\Delta Q=1 \times C_{P} \times \Delta T \Rightarrow \Delta Q=C_{P} \Delta T$ .............(3) If $\Delta V$ is the change in volume at constant pressure, hen, $\Delta W=P \Delta V$ From first law, $\Delta Q=\Delta W+\Delta U$ $\Rightarrow \Delta Q=P \Delta V+\Delta U \quad \Rightarrow C_{P} \Delta T=P \Delta V+\Delta U$ $\Rightarrow P \Delta V+C_{V} \Delta T=C_{P} \Delta T[\text { From equation }(2)]$ $\Rightarrow\left(C_{P}-C_{V}\right) \Delta T=P \Delta V$ ...........(4) $\begin{array}{l}{\text { But from ideal gas equation, }} \\ {P V=R T \quad \text { ideal }} \\ {\text { From equation }(4), \text { we have, }}\end{array} \Rightarrow P \Delta V=R \Delta T$ $\left(C_{P}-C_{V}\right) \Delta T=R \Delta T$ This is known as Mayor's equation. Note (1) In above equation units of all quantities $\left(C_{P}, C_{V} \text { and } R\right)$ are Joule $m o l^{-1} K^{-1} .$ If $C_{P}$ and $d$ $C_{V}$ are expressed in units calorie $m o l^{-1} K^{-1},$ then $C_{P}-C_{V}=\frac{R}{J}$ (2) In above relation is expressed for one gram of gas, then, $C_{P}-C_{V}=\frac{r}{J}$ (3) As value of molar gas constant $R$ is $(+v e) 8.31$ Jmol $^{-1} K^{-1}$, it implies that $C_{P}>C_{V}$ and their difference in equal to $8.31 J \mathrm{mol}^{-1} K^{-1}$.

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