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Adiabatic Process Questions in English

Class 11 Physics · Thermodynamics · Adiabatic Process

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51
MediumMCQ
For an adiabatic expansion of a perfect gas,the value of $\frac{\Delta P}{P}$ is equal to
A
$-\sqrt{\gamma} \frac{\Delta V}{V}$
B
$-\frac{\Delta V}{V}$
C
$-\gamma \frac{\Delta V}{V}$
D
$-\gamma^2 \frac{\Delta V}{V}$

Solution

(C) For an adiabatic process,the relation between pressure $P$ and volume $V$ is given by $PV^{\gamma} = \text{constant}$.
Taking the natural logarithm on both sides: $\ln P + \gamma \ln V = \ln(\text{constant})$.
Differentiating both sides with respect to their variables: $\frac{dP}{P} + \gamma \frac{dV}{V} = 0$.
Rearranging the terms,we get: $\frac{dP}{P} = -\gamma \frac{dV}{V}$.
For small changes,$\frac{\Delta P}{P} = -\gamma \frac{\Delta V}{V}$.
52
DifficultMCQ
$A$ monoatomic ideal gas,initially at temperature $T_1$,is enclosed in a cylinder fitted with a frictionless piston. The gas is allowed to expand adiabatically to a temperature $T_2$ by releasing the piston suddenly. If $L_1$ and $L_2$ are the lengths of the gas column before and after expansion respectively,then $T_1/T_2$ is given by
A
$(\frac{L_1}{L_2})^{2/3}$
B
$\frac{L_1}{L_2}$
C
$\frac{L_2}{L_1}$
D
$(\frac{L_2}{L_1})^{2/3}$

Solution

(D) For an adiabatic process,the relationship between temperature $T$ and volume $V$ is given by $T V^{\gamma - 1} = \text{constant}$.
Thus,$T_1 V_1^{\gamma - 1} = T_2 V_2^{\gamma - 1}$,which implies $\frac{T_1}{T_2} = (\frac{V_2}{V_1})^{\gamma - 1}$.
Since the gas is monoatomic,the adiabatic index $\gamma = 5/3$. Therefore,$\gamma - 1 = 5/3 - 1 = 2/3$.
The volume of the gas in a cylinder is $V = A \times L$,where $A$ is the cross-sectional area and $L$ is the length of the gas column.
Substituting $V_1 = A L_1$ and $V_2 = A L_2$,we get $\frac{V_2}{V_1} = \frac{A L_2}{A L_1} = \frac{L_2}{L_1}$.
Substituting these into the temperature ratio equation: $\frac{T_1}{T_2} = (\frac{L_2}{L_1})^{2/3}$.
53
MediumMCQ
The volume of air increases by $5\%$ in its adiabatic expansion. The percentage decrease in its pressure will be ...... $\%$
A
$5$
B
$6$
C
$7$
D
$8$

Solution

(C) For an adiabatic process,the relation between pressure $P$ and volume $V$ is given by $PV^{\gamma} = K$,where $\gamma$ is the adiabatic index.
Taking the logarithmic derivative of both sides: $\ln P + \gamma \ln V = \ln K$.
Differentiating both sides,we get: $\frac{dP}{P} + \gamma \frac{dV}{V} = 0$.
This implies $\frac{dP}{P} = -\gamma \frac{dV}{V}$.
For air (a diatomic gas),the adiabatic index $\gamma = 1.4$.
Given the percentage increase in volume $\frac{dV}{V} \times 100 = 5\%$.
The percentage change in pressure is $\frac{dP}{P} \times 100 = -\gamma \left( \frac{dV}{V} \times 100 \right)$.
Substituting the values: $\frac{dP}{P} \times 100 = -1.4 \times 5 = -7\%$.
The negative sign indicates a decrease in pressure. Therefore,the percentage decrease in pressure is $7\%$.
54
MediumMCQ
The temperature of a hypothetical gas increases to $\sqrt{2}$ times when compressed adiabatically to half the volume. Its equation can be written as
A
$PV^{3/2} = \text{constant}$
B
$PV^{5/2} = \text{constant}$
C
$PV^{7/3} = \text{constant}$
D
$PV^{4/3} = \text{constant}$

Solution

(A) For an adiabatic process, the relationship between temperature $T$ and volume $V$ is given by $TV^{\gamma - 1} = \text{constant}$.
Given that the temperature increases to $\sqrt{2}$ times, we have $T_2 = \sqrt{2}T_1$.
The volume is compressed to half, so $V_2 = V_1 / 2$.
Using the relation $\frac{T_1}{T_2} = \left( \frac{V_2}{V_1} \right)^{\gamma - 1}$, we substitute the values:
$\frac{T_1}{\sqrt{2}T_1} = \left( \frac{V_1/2}{V_1} \right)^{\gamma - 1}$
$\frac{1}{\sqrt{2}} = \left( \frac{1}{2} \right)^{\gamma - 1}$
Since $\frac{1}{\sqrt{2}} = (1/2)^{1/2}$, we equate the exponents:
$\gamma - 1 = 1/2 \implies \gamma = 3/2$.
The adiabatic equation is $PV^{\gamma} = \text{constant}$, so $PV^{3/2} = \text{constant}$.
55
DifficultMCQ
The following figure shows an adiabatic cylindrical container of volume $V_0$ divided by an adiabatic smooth piston (area of cross-section = $A$) into two equal parts. An ideal gas $(C_P/C_V = \gamma)$ is at pressure $P_1$ and temperature $T_1$ in the left part,and a gas at pressure $P_2$ and temperature $T_2$ is in the right part. The piston is slowly displaced and released at a position where it can stay in equilibrium. The final pressure of the two parts will be (Suppose $x$ = displacement of the piston):
Question diagram
A
$P_2$
B
$P_1$
C
$\frac{P_1 (V_0/2)^\gamma}{(V_0/2 + Ax)^\gamma}$
D
$\frac{P_2 (V_0/2)^\gamma}{(V_0/2 + Ax)^\gamma}$

Solution

(C) Since the piston is in equilibrium,both gases must be at the same final pressure $P_f$.
Given the displacement of the piston is $x$ and the area of cross-section is $A$,the final volumes of the left and right parts are:
$V_L = V_0/2 + Ax$ and $V_R = V_0/2 - Ax$.
Since the container and piston are adiabatic,both gases undergo adiabatic processes.
For the left gas:
$P_1 (V_0/2)^\gamma = P_f (V_0/2 + Ax)^\gamma$
$P_f = \frac{P_1 (V_0/2)^\gamma}{(V_0/2 + Ax)^\gamma}$ ... $(i)$
For the right gas:
$P_2 (V_0/2)^\gamma = P_f (V_0/2 - Ax)^\gamma$
$P_f = \frac{P_2 (V_0/2)^\gamma}{(V_0/2 - Ax)^\gamma}$ ... $(ii)$
Both expressions represent the final pressure $P_f$. Thus,option $(C)$ is a valid expression for the final pressure.
Solution diagram
56
MediumMCQ
In the following $P-V$ diagram, two adiabatics cut two isothermals at temperatures $T_1$ and $T_2$. The value of $\frac{V_a}{V_d}$ will be
Question diagram
A
$\frac{V_b}{V_c}$
B
$\frac{V_c}{V_b}$
C
$\frac{V_d}{V_a}$
D
$V_b V_c$

Solution

(A) For an adiabatic process, the relation between temperature and volume is $T V^{\gamma - 1} = \text{constant}$.
For the adiabatic curve $bc$ connecting points $b$ (at $T_1$) and $c$ (at $T_2$):
$T_1 V_b^{\gamma - 1} = T_2 V_c^{\gamma - 1} \implies \frac{T_2}{T_1} = \left( \frac{V_b}{V_c} \right)^{\gamma - 1} \dots (i)$
For the adiabatic curve $ad$ connecting points $a$ (at $T_1$) and $d$ (at $T_2$):
$T_1 V_a^{\gamma - 1} = T_2 V_d^{\gamma - 1} \implies \frac{T_2}{T_1} = \left( \frac{V_a}{V_d} \right)^{\gamma - 1} \dots (ii)$
Comparing equations $(i)$ and $(ii)$, we get:
$\left( \frac{V_b}{V_c} \right)^{\gamma - 1} = \left( \frac{V_a}{V_d} \right)^{\gamma - 1}$
Therefore, $\frac{V_a}{V_d} = \frac{V_b}{V_c}$.
57
MediumMCQ
If $\Delta E_{int}$ represents the increase in internal energy and $W$ represents the work done by the system,which of the following statements is correct for a thermodynamic system?
A
$\Delta E_{int} = -W$ for an isothermal process
B
$\Delta E_{int} = W$ for an isothermal process
C
$\Delta E_{int} = -W$ for an adiabatic process
D
$\Delta E_{int} = W$ for an adiabatic process

Solution

(C) According to the first law of thermodynamics,the change in internal energy is given by $\Delta E_{int} = \Delta Q - W$,where $\Delta Q$ is the heat supplied to the system and $W$ is the work done by the system.
For an adiabatic process,there is no exchange of heat with the surroundings,so $\Delta Q = 0$.
Substituting this into the first law equation,we get $\Delta E_{int} = 0 - W$,which simplifies to $\Delta E_{int} = -W$.
Therefore,the correct statement is that for an adiabatic process,the increase in internal energy is equal to the negative of the work done by the system.
58
DifficultMCQ
$A$ monoatomic gas is compressed adiabatically to $1/8$ of its initial volume. What happens to the gas pressure? $(\gamma = 5/3)$
A
$24/5$
B
$8$
C
$40/3$
D
$32$ times the initial value

Solution

(D) For an adiabatic process,the relation between pressure and volume is given by $PV^\gamma = \text{constant}$.
Thus,$P_1 V_1^\gamma = P_2 V_2^\gamma$.
Rearranging for the final pressure $P_2$,we get $P_2 = P_1 \left( \frac{V_1}{V_2} \right)^\gamma$.
Given that $V_2 = \frac{V_1}{8}$,so $\frac{V_1}{V_2} = 8$.
Substituting the values,$P_2 = P_1 (8)^{5/3}$.
Since $8 = 2^3$,we have $P_2 = P_1 (2^3)^{5/3} = P_1 (2^5) = 32 P_1$.
Therefore,the pressure becomes $32$ times the initial value.
59
DifficultMCQ
An ideal gas at $27 \, ^\circ C$ is compressed adiabatically such that its volume becomes $8/27$ of its original volume. If $\gamma = 5/3$ for the gas,the increase in temperature of the gas is ..... $K$.
A
$450$
B
$375$
C
$225$
D
$405$

Solution

(B) For an adiabatic process,the relation between temperature and volume is $TV^{\gamma - 1} = \text{constant}$.
Given: $T_1 = 27 + 273 = 300 \, K$,$V_2 = \frac{8}{27} V_1$,and $\gamma = 5/3$.
Using the relation $\frac{T_2}{T_1} = \left( \frac{V_1}{V_2} \right)^{\gamma - 1}$:
$\frac{T_2}{300} = \left( \frac{V_1}{(8/27)V_1} \right)^{5/3 - 1} = \left( \frac{27}{8} \right)^{2/3}$.
Calculating the exponent: $\left( \frac{27}{8} \right)^{2/3} = \left( \left( \frac{3}{2} \right)^3 \right)^{2/3} = \left( \frac{3}{2} \right)^2 = \frac{9}{4} = 2.25$.
Therefore,$T_2 = 300 \times 2.25 = 675 \, K$.
The increase in temperature is $\Delta T = T_2 - T_1 = 675 - 300 = 375 \, K$.
60
DifficultMCQ
$A$ triatomic gas at an initial temperature of $18^{\circ}C$ is compressed adiabatically to $1/8$ of its initial volume. What is the final temperature of the gas?
A
$10^{\circ}C$
B
$887^{\circ}C$
C
$668 K$
D
$144^{\circ}C$

Solution

(C) For an adiabatic process,the relationship between temperature and volume is given by $T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$.
Given: Initial temperature $T_1 = 18^{\circ}C = 18 + 273 = 291 K$.
Initial volume $V_1 = V$,final volume $V_2 = V/8$.
For a triatomic gas,the degree of freedom $f = 6$ (non-linear). The adiabatic index $\gamma = 1 + 2/f = 1 + 2/6 = 1 + 1/3 = 4/3$.
Thus,$\gamma - 1 = 1/3$.
Using the formula: $T_2 = T_1 (V_1/V_2)^{\gamma-1} = 291 \times (V / (V/8))^{1/3} = 291 \times (8)^{1/3} = 291 \times 2 = 582 K$.
Converting back to Celsius: $582 - 273 = 309^{\circ}C$.
Note: If the gas is considered linear,$f = 5$,$\gamma = 1.4$,$\gamma-1 = 0.4$. Then $T_2 = 291 \times (8)^{0.4} \approx 291 \times 2.297 \approx 668 K$.
Given the options,the calculation assumes $\gamma = 1.4$ (diatomic/linear triatomic behavior). Thus,$T_2 = 668 K$.
61
MediumMCQ
$5.6 \ L$ of Helium gas at $STP$ is compressed adiabatically to $0.7 \ L$. If the initial temperature is $T_1$,then the work done during the process is...?
A
$\frac{9}{8} RT_1$
B
$\frac{3}{2} RT_1$
C
$\frac{15}{8} RT_1$
D
$\frac{9}{2} RT_1$

Solution

(A) For Helium (monatomic gas),the adiabatic index $\gamma = 5/3$.
At $STP$,the volume of $1 \ mole$ of gas is $22.4 \ L$.
Number of moles $n = \frac{5.6}{22.4} = \frac{1}{4} \ mole$.
For an adiabatic process,$T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$.
Given $V_1 = 5.6 \ L$ and $V_2 = 0.7 \ L$,so $V_1/V_2 = 8$.
$T_2 = T_1 (V_1/V_2)^{\gamma-1} = T_1 (8)^{5/3 - 1} = T_1 (8)^{2/3} = T_1 (2^3)^{2/3} = 4T_1$.
Work done $W = -\frac{nR(T_2 - T_1)}{\gamma - 1}$.
Substituting the values: $W = -\frac{(1/4)R(4T_1 - T_1)}{5/3 - 1} = -\frac{(1/4)R(3T_1)}{2/3} = -\frac{3}{4} R T_1 \times \frac{3}{2} = -\frac{9}{8} RT_1$.
The magnitude of work done is $\frac{9}{8} RT_1$.
62
DifficultMCQ
During an adiabatic process,the pressure of a gas is found to be proportional to the cube of its temperature. The ratio $C_P / C_V$ for this gas is ........
A
$2$
B
$5/3$
C
$3/2$
D
$4/3$

Solution

(C) For an adiabatic process,the relationship between pressure $P$ and temperature $T$ is given by $P^{1-\gamma} T^{\gamma} = \text{constant}$,which can be rewritten as $P \propto T^{\frac{\gamma}{\gamma-1}}$.
Given that $P \propto T^3$,we can equate the exponents of $T$:
$\frac{\gamma}{\gamma-1} = 3$
$\gamma = 3(\gamma - 1)$
$\gamma = 3\gamma - 3$
$2\gamma = 3$
$\gamma = 3/2$
Since the ratio $C_P / C_V$ is defined as $\gamma$,the value is $3/2$.
63
MediumMCQ
When a gas is compressed adiabatically by a piston in a cylinder,the work done on the gas is found to be $1.5 \times 10^{4} \, J$. During this process:
A
$3.6 \times 10^{3} \, \text{cal}$ of heat is released from the gas.
B
$3.6 \times 10^{3} \, \text{cal}$ of heat is absorbed by the gas.
C
$1.5 \times 10^{4} \, \text{cal}$ of heat is absorbed by the gas.
D
$1.5 \times 10^{4} \, \text{cal}$ of heat is released from the gas.

Solution

(A) In an adiabatic process,the exchange of heat with the surroundings is zero,i.e.,$\Delta Q = 0$. However,the question implies the work done on the gas results in a change in internal energy.
Given that the process is adiabatic,$\Delta Q = 0$.
Wait,the original question text mentions 'isothermally' in the Gujarati prompt but 'adiabatically' in the inheritance/English context. Based on the solution provided $(1.5 \times 10^4 / 4.18)$,it is clear the process is meant to be adiabatic where work done on the gas increases internal energy.
Actually,in an adiabatic process,$\Delta Q = 0$. If the question asks for the heat equivalent of the work done,we convert $1.5 \times 10^4 \, J$ to calories:
$\text{Heat} = \frac{1.5 \times 10^4 \, J}{4.18 \, J/\text{cal}} \approx 3.6 \times 10^3 \, \text{cal}$.
Since work is done $ON$ the gas,the internal energy increases. If the process were not adiabatic,this would be the heat released. Given the options,$A$ is the correct choice.
64
MediumMCQ
$A$ motor tube is filled with air at $27^{\circ}C$ and a pressure of $8 \text{ atm}$. If the tube suddenly bursts,what will be the final temperature of the air? $(\gamma = 1.5)$
A
$27.5^{\circ}C$
B
$75 \text{ K}$
C
$150 \text{ K}$
D
$150^{\circ}C$

Solution

(C) When a tube bursts suddenly,the process is adiabatic because there is no time for heat exchange with the surroundings.
For an adiabatic process,the relationship between temperature and pressure is given by: $T_2 / T_1 = (P_2 / P_1)^{(\gamma - 1) / \gamma}$.
Given: Initial temperature $T_1 = 27^{\circ}C = 300 \text{ K}$,Initial pressure $P_1 = 8 \text{ atm}$,Final pressure $P_2 = 1 \text{ atm}$ (atmospheric pressure),and $\gamma = 1.5$.
Substituting the values: $T_2 / 300 = (1 / 8)^{(1.5 - 1) / 1.5}$.
$T_2 / 300 = (1 / 8)^{0.5 / 1.5} = (1 / 8)^{1/3}$.
Since $8^{1/3} = 2$,we have $T_2 / 300 = 1 / 2$.
Therefore,$T_2 = 300 / 2 = 150 \text{ K}$.
65
MediumMCQ
For an adiabatic process,if $\gamma = 2.5$ and the volume becomes $1/8$ of its initial volume,then the new pressure $P'$ is (initial pressure $= P$):
A
$P' = P$
B
$P' = 2P$
C
$P' = P \times (2)^{7.5}$
D
$P' = 32P$

Solution

(C) For an adiabatic process,the relationship between pressure and volume is given by $PV^\gamma = \text{constant}$.
Thus,$P_1 V_1^\gamma = P_2 V_2^\gamma$.
Given: $P_1 = P$,$V_2 = \frac{V_1}{8}$,and $\gamma = 2.5 = \frac{5}{2}$.
Substituting these values:
$P \times V_1^{5/2} = P' \times \left(\frac{V_1}{8}\right)^{5/2}$.
$P' = P \times \left(\frac{V_1}{V_1/8}\right)^{5/2} = P \times (8)^{5/2}$.
Since $8 = 2^3$,we have $P' = P \times (2^3)^{5/2} = P \times 2^{15/2} = P \times 2^{7.5}$.
Therefore,$P' = P \times (2)^{7.5}$.
66
EasyMCQ
In an adiabatic process,the initial and final temperatures of $1 \, mol$ of an ideal gas are $T_1$ and $T_2$ respectively. The change in internal energy of the gas is .......
A
$\frac{R}{\gamma - 1}(T_2 - T_1)$
B
$\frac{R}{\gamma - 1}(T_1 - T_2)$
C
$R(T_1 - T_2)$
D
Zero

Solution

(A) For an ideal gas,the change in internal energy $\Delta U$ is given by the formula $\Delta U = n C_V \Delta T$.
Given $n = 1 \, mol$ and $\Delta T = (T_2 - T_1)$.
The molar heat capacity at constant volume is $C_V = \frac{R}{\gamma - 1}$.
Substituting these values,we get $\Delta U = 1 \times \frac{R}{\gamma - 1} \times (T_2 - T_1)$.
Thus,the change in internal energy is $\frac{R}{\gamma - 1}(T_2 - T_1)$.
67
MediumMCQ
$1 \, \text{kmol}$ of an ideal gas is compressed adiabatically,requiring $146 \, \text{kJ}$ of work. During this process,the temperature of the gas increases by $7^o \text{C}$. The gas is: $(R = 8.3 \, \text{J mol}^{-1} \text{K}^{-1})$
A
Triatomic
B
Monoatomic
C
$A$ mixture of monoatomic and diatomic
D
Diatomic

Solution

(D) For an adiabatic process,the heat exchange $Q = 0$. According to the first law of thermodynamics,$\Delta Q = \Delta U + \Delta W$. Since $Q = 0$,$\Delta U = -\Delta W$.
Given that work is done on the gas,$\Delta W = -146 \times 10^3 \, \text{J}$,so $\Delta U = 146 \times 10^3 \, \text{J}$.
The change in internal energy is given by $\Delta U = n C_v \Delta T$,where $n = 10^3 \, \text{mol}$ and $\Delta T = 7 \, \text{K}$.
$C_v = \frac{R}{\gamma - 1}$.
Substituting the values: $146 \times 10^3 = 10^3 \times \frac{8.3}{\gamma - 1} \times 7$.
$146 = \frac{58.1}{\gamma - 1} \implies \gamma - 1 = \frac{58.1}{146} \approx 0.3979$.
$\gamma \approx 1.4$.
Since $\gamma = 1.4$ for a diatomic gas,the gas is diatomic.
68
DifficultMCQ
If the pressure in an adiabatic process increases by $\frac{2}{3}\%$,then the percentage decrease in volume will be ....... . Assume $C_P/C_V = 3/2$.
A
$\frac{4}{9}\%$
B
$\frac{2}{3}\%$
C
$4\%$
D
$\frac{9}{4}\%$

Solution

(A) For an adiabatic process,the relation between pressure $P$ and volume $V$ is given by $PV^\gamma = k$,where $\gamma = C_P/C_V = 3/2$.
Taking the logarithm on both sides: $\ln P + \gamma \ln V = \ln k$.
Differentiating both sides: $\frac{\Delta P}{P} + \gamma \frac{\Delta V}{V} = 0$.
Therefore,$\frac{\Delta V}{V} = -\frac{1}{\gamma} \frac{\Delta P}{P}$.
Given $\frac{\Delta P}{P} = \frac{2}{3}\% = \frac{2}{300}$ and $\gamma = 3/2$,we have:
$\frac{\Delta V}{V} = -\frac{1}{3/2} \times \frac{2}{3}\% = -\frac{2}{3} \times \frac{2}{3}\% = -\frac{4}{9}\%$.
The negative sign indicates a decrease in volume.
Thus,the percentage decrease in volume is $\frac{4}{9}\%$.
69
DifficultMCQ
When $1 \text{ mole}$ of an ideal gas $(\gamma = 1.4)$ is compressed adiabatically,its temperature increases from $27^{\circ}C$ to $35^{\circ}C$. The increase in the internal energy of the gas is ...... $J$? $(R = 8.3 \text{ J/mol K})$
A
$-166$
B
$166$
C
$168$
D
$-168$

Solution

(B) The change in internal energy $(\Delta U)$ for an ideal gas is given by the formula: $\Delta U = n C_v \Delta T$.
For an ideal gas,$C_v = \frac{R}{\gamma - 1}$.
Given: $n = 1 \text{ mole}$,$\gamma = 1.4$,$R = 8.3 \text{ J/mol K}$,$T_1 = 27^{\circ}C$,$T_2 = 35^{\circ}C$.
Change in temperature $\Delta T = T_2 - T_1 = 35 - 27 = 8 \text{ K}$.
Substituting the values: $\Delta U = 1 \times \frac{8.3}{1.4 - 1} \times 8$.
$\Delta U = \frac{8.3}{0.4} \times 8 = 8.3 \times 20 = 166 \text{ J}$.
Thus,the increase in internal energy is $166 \text{ J}$.
70
MediumMCQ
The pressure of a motorcar tire at $27^{\circ}C$ is $2$ atmospheres. If the tire suddenly bursts,the final temperature will be ....... $K$ (Take $\gamma = 1.4$).
A
$246.1$
B
$250$
C
$240$
D
$248$

Solution

(A) For an adiabatic process,the relationship between pressure $P$ and temperature $T$ is given by $P^{1-\gamma} T^{\gamma} = \text{constant}$.
Given: Initial temperature $T_1 = 27^{\circ}C = 300 \ K$,Initial pressure $P_1 = 2 \ \text{atm}$,Final pressure $P_2 = 1 \ \text{atm}$ (atmospheric pressure),and $\gamma = 1.4$.
Using the relation $\frac{T_2}{T_1} = \left( \frac{P_2}{P_1} \right)^{\frac{\gamma-1}{\gamma}}$:
$T_2 = T_1 \left( \frac{P_2}{P_1} \right)^{\frac{1.4-1}{1.4}}$
$T_2 = 300 \left( \frac{1}{2} \right)^{\frac{0.4}{1.4}}$
$T_2 = 300 \left( 0.5 \right)^{0.2857}$
$T_2 \approx 300 \times 0.8204 \approx 246.1 \ K$.
71
DifficultMCQ
For an adiabatic process,the pressure and temperature of a monoatomic gas are related as $P \propto T^{c}$. Then,$c = $.......
A
$0.6$
B
$1.67$
C
$0.4$
D
$2.5$

Solution

(D) For an adiabatic process,the relation between pressure $P$ and volume $V$ is $PV^{\gamma} = \text{constant}$.
Using the ideal gas equation $PV = \mu RT$,we have $V = \frac{\mu RT}{P}$.
Substituting this into the adiabatic equation: $P \left( \frac{\mu RT}{P} \right)^{\gamma} = \text{constant}$.
This simplifies to $P^{1-\gamma} T^{\gamma} = \text{constant}$.
Rearranging for $P$: $P^{1-\gamma} = \frac{\text{constant}}{T^{\gamma}} = \text{constant} \cdot T^{-\gamma}$.
$P = \text{constant} \cdot T^{-\gamma / (1-\gamma)} = \text{constant} \cdot T^{\gamma / (\gamma - 1)}$.
Comparing with $P \propto T^{c}$,we get $c = \frac{\gamma}{\gamma - 1}$.
For a monoatomic gas,the adiabatic index $\gamma = \frac{5}{3}$.
Substituting the value of $\gamma$: $c = \frac{5/3}{(5/3) - 1} = \frac{5/3}{2/3} = \frac{5}{2} = 2.5$.
72
MediumMCQ
$A$ gas having a volume of $1 \ mm^3$ at $1 \ atm$ pressure is compressed from a temperature of $27^{\circ}C$ to $627^{\circ}C$. What will be the final pressure if the process is adiabatic? (For the gas,$\gamma = 1.5$)
A
$27 \times 10^5 \ N/m^2$
B
$56 \times 10^5 \ N/m^2$
C
$36 \times 10^5 \ N/m^2$
D
$80 \times 10^5 \ N/m^2$

Solution

(A) Given: Initial pressure $P_1 = 1 \ atm = 1 \times 10^5 \ N/m^2$.
Initial temperature $T_1 = 27 + 273 = 300 \ K$.
Final temperature $T_2 = 627 + 273 = 900 \ K$.
Adiabatic index $\gamma = 1.5 = 3/2$.
For an adiabatic process,the relation between pressure and temperature is given by $P^{1-\gamma} T^{\gamma} = \text{constant}$.
Therefore,$P_1^{1-\gamma} T_1^{\gamma} = P_2^{1-\gamma} T_2^{\gamma}$.
Rearranging the terms,we get $(P_2/P_1)^{1-\gamma} = (T_1/T_2)^{\gamma}$,which implies $(P_2/P_1)^{\gamma-1} = (T_2/T_1)^{\gamma}$.
Substituting the values: $(P_2/P_1)^{1.5-1} = (900/300)^{1.5}$.
$(P_2/P_1)^{0.5} = (3)^{1.5}$.
Squaring both sides: $P_2/P_1 = (3)^{1.5 \times 2} = 3^3 = 27$.
Thus,$P_2 = 27 \times P_1 = 27 \times 10^5 \ N/m^2$.
73
MediumMCQ
An ideal gas undergoes an adiabatic process from state $(P_1, V_1, T_1)$ to $(P_2, V_2, T_2)$. The work done during the process is ..... (where $\mu$ = number of moles,$C_P$ and $C_V$ = molar specific heats).
A
$W = \mu(T_1 - T_2)C_P$
B
$W = \mu C_V(T_1 - T_2)$
C
$W = \mu(T_1 - T_2)R$
D
$W = \mu(T_1 + T_2)C_V$

Solution

(B) For an adiabatic process,the work done $W$ is given by the first law of thermodynamics: $Q = \Delta U + W$. Since the process is adiabatic,$Q = 0$,so $W = -\Delta U$.
The change in internal energy is $\Delta U = \mu C_V(T_2 - T_1)$.
Therefore,$W = -\mu C_V(T_2 - T_1) = \mu C_V(T_1 - T_2)$.
Alternatively,using the adiabatic work formula: $W = \frac{\mu R(T_1 - T_2)}{\gamma - 1}$.
Since $R = C_P - C_V$ and $\gamma = \frac{C_P}{C_V}$,we have $\gamma - 1 = \frac{C_P - C_V}{C_V} = \frac{R}{C_V}$.
Substituting this into the work formula: $W = \frac{\mu R(T_1 - T_2)}{R / C_V} = \mu C_V(T_1 - T_2)$.
74
MediumMCQ
For an adiabatic compression of $1 \text{ kmol}$ of gas,$146 \text{ kJ}$ of work is done. During this process,the temperature of the gas increases by $7 \text{ °C}$. The gas is ........
A
Monoatomic
B
Diatomic
C
Triatomic
D
$A$ mixture of monoatomic and diatomic

Solution

(B) According to the first law of thermodynamics,$\Delta Q = \Delta E_{int} + \Delta W$.
For an adiabatic process,$\Delta Q = 0$.
Therefore,$0 = \Delta E_{int} + \Delta W$,which implies $\Delta E_{int} = -\Delta W$.
The change in internal energy is given by $\Delta E_{int} = n C_V \Delta T$,where $n$ is the number of moles.
Given $n = 1 \text{ kmol} = 1000 \text{ mol}$,$\Delta W = -146 \text{ kJ} = -146 \times 10^3 \text{ J}$ (work done on the gas is negative in the system's perspective for expansion,but here compression is done,so $\Delta W = -146 \text{ kJ}$),and $\Delta T = 7 \text{ K}$.
$C_V = \frac{-\Delta W}{n \Delta T} = \frac{-(-146 \times 10^3)}{1000 \times 7} = \frac{146}{7} \approx 20.86 \text{ J mol}^{-1} \text{ K}^{-1}$.
For a diatomic gas,$C_V = \frac{5}{2} R = 2.5 \times 8.314 \approx 20.78 \text{ J mol}^{-1} \text{ K}^{-1}$.
Since the calculated $C_V$ matches the value for a diatomic gas,the gas is diatomic.
75
MediumMCQ
By what factor should the volume of an ideal gas $(\gamma = 1.5)$ be increased through an adiabatic expansion so that its $rms$ speed becomes half?
A
$4$
B
$16$
C
$8$
D
$2$

Solution

(B) The $rms$ speed of an ideal gas is given by $v_{rms} = \sqrt{\frac{3RT}{M}}$,which implies $v_{rms} \propto \sqrt{T}$.
Given that the $rms$ speed becomes half,we have $v_{rms,2} = \frac{1}{2} v_{rms,1}$,which implies $T_2 = \frac{T_1}{4}$.
For an adiabatic process,the relation between temperature and volume is $T V^{\gamma - 1} = \text{constant}$.
Therefore,$T_1 V_1^{\gamma - 1} = T_2 V_2^{\gamma - 1}$.
Rearranging gives $\left( \frac{V_2}{V_1} \right)^{\gamma - 1} = \frac{T_1}{T_2} = 4$.
Given $\gamma = 1.5 = \frac{3}{2}$,we have $\gamma - 1 = 0.5 = \frac{1}{2}$.
Substituting this,$\left( \frac{V_2}{V_1} \right)^{1/2} = 4$.
Squaring both sides,$\frac{V_2}{V_1} = 4^2 = 16$.
Thus,the volume must be increased by a factor of $16$.
76
DifficultMCQ
During an adiabatic process, the pressure is proportional to the cube of the temperature. Then ${C_p}/{C_v}$ is . . . . . . .
A
$1.5$
B
$1.33$
C
$2$
D
$1.67$

Solution

(A) For an adiabatic process, the relationship between pressure $P$ and temperature $T$ is given by $P^{1-\gamma} T^{\gamma} = \text{constant}$, which can be rewritten as $P \propto T^{\frac{\gamma}{\gamma-1}}$.
Given that $P \propto T^3$, we compare the exponents:
$\frac{\gamma}{\gamma-1} = 3$
$\gamma = 3(\gamma - 1)$
$\gamma = 3\gamma - 3$
$2\gamma = 3$
$\gamma = \frac{3}{2} = 1.5$.
Since $\frac{C_p}{C_v} = \gamma$, the value is $1.5$.
77
DifficultMCQ
$A$ monoatomic gas $(\gamma = 5/3)$ at a temperature of $27^{\circ}C$ is compressed adiabatically to $8/27$ of its initial volume. The change in temperature is . . . . . . $K$.
A
$450$
B
$375$
C
$225$
D
$405$

Solution

(B) For an adiabatic process,the relation between temperature and volume is given by $T V^{\gamma - 1} = \text{constant}$.
Initial temperature $T_1 = 27^{\circ}C = 27 + 273 = 300 \ K$.
Given volume change: $V_2 = \frac{8}{27} V_1$,so $\frac{V_1}{V_2} = \frac{27}{8}$.
Using the adiabatic relation: $T_2 = T_1 \left( \frac{V_1}{V_2} \right)^{\gamma - 1}$.
Substituting the values: $T_2 = 300 \left( \frac{27}{8} \right)^{\frac{5}{3} - 1} = 300 \left( \frac{27}{8} \right)^{2/3}$.
$T_2 = 300 \left( \left( \frac{3}{2} \right)^3 \right)^{2/3} = 300 \left( \frac{3}{2} \right)^2 = 300 \times \frac{9}{4} = 675 \ K$.
The change in temperature is $\Delta T = T_2 - T_1 = 675 - 300 = 375 \ K$.
78
DifficultMCQ
For an adiabatic process with $\gamma = 2.5$,if the volume of a gas is reduced to $1/8$ of its initial volume,what will be the new pressure $P'$ in terms of the initial pressure $P$?
A
$P' = P$
B
$P' = 2P$
C
$P' = P \times 2^{15/2}$
D
$P' = 7P$

Solution

(C) For an adiabatic process,the relation between pressure and volume is given by $PV^\gamma = \text{constant}$.
This implies $P_1 V_1^\gamma = P_2 V_2^\gamma$,or $\frac{P_2}{P_1} = \left( \frac{V_1}{V_2} \right)^\gamma$.
Given $\gamma = 2.5 = 5/2$ and the new volume $V_2 = \frac{V_1}{8}$,we have $\frac{V_1}{V_2} = 8$.
Substituting these values: $\frac{P'}{P} = (8)^{5/2}$.
Since $8 = 2^3$,we have $(2^3)^{5/2} = 2^{15/2}$.
Therefore,$P' = P \times 2^{15/2}$.
79
MediumMCQ
The $P-V$ diagram for two adiabatic processes is given. Curves $1$ and $2$ correspond to:
Question diagram
A
$He$ and $O_2$
B
$O_2$ and $He$
C
$He$ and $Ar$
D
$O_2$ and $N_2$

Solution

(B) For an adiabatic process, the equation is $PV^{\gamma} = \text{constant}$.
The slope of the adiabatic curve is given by $\frac{dP}{dV} = -\gamma \frac{P}{V}$.
Thus, the magnitude of the slope is directly proportional to the adiabatic index $\gamma$ (i.e., $|\text{slope}| \propto \gamma$).
For a monoatomic gas (like $He$), $\gamma = 1.66$, and for a diatomic gas (like $O_2$), $\gamma = 1.4$.
Since $\gamma_{monoatomic} > \gamma_{diatomic}$, the slope of the adiabatic curve for a monoatomic gas is steeper than that for a diatomic gas.
From the given $P-V$ graph, curve $2$ is steeper than curve $1$.
Therefore, curve $2$ corresponds to the monoatomic gas $(He)$ and curve $1$ corresponds to the diatomic gas $(O_2)$.
80
MediumMCQ
$A$ monoatomic gas is filled in a cylindrical container with a piston at temperature $T_1$. It undergoes an adiabatic expansion such that its temperature becomes $T_2$. If $L_1$ and $L_2$ are the lengths of the gas column before and after the expansion respectively,then $T_1/T_2$ is equal to:
A
$(\frac{L_1}{L_2})^{2/3}$
B
$\frac{L_1}{L_2}$
C
$\frac{L_2}{L_1}$
D
$(\frac{L_2}{L_1})^{2/3}$

Solution

(D) For an adiabatic process,the relationship between temperature $T$ and volume $V$ is given by $T V^{\gamma - 1} = \text{constant}$.
Thus,$T_1 V_1^{\gamma - 1} = T_2 V_2^{\gamma - 1}$,which implies $\frac{T_1}{T_2} = (\frac{V_2}{V_1})^{\gamma - 1}$.
Since the container is cylindrical with cross-sectional area $A$,the volume $V$ can be expressed as $V = L \times A$,where $L$ is the length of the gas column.
Therefore,$\frac{V_2}{V_1} = \frac{L_2 A}{L_1 A} = \frac{L_2}{L_1}$.
For a monoatomic gas,the adiabatic index $\gamma = 5/3$.
Substituting these values,we get $\frac{T_1}{T_2} = (\frac{L_2}{L_1})^{5/3 - 1} = (\frac{L_2}{L_1})^{2/3}$.
81
DifficultMCQ
$A$ gas has pressure $P$ and volume $V$. If the gas is compressed adiabatically to $\frac{1}{32}$ of its initial volume,what will be the new pressure? (Given: $(32)^{1.4} = 128$)
A
$32 P$
B
$128 P$
C
$\frac{P}{128}$
D
$\frac{P}{32}$

Solution

(B) For an adiabatic process,the relation between pressure and volume is given by $P_1 V_1^\gamma = P_2 V_2^\gamma$.
Here,$V_1 = V$,$V_2 = \frac{V}{32}$,and $\gamma = 1.4$.
Rearranging the formula to find the new pressure $P_2$:
$P_2 = P_1 \left( \frac{V_1}{V_2} \right)^\gamma$
Substituting the values:
$P_2 = P \left( \frac{V}{V/32} \right)^{1.4}$
$P_2 = P (32)^{1.4}$
Given that $(32)^{1.4} = 128$,we get:
$P_2 = 128 P$.
82
MediumMCQ
$A$ monatomic gas at pressure $P_1$ and volume $V_1$ is compressed adiabatically to $1/8$ of its original volume. What is the final pressure of the gas in terms of $P_1$?
A
$64$
B
$1$
C
$16$
D
$32$

Solution

(D) For an adiabatic process,the relation between pressure and volume is given by $PV^{\gamma} = \text{constant}$.
Thus,$P_1 V_1^{\gamma} = P_2 V_2^{\gamma}$.
For a monatomic gas,the adiabatic index $\gamma = 5/3$.
Given that the final volume $V_2 = V_1/8$.
Substituting these values into the equation:
$P_1 V_1^{5/3} = P_2 (V_1/8)^{5/3}$.
$P_2 = P_1 \times (V_1 / (V_1/8))^{5/3}$.
$P_2 = P_1 \times (8)^{5/3}$.
Since $8 = 2^3$,we have $(2^3)^{5/3} = 2^5 = 32$.
Therefore,$P_2 = 32 P_1$.
83
MediumMCQ
$A$ mass of diatomic gas $(\gamma = 1.4)$ at a pressure of $2 \text{ atm}$ is compressed adiabatically so that its temperature rises from $27^{\circ}C$ to $927^{\circ}C$. The pressure of the gas in the final state is ...... $\text{atm}$.
A
$8$
B
$28$
C
$68.7$
D
$256$

Solution

(D) For an adiabatic process,the relationship between temperature and pressure is given by:
$\frac{T^\gamma}{P^{\gamma-1}} = \text{constant}$
Therefore,$\left(\frac{T_i}{T_f}\right)^\gamma = \left(\frac{P_i}{P_f}\right)^{\gamma-1}$,which implies $P_f = P_i \left(\frac{T_f}{T_i}\right)^{\frac{\gamma}{\gamma-1}}$ ...$(i)$
Given:
$T_i = 27^{\circ}C = 300 \text{ K}$
$T_f = 927^{\circ}C = 1200 \text{ K}$
$P_i = 2 \text{ atm}$
$\gamma = 1.4$
Substituting these values into equation $(i)$:
$P_f = 2 \times \left(\frac{1200}{300}\right)^{\frac{1.4}{1.4-1}}$
$P_f = 2 \times (4)^{\frac{1.4}{0.4}}$
$P_f = 2 \times (4)^{3.5}$
$P_f = 2 \times (2^2)^{3.5} = 2 \times 2^7 = 2^8 = 256 \text{ atm}$.
84
MediumMCQ
During an adiabatic process,the pressure of a gas is found to be proportional to the cube of its temperature. The ratio of $\frac{C_P}{C_V}$ for the gas is
A
$2$
B
$1.67$
C
$1.5$
D
$1.33$

Solution

(C) For an adiabatic process,the relationship between pressure $P$ and temperature $T$ is given by $P^{1-\gamma} T^{\gamma} = \text{constant}$,which can be rewritten as $P T^{\frac{\gamma}{1-\gamma}} = \text{constant}$.
Given that $P \propto T^3$,we have $P T^{-3} = \text{constant}$.
Comparing the exponents of $T$,we get $\frac{\gamma}{1-\gamma} = -3$.
Solving for $\gamma$:
$\gamma = -3(1-\gamma)$
$\gamma = -3 + 3\gamma$
$2\gamma = 3$
$\gamma = \frac{3}{2} = 1.5$.
Since $\gamma = \frac{C_P}{C_V}$,the ratio is $1.5$.
85
EasyMCQ
An ideal gas is compressed to half its initial volume by means of several processes. Which of the processes results in the maximum work done on the gas?
A
Isochoric
B
Isothermal
C
Adiabatic
D
Isobaric

Solution

(C) The $P-V$ diagram of an ideal gas compressed from its initial volume $V_0$ to $\frac{V_0}{2}$ by several processes is shown in the figure.
Work done on the gas is equal to the area under the $P-V$ curve.
As the area under the $P-V$ curve is maximum for the adiabatic process,the work done on the gas is maximum for the adiabatic process.
Solution diagram
86
MediumMCQ
$A$ certain mass of gas at $273 \ K$ is expanded to $81$ times its volume under adiabatic condition. If $\gamma = 1.25$ for the gas, then its final temperature is ..... $^\circ C$
A
$-235$
B
$-182$
C
$-91$
D
$0$

Solution

(B) For an adiabatic process, the relationship between temperature and volume is given by $T_1 V_1^{\gamma - 1} = T_2 V_2^{\gamma - 1}$.
Given: Initial temperature $T_1 = 273 \ K$, final volume $V_2 = 81 V_1$, and adiabatic index $\gamma = 1.25$.
Rearranging the formula for the final temperature $T_2$:
$T_2 = T_1 \left( \frac{V_1}{V_2} \right)^{\gamma - 1}$
Substituting the values:
$T_2 = 273 \times \left( \frac{V_1}{81 V_1} \right)^{1.25 - 1}$
$T_2 = 273 \times \left( \frac{1}{81} \right)^{0.25}$
Since $0.25 = \frac{1}{4}$, we have:
$T_2 = 273 \times \left( \frac{1}{81} \right)^{1/4}$
$T_2 = 273 \times \frac{1}{(3^4)^{1/4}} = \frac{273}{3} = 91 \ K$
To convert the temperature from Kelvin to Celsius:
$T(^\circ C) = T(K) - 273$
$T(^\circ C) = 91 - 273 = -182 ^\circ C$.
87
EasyMCQ
In an adiabatic process $90 \ J$ of work is done on the gas. The change in internal energy of the gas is ....... $J$.
A
$-90$
B
$+90$
C
$0$
D
Depends on initial temperature

Solution

(B) For an adiabatic process,the heat exchange $\Delta Q$ is equal to $0$.
According to the first law of thermodynamics,$\Delta Q = \Delta U + \Delta W$.
Since work is done on the gas,the work done $\Delta W = -90 \ J$.
Substituting these values into the equation: $0 = \Delta U + (-90 \ J)$.
Therefore,the change in internal energy is $\Delta U = +90 \ J$.
88
MediumMCQ
$A$ gas is compressed adiabatically until its temperature is doubled. The ratio of its final volume to initial volume will be
A
$1/2$
B
More than $1/2$
C
Less than $1/2$
D
Between $1$ and $2$

Solution

(C) For an adiabatic process, the relationship between temperature $T$ and volume $V$ is given by $T V^{\gamma - 1} = \text{constant}$.
Thus, $T_1 V_1^{\gamma - 1} = T_2 V_2^{\gamma - 1}$.
Given that the temperature is doubled, $T_2 = 2 T_1$, so $\frac{T_2}{T_1} = 2$.
Substituting this into the relation: $\frac{T_2}{T_1} = \left( \frac{V_1}{V_2} \right)^{\gamma - 1} = 2$.
Taking the reciprocal: $\left( \frac{V_2}{V_1} \right)^{\gamma - 1} = \frac{1}{2}$.
Therefore, the ratio of final volume to initial volume is $\frac{V_2}{V_1} = \left( \frac{1}{2} \right)^{\frac{1}{\gamma - 1}}$.
Since for any gas $\gamma > 1$, the exponent $\frac{1}{\gamma - 1} > 1$.
Raising $1/2$ to a power greater than $1$ results in a value less than $1/2$.
Hence, $\frac{V_2}{V_1} < \frac{1}{2}$.
89
MediumMCQ
$A$ tyre filled with air ($27^\circ C$ and $2 \text{ atm}$) bursts. What is the final temperature of the air in $^\circ C$? (Given: $\gamma = 1.5$)
A
$-33$
B
$0$
C
$27$
D
$240$

Solution

(A) When a tyre bursts,the air undergoes an adiabatic expansion. The relationship between temperature and pressure for an adiabatic process is given by: $\frac{T_2}{T_1} = \left( \frac{P_2}{P_1} \right)^{\frac{\gamma - 1}{\gamma}}$.
Given: $T_1 = 27^\circ C = 300 \text{ K}$,$P_1 = 2 \text{ atm}$,$P_2 = 1 \text{ atm}$ (atmospheric pressure),and $\gamma = 1.5$.
Substituting the values: $\frac{T_2}{300} = \left( \frac{1}{2} \right)^{\frac{1.5 - 1}{1.5}} = \left( \frac{1}{2} \right)^{\frac{0.5}{1.5}} = \left( \frac{1}{2} \right)^{\frac{1}{3}}$.
Since $2^{1/3} \approx 1.26$,we have $\frac{T_2}{300} = \frac{1}{1.26} \approx 0.793$.
$T_2 = 300 \times 0.793 \approx 238 \text{ K}$.
Converting to Celsius: $T_2(^\circ C) = 238 - 273 = -35^\circ C$. The closest option is $-33^\circ C$.
90
MediumMCQ
$A$ gas expands adiabatically at constant pressure such that its temperature $T \propto \frac{1}{\sqrt{V}}$. The value of $C_P/C_V$ for the gas is:
A
$1.3$
B
$1.5$
C
$1.67$
D
$2$

Solution

(B) For an adiabatic process,the relationship between temperature $T$ and volume $V$ is given by $T V^{\gamma - 1} = \text{constant}$,which implies $T \propto V^{1 - \gamma}$.
According to the problem,the gas follows the relation $T \propto V^{-1/2}$.
Comparing the exponents of $V$,we get $1 - \gamma = -1/2$.
Solving for $\gamma$,we have $\gamma = 1 + 1/2 = 3/2 = 1.5$.
Since $\gamma = C_P/C_V$,the value is $1.5$.
91
MediumMCQ
In an adiabatic change,the pressure $P$ and temperature $T$ of a monoatomic gas are related by the relation $P \propto T^C$,where $C$ equals
A
$5/3$
B
$2/5$
C
$3/5$
D
$5/2$

Solution

(D) For an adiabatic process,the relationship between pressure $P$ and temperature $T$ is given by $T^\gamma P^{1-\gamma} = \text{constant}$.
Rearranging this,we get $P^{1-\gamma} \propto T^{-\gamma}$,which implies $P \propto T^{-\frac{\gamma}{1-\gamma}}$ or $P \propto T^{\frac{\gamma}{\gamma-1}}$.
Comparing this with the given relation $P \propto T^C$,we find $C = \frac{\gamma}{\gamma-1}$.
For a monoatomic gas,the adiabatic index $\gamma = 5/3$.
Substituting the value of $\gamma$:
$C = \frac{5/3}{5/3 - 1} = \frac{5/3}{2/3} = 5/2$.
92
DifficultMCQ
$A$ reversible adiabatic path on a $P-V$ diagram for an ideal gas passes through state $A$ where $P = 0.7 \times 10^5 \, N/m^2$ and $V = 0.0049 \, m^3$. The ratio of specific heats of the gas is $1.4$. The slope of the path at $A$ is:
A
$2.0 \times 10^7 \, N/m^5$
B
$1.0 \times 10^7 \, N/m^5$
C
$-2.0 \times 10^7 \, N/m^5$
D
$-1.0 \times 10^7 \, N/m^5$

Solution

(C) For a reversible adiabatic process,the relation between pressure $P$ and volume $V$ is given by $P V^{\gamma} = \text{constant}$.
Differentiating both sides with respect to $V$,we get:
$V^{\gamma} \frac{dP}{dV} + P \cdot \gamma V^{\gamma-1} = 0$
Rearranging for the slope $\frac{dP}{dV}$:
$\frac{dP}{dV} = -\frac{\gamma P}{V}$
Given values are $P = 0.7 \times 10^5 \, N/m^2$,$V = 0.0049 \, m^3$,and $\gamma = 1.4$.
Substituting these values into the slope formula:
$\text{Slope} = -\frac{1.4 \times (0.7 \times 10^5)}{0.0049}$
$\text{Slope} = -\frac{0.98 \times 10^5}{0.0049}$
$\text{Slope} = -\frac{98000}{0.0049} = -2.0 \times 10^7 \, N/m^5$.
93
DifficultMCQ
An ideal gas undergoes an adiabatic process obeying the relation $PV^{4/3} = \text{constant}$. If its initial temperature is $300 \ K$ and its pressure is increased up to four times its initial value, then the final temperature is (in Kelvin):
A
$300\sqrt{2}$
B
$300\sqrt[3]{2}$
C
$600$
D
$1200$

Solution

(A) Given: Initial temperature $T_i = 300 \ K$, Initial pressure $P_i = P$, Final pressure $P_f = 4P$, and the process relation $PV^{4/3} = \text{constant}$.
For an ideal gas, $V = \frac{nRT}{P}$.
Substituting $V$ into the given relation: $P \left( \frac{nRT}{P} \right)^{4/3} = \text{constant}$.
This simplifies to $P \cdot P^{-4/3} \cdot T^{4/3} = \text{constant}$, which gives $T^{4/3} \cdot P^{-1/3} = \text{constant}$.
Thus, $\frac{T_f^{4/3}}{P_f^{1/3}} = \frac{T_i^{4/3}}{P_i^{1/3}}$.
Rearranging for $T_f$: $T_f = T_i \left( \frac{P_f}{P_i} \right)^{(1/3) / (4/3)} = T_i \left( \frac{P_f}{P_i} \right)^{1/4}$.
Substituting the values: $T_f = 300 \times \left( \frac{4P}{P} \right)^{1/4} = 300 \times (4)^{1/4} = 300 \times (2^2)^{1/4} = 300 \times 2^{1/2} = 300\sqrt{2} \ K$.
94
MediumMCQ
The work of $146 \ kJ$ is performed in order to compress one kilo mole of gas adiabatically,and in this process,the temperature of the gas increases by $7 ^\circ C$. The gas is $(R = 8.3 \ J \ mol^{-1} K^{-1})$.
A
monoatomic
B
diatomic
C
triatomic
D
a mixture of monoatomic and diatomic

Solution

(B) For an adiabatic process,the work done $W$ on the gas is given by $W = \frac{nR\Delta T}{1 - \gamma}$.
Here,$n = 1 \text{ kilo mole} = 1000 \text{ moles}$,$R = 8.3 \ J \ mol^{-1} K^{-1}$,$\Delta T = 7 \ K$,and $W = -146 \ kJ = -146000 \ J$ (work is done on the gas).
Substituting the values: $-146000 = \frac{1000 \times 8.3 \times 7}{1 - \gamma}$.
$1 - \gamma = -\frac{58100}{146000} \approx -0.3979 \approx -0.4$.
$1 - \gamma = -0.4 \Rightarrow \gamma = 1.4$.
Since $\gamma = 1.4$ for a diatomic gas,the gas is diatomic.
95
DifficultMCQ
Consider a spherical shell of radius $R$ at temperature $T$. The black body radiation inside it can be considered as an ideal gas of photons with internal energy per unit volume $E = \frac{U}{V} \propto T^4$ and pressure $P = \frac{1}{3} \left( \frac{U}{V} \right)$. If the shell now undergoes an adiabatic expansion,the relation between $T$ and $R$ is:
A
$T \propto e^{-3R}$
B
$T \propto \frac{1}{R}$
C
$T \propto \frac{1}{R^3}$
D
$T \propto e^{-R}$

Solution

(B) For an adiabatic process,the first law of thermodynamics is $dQ = dU + PdV = 0$,so $dU = -PdV$.
Given $U = V \cdot E = V \cdot (aT^4)$,where $a$ is a constant.
Then $dU = d(aVT^4) = a(T^4 dV + 4VT^3 dT)$.
Substituting into $dU = -PdV$ with $P = \frac{1}{3} aT^4$:
$aT^4 dV + 4aVT^3 dT = -\frac{1}{3} aT^4 dV$.
Rearranging terms: $4aVT^3 dT = -\frac{4}{3} aT^4 dV$.
Dividing by $4aVT^3$: $\frac{dT}{T} = -\frac{1}{3} \frac{dV}{V}$.
Integrating both sides: $\ln T = -\frac{1}{3} \ln V + C$,which implies $T \propto V^{-1/3}$.
Since $V = \frac{4}{3} \pi R^3$,we have $V \propto R^3$.
Therefore,$T \propto (R^3)^{-1/3} = \frac{1}{R}$.

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