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Adiabatic Process Questions in English

Class 11 Physics · Thermodynamics · Adiabatic Process

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251
EasyMCQ
In a thermodynamic process,there is no exchange of heat between the system and surroundings. Then the thermodynamic process is
A
isothermal
B
isobaric
C
isochoric
D
adiabatic

Solution

(D) thermodynamic process in which there is no exchange of heat between the system and the surroundings is defined as an adiabatic process.
In this process,the heat exchange $dQ = 0$.
According to the first law of thermodynamics,$dQ = dU + dW$,so for an adiabatic process,$dU = -dW$.
252
EasyMCQ
In an adiabatic expansion of an ideal gas,the product of pressure and volume:
A
decreases
B
increases
C
remains constant
D
at first increases and then decreases

Solution

(A) For an adiabatic process,the system does work on the surroundings at the expense of its internal energy.
Since the internal energy of an ideal gas depends only on its temperature $(U = n C_v T)$,a decrease in internal energy implies a decrease in temperature $(T)$.
According to the ideal gas equation,$p V = n R T$.
Since $n$ and $R$ are constants and the temperature $T$ decreases during adiabatic expansion,the product $p V$ must also decrease.
253
EasyMCQ
$A$ graph of pressure versus volume for an ideal gas for different processes is as shown. In the graph,curve $OC$ represents:
Question diagram
A
isochoric process
B
isothermal process
C
isobaric process
D
adiabatic process

Solution

(D) $(i)$ Curve $OA$ represents an isobaric process (since pressure is constant).
(ii) Curve $OB$ represents an isothermal process.
(iii) Curve $OC$ represents an adiabatic process,as the slope of an adiabatic process is steeper than that of an isothermal process.
(iv) Curve $OD$ represents an isochoric process (since volume is constant).
Solution diagram
254
MediumMCQ
During an adiabatic process, the cube of the pressure is found to be inversely proportional to the fourth power of the volume. Then the ratio of specific heats is
A
$1$
B
$1.33$
C
$1.67$
D
$1.4$

Solution

(B) The equation for an adiabatic process is given by $P V^{\gamma} = \text{constant}$.
Given that the cube of the pressure is inversely proportional to the fourth power of the volume, we have $P^3 \propto V^{-4}$, which implies $P^3 V^4 = \text{constant}$.
Taking the cube root of both sides, we get $(P^3 V^4)^{1/3} = \text{constant}^{1/3}$, which simplifies to $P V^{4/3} = \text{constant}$.
Comparing this with the standard adiabatic equation $P V^{\gamma} = \text{constant}$, we identify $\gamma = 4/3$.
Thus, the ratio of specific heats is $\gamma = 1.33$.
255
MediumMCQ
$A$ cycle tyre bursts suddenly. What is the type of this process?
A
Isothermal
B
Adiabatic
C
Isochoric
D
Isobaric

Solution

(B) When a cycle tyre bursts suddenly,the process is adiabatic.
This is because the expansion of air occurs very rapidly,leaving no time for heat exchange between the system (the air inside the tyre) and the surroundings.
Since $dQ = 0$,the process satisfies the condition for an adiabatic process.
256
EasyMCQ
For $1 \text{ mole}$ of an ideal gas,during an adiabatic process,the square of the pressure of a gas is found to be proportional to the cube of its absolute temperature. The specific heat of the gas at constant volume is ($R$ is universal gas constant)
A
$3 R$
B
$\frac{R}{2}$
C
$\frac{R}{3}$
D
$2 R$

Solution

(B) For an adiabatic process,the relation between pressure $P$ and temperature $T$ is given by $P^{1-\gamma} T^{\gamma} = \text{constant}$.
Given that $P^2 \propto T^3$,we can write $P \propto T^{3/2}$,which implies $P T^{-3/2} = \text{constant}$.
Comparing $P T^{-3/2} = \text{constant}$ with $P^{1-\gamma} T^{\gamma} = \text{constant}$,we rewrite the given relation as $P T^{-3/2} = \text{constant}$,or $P^{1} T^{-3/2} = \text{constant}$.
Raising to the power of $\frac{1}{1-\gamma}$,we get $P T^{\frac{-3/2}{1-\gamma}} = \text{constant}$.
Comparing the exponents of $T$,we have $\frac{\gamma}{1-\gamma} = -\frac{3}{2}$.
$2\gamma = -3(1-\gamma) \implies 2\gamma = -3 + 3\gamma \implies \gamma = 3$.
For an ideal gas,the molar specific heat at constant volume is $C_V = \frac{R}{\gamma - 1}$.
Substituting $\gamma = 3$,we get $C_V = \frac{R}{3-1} = \frac{R}{2}$.
257
EasyMCQ
Assertion $(A)$: When an ideal gas is compressed adiabatically,its temperature and the average kinetic energy of the gas molecules increase.
Reason $(R)$: The kinetic energy increases because of collisions of molecules with the moving parts of the wall.
A
$A$ and $R$ are true and $R$ is the correct explanation of $A$.
B
$A$ and $R$ are true but $R$ is not the correct explanation of $A$.
C
$A$ is true and $R$ is false.
D
$A$ is false and $R$ is true.

Solution

(A) In an adiabatic compression,work is done on the gas,which increases its internal energy. Since the internal energy of an ideal gas is directly proportional to its absolute temperature $(U \propto T)$,the temperature of the gas increases.
According to the kinetic theory of gases,the average kinetic energy of gas molecules is given by $K_{avg} = \frac{3}{2} k_B T$. As the temperature $T$ increases,the average kinetic energy also increases.
This increase in kinetic energy occurs because,during compression,the molecules collide with the moving piston (wall). When a molecule hits a wall moving towards it,the molecule gains speed (and thus kinetic energy) due to the elastic collision,similar to a ball bouncing off a moving bat. Therefore,both the assertion and the reason are true,and the reason correctly explains the assertion.
258
EasyMCQ
Specific heat of a gas undergoing adiabatic change is
A
zero
B
infinite
C
positive
D
negative

Solution

(A) The molar specific heat capacity $C$ is defined as the amount of heat required to raise the temperature of $1 \text{ mole}$ of a gas by $1 \text{ K}$.
Mathematically,$C = \frac{\Delta Q}{n \Delta T}$.
In an adiabatic process,the system is thermally insulated from its surroundings,meaning no heat is exchanged with the environment.
Therefore,the heat change $\Delta Q = 0$.
Substituting this into the formula,we get $C = \frac{0}{n \Delta T} = 0$.
Thus,the specific heat of a gas undergoing an adiabatic change is $0$.
259
MediumMCQ
If a gas of volume $400 \ cc$ at an initial pressure $P$ is suddenly compressed to $100 \ cc$,then its final pressure is (The ratio of the specific heat capacities of the gas at constant pressure and constant volume is $1.5$).
A
$P/32$
B
$8P$
C
$32P$
D
$16P$

Solution

(B) sudden compression of a gas is an adiabatic process.
For an adiabatic process,the relation between pressure $P$ and volume $V$ is given by $P_1 V_1^\gamma = P_2 V_2^\gamma$,where $\gamma$ is the adiabatic index (ratio of specific heat capacities).
Given:
Initial pressure $P_1 = P$
Initial volume $V_1 = 400 \ cc$
Final volume $V_2 = 100 \ cc$
Adiabatic index $\gamma = 1.5 = 3/2$
Substituting the values into the adiabatic equation:
$P \times (400)^{3/2} = P_2 \times (100)^{3/2}$
$P_2 = P \times (400/100)^{3/2}$
$P_2 = P \times (4)^{3/2}$
$P_2 = P \times (2^2)^{3/2}$
$P_2 = P \times 2^3$
$P_2 = 8P$
Therefore,the final pressure is $8P$.
260
MediumMCQ
During adiabatic expansion,if the temperature of $3$ moles of a diatomic gas decreases by $50^{\circ} C$,then the work done by the gas is (where $R$ is the Universal gas constant). (in $R$)
A
$375$
B
$750$
C
$1500$
D
$825$

Solution

(A) For an adiabatic process,the first law of thermodynamics states that $Q = \Delta U + W$. Since the process is adiabatic,$Q = 0$,so $W = -\Delta U$.
The change in internal energy is given by $\Delta U = n C_v \Delta T$.
For a diatomic gas,the molar heat capacity at constant volume is $C_v = \frac{5}{2} R$.
Given: $n = 3$ moles,$\Delta T = -50^{\circ} C$ (since temperature decreases).
Therefore,$\Delta U = 3 \times (\frac{5}{2} R) \times (-50) = 3 \times 2.5 R \times (-50) = -375 R$.
The work done by the gas is $W = -\Delta U = -(-375 R) = 375 R$.
261
MediumMCQ
If the given graph shows the logarithmic values of pressure $(P)$ and volume $(V)$ of an ideal gas, then the ratio of the specific heat capacities of the gas is
Question diagram
A
$1.5$
B
$1.2$
C
$1.4$
D
$1.3$

Solution

(C) For an adiabatic process, the relation between pressure $(P)$ and volume $(V)$ is given by $PV^{\gamma} = \text{constant}$.
Taking the logarithm on both sides, we get $\log P + \gamma \log V = \text{constant}$, which can be written as $\log P = -\gamma \log V + C$.
This is the equation of a straight line $y = mx + c$, where the slope $m = -\gamma$.
From the graph, the slope is calculated as:
$m = \frac{\log P_2 - \log P_1}{\log V_2 - \log V_1} = \frac{2.20 - 2.48}{1.4 - 1.2} = \frac{-0.28}{0.2} = -1.4$.
Since $m = -\gamma$, we have $-\gamma = -1.4$, which implies $\gamma = 1.4$.
Therefore, the ratio of the specific heat capacities is $1.4$.
262
MediumMCQ
When an ideal diatomic gas undergoes adiabatic expansion,if the increase in its volume is $0.5 \%$,then the change in the pressure of the gas is
A
$+0.5 \%$
B
$-0.5 \%$
C
$-0.7 \%$
D
$+0.7 \%$

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: $\ln P + \gamma \ln V = \text{constant}$.
Differentiating both sides: $\frac{dP}{P} + \gamma \frac{dV}{V} = 0$.
Therefore,the fractional change in pressure is $\frac{dP}{P} = -\gamma \frac{dV}{V}$.
For a diatomic gas,the adiabatic exponent $\gamma = \frac{7}{5} = 1.4$.
Given the increase in volume $\frac{dV}{V} = 0.5 \% = 0.005$.
Substituting the values: $\frac{dP}{P} = -1.4 \times 0.5 \% = -0.7 \%$.
Thus,the change in pressure is $-0.7 \%$.
263
MediumMCQ
If $5.6$ litres of a monoatomic gas at $STP$ is adiabatically compressed to $0.7$ litres,then the work done on the gas is nearly ($R$ = Universal gas constant). (in $R$)
A
$307$
B
$357$
C
$367$
D
$407$

Solution

(A) For a monoatomic gas,the adiabatic index $\gamma = 5/3$. At $STP$,$1$ mole of gas occupies $22.4$ litres. Therefore,the number of moles $n = 5.6 / 22.4 = 0.25$ moles.
For an adiabatic process,$T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$.
Given $V_1 = 5.6$ $L$,$V_2 = 0.7$ $L$,and $T_1 = 273$ $K$.
$T_2 = T_1 (V_1/V_2)^{\gamma-1} = 273 \times (5.6/0.7)^{5/3-1} = 273 \times (8)^{2/3} = 273 \times 4 = 1092$ $K$.
The work done on the gas in an adiabatic process is $W = -\Delta U = -n C_v (T_2 - T_1)$.
For a monoatomic gas,$C_v = 3R/2$.
$W = -0.25 \times (3R/2) \times (1092 - 273) = -0.375 R \times 819 = -307.125 R$.
The work done on the gas is the magnitude,which is approximately $307 R$.
264
MediumMCQ
$A$ monatomic gas at a pressure of $100 \text{ kPa}$ expands adiabatically such that its final volume becomes $8$ times its initial volume. If the work done during the process is $180 \text{ J}$, then the initial volume of the gas is (in $\text{ cm}^3$)
A
$1600$
B
$800$
C
$1200$
D
$2000$

Solution

(A) For an adiabatic process, the work done $W$ is given by the formula: $W = \frac{P_i V_i - P_f V_f}{\gamma - 1}$.
Since $P_i V_i^\gamma = P_f V_f^\gamma$, we have $P_f = P_i (V_i / V_f)^\gamma$.
Given $V_f = 8 V_i$ and for a monatomic gas $\gamma = 5/3$, we get $P_f = P_i (1/8)^{5/3} = P_i / 32$.
Substituting these into the work formula: $W = \frac{P_i V_i - (P_i / 32)(8 V_i)}{5/3 - 1} = \frac{P_i V_i - P_i V_i / 4}{2/3} = \frac{(3/4) P_i V_i}{2/3} = \frac{9}{8} P_i V_i$.
Given $W = 180 \text{ J}$ and $P_i = 100 \times 10^3 \text{ Pa}$, we have $180 = \frac{9}{8} \times 10^5 \times V_i$.
$V_i = \frac{180 \times 8}{9 \times 10^5} = \frac{160}{10^5} = 1.6 \times 10^{-3} \text{ m}^3$.
Converting to $\text{cm}^3$: $1.6 \times 10^{-3} \times 10^6 \text{ cm}^3 = 1600 \text{ cm}^3$.
265
MediumMCQ
$A$ certain volume of a gas at $300 \ K$ expands adiabatically until its volume is doubled. The resultant fall in temperature of the gas is nearly (The ratio of the specific heats of the gas $\gamma = 1.5$) (in $K$)
A
$88$
B
$77$
C
$67$
D
$54$

Solution

(A) For an adiabatic process,the relationship between temperature $T$ and volume $V$ is given by $TV^{\gamma-1} = \text{constant}$.
Given: $T_1 = 300 \ K$,$V_2 = 2V_1$,and $\gamma = 1.5$.
Using the adiabatic relation: $T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$.
Substituting the values: $300 \times (V_1)^{1.5-1} = T_2 \times (2V_1)^{1.5-1}$.
$300 \times (V_1)^{0.5} = T_2 \times (2)^{0.5} \times (V_1)^{0.5}$.
$T_2 = \frac{300}{\sqrt{2}} = \frac{300}{1.414} \approx 212.13 \ K$.
The fall in temperature is $\Delta T = T_1 - T_2 = 300 - 212.13 = 87.87 \ K$.
Rounding to the nearest integer,the fall in temperature is approximately $88 \ K$.
266
MediumMCQ
Initially,the pressure of $1 \text{ mole}$ of an ideal gas is $10^5 \text{ Nm}^{-2}$ and its volume is $16 \text{ litres}$. When it is adiabatically compressed,its final volume is $2 \text{ litres}$. Calculate the work done on the gas. [Given: Molar specific heat at constant volume $C_v = \frac{3R}{2}$] (in $\text{ kJ}$)
A
$72$
B
$7.2$
C
$720$
D
$360$

Solution

(B) Given: $P_1 = 10^5 \text{ Nm}^{-2}$,$V_1 = 16 \text{ L} = 16 \times 10^{-3} \text{ m}^3$,$V_2 = 2 \text{ L} = 2 \times 10^{-3} \text{ m}^3$,$C_v = \frac{3R}{2}$.
For an ideal gas,$C_p = C_v + R = \frac{3R}{2} + R = \frac{5R}{2}$.
The adiabatic index $\gamma = \frac{C_p}{C_v} = \frac{5/2 R}{3/2 R} = \frac{5}{3}$.
For an adiabatic process,$P_1 V_1^\gamma = P_2 V_2^\gamma$.
$P_2 = P_1 \left( \frac{V_1}{V_2} \right)^\gamma = 10^5 \left( \frac{16}{2} \right)^{5/3} = 10^5 \times (8)^{5/3} = 10^5 \times (2^3)^{5/3} = 10^5 \times 2^5 = 32 \times 10^5 \text{ Nm}^{-2}$.
Work done by the gas $W = \frac{P_1 V_1 - P_2 V_2}{\gamma - 1}$.
$W = \frac{(10^5 \times 16 \times 10^{-3}) - (32 \times 10^5 \times 2 \times 10^{-3})}{5/3 - 1} = \frac{1600 - 6400}{2/3} = \frac{-4800}{2/3} = -4800 \times \frac{3}{2} = -7200 \text{ J} = -7.2 \text{ kJ}$.
Since the work done by the gas is $-7.2 \text{ kJ}$,the work done on the gas is $7.2 \text{ kJ}$.
267
MediumMCQ
When $2 \text{ moles}$ of a monatomic gas expands adiabatically from a temperature of $80^{\circ} C$ to $50^{\circ} C$,the work done is $W$. The work done when $3 \text{ moles}$ of a diatomic gas expands adiabatically from $50^{\circ} C$ to $20^{\circ} C$ is: (in $W$)
A
$7$
B
$5$
C
$2.5$
D
$3.5$

Solution

(C) For an adiabatic process,the work done is given by $W = -\Delta U = -\frac{f}{2} nR \Delta T = \frac{f}{2} nR (T_i - T_f)$.
For the monatomic gas: $n_1 = 2$,$f_1 = 3$,$T_{i1} = 80^{\circ} C$,$T_{f1} = 50^{\circ} C$.
$W = \frac{3}{2} \times 2 \times R \times (80 - 50) = 3R \times 30 = 90R$.
For the diatomic gas: $n_2 = 3$,$f_2 = 5$,$T_{i2} = 50^{\circ} C$,$T_{f2} = 20^{\circ} C$.
$W' = \frac{5}{2} \times 3 \times R \times (50 - 20) = \frac{15}{2} R \times 30 = 15R \times 15 = 225R$.
Now,find the ratio: $\frac{W'}{W} = \frac{225R}{90R} = \frac{225}{90} = 2.5$.
Therefore,$W' = 2.5 W$.
268
EasyMCQ
$A$ monatomic gas at $630 \,K$ expands adiabatically to $27$ times its initial volume. The final temperature of the gas is (in $\,K$)
A
$30$
B
$130$
C
$170$
D
$70$

Solution

(D) For a monatomic gas, the degrees of freedom $f=3$.
The adiabatic index $\gamma = 1 + \frac{2}{f} = 1 + \frac{2}{3} = \frac{5}{3}$.
For an adiabatic process, the relationship between temperature $T$ and volume $V$ is given by $T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$.
Given $T_1 = 630 \,K$, $V_1 = V$, and $V_2 = 27V$.
Substituting these values:
$630 \times V^{\frac{5}{3}-1} = T_2 \times (27V)^{\frac{5}{3}-1}$
$630 = T_2 \times (27)^{\frac{2}{3}}$
$630 = T_2 \times (3^3)^{\frac{2}{3}}$
$630 = T_2 \times 3^2$
$630 = T_2 \times 9$
$T_2 = \frac{630}{9} = 70 \,K$.
269
EasyMCQ
$A$ work of $166.28 \ J$ is done to adiabatically compress one mole of a gas. If the increase in the temperature of the gas is $8^{\circ} C$,the gas is $\left(R=8.314 \ J \ mol^{-1} \ K^{-1}\right)$
A
monatomic
B
diatomic
C
polyatomic
D
mixture of diatomic and polyatomic

Solution

(B) Work done on the system,$W = 166.28 \ J$.
Increase in temperature,$\Delta T = 8^{\circ} C = 8 \ K$.
For an adiabatic process,the work done on the gas is given by $W = \frac{nR\Delta T}{\gamma - 1}$.
Given $n = 1 \ mol$ and $R = 8.314 \ J \ mol^{-1} \ K^{-1}$.
Substituting the values: $166.28 = \frac{1 \times 8.314 \times 8}{\gamma - 1}$.
$\gamma - 1 = \frac{66.512}{166.28} = 0.4$.
$\gamma = 1.4$.
Since the adiabatic index $\gamma = 1.4$ corresponds to a diatomic gas,the gas is diatomic.
270
EasyMCQ
The relation between absolute temperature $(T)$ and pressure $(P)$ of a gas in an adiabatic process is
A
$P^{\gamma} T^{1-\gamma} = \text{constant}$
B
$P^{1-\gamma} T^{\gamma} = \text{constant}$
C
$P^{\gamma-1} T^{\gamma} = \text{constant}$
D
$P^{\gamma} T^{\gamma-1} = \text{constant}$

Solution

(B) For an adiabatic process,the relation between pressure $(P)$ and volume $(V)$ is given by $PV^{\gamma} = \text{constant}$.
From the ideal gas equation,we know that $PV = nRT$,which implies $V = \frac{nRT}{P}$.
Substituting the expression for $V$ into the adiabatic equation:
$P \left( \frac{nRT}{P} \right)^{\gamma} = \text{constant}$.
Since $n$ and $R$ are constants,we can write:
$P \cdot \frac{T^{\gamma}}{P^{\gamma}} = \text{constant}$.
$P^{1-\gamma} T^{\gamma} = \text{constant}$.
271
EasyMCQ
In a thermodynamic process,if $\Delta W$ and $\Delta U$ are the work done and the change in the internal energy of a system respectively,then:
A
$\Delta U = \Delta W$ in an adiabatic process
B
$\Delta U = \Delta W$ in an isothermal process
C
$\Delta U = -\Delta W$ in an isothermal process
D
$\Delta U = -\Delta W$ in an adiabatic process

Solution

(D) According to the first law of thermodynamics,the heat supplied to a system $(\Delta Q)$ is equal to the sum of the change in internal energy $(\Delta U)$ and the work done by the system $(\Delta W)$:
$\Delta Q = \Delta U + \Delta W$
In an adiabatic process,there is no exchange of heat between the system and the surroundings,so $\Delta Q = 0$.
Substituting this into the first law equation:
$0 = \Delta U + \Delta W$
Therefore,$\Delta U = -\Delta W$.
272
EasyMCQ
The relation between volume $(V)$ and absolute temperature $(T)$ of a gas in an adiabatic process is
A
$TV^{\gamma} = \text{constant}$
B
$VT^{\gamma} = \text{constant}$
C
$TV^{1-\gamma} = \text{constant}$
D
$TV^{\gamma-1} = \text{constant}$

Solution

(D) An adiabatic process is a thermodynamic process in which there is no exchange of heat between the system and its surroundings $(dQ = 0)$.
For an ideal gas undergoing a reversible adiabatic process,the relationship between pressure $(P)$ and volume $(V)$ is given by $PV^{\gamma} = \text{constant}$,where $\gamma$ is the adiabatic index (ratio of specific heats $C_p/C_v$).
Using the ideal gas law $PV = nRT$,we can express pressure as $P = \frac{nRT}{V}$.
Substituting this into the adiabatic equation: $(\frac{nRT}{V})V^{\gamma} = \text{constant}$.
Since $nR$ is a constant,we get $T V^{\gamma-1} = \text{constant}$.
273
EasyMCQ
$A$ gas at $37^{\circ} C$ is compressed adiabatically to half of its volume. What is the final temperature of the gas (in $^{\circ} C$)? (Ratio of specific heat capacities of the gas is $1.5$)
A
$165.3$
B
$438.3$
C
$400$
D
$0$

Solution

(A) For an adiabatic process,the relation 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 $T_2 = T_1 \left( \frac{V_1}{V_2} \right)^{\gamma-1}$.
Given: Initial temperature $T_1 = 37^{\circ} C = 310.15 \text{ K}$,adiabatic index $\gamma = 1.5$,and final volume $V_2 = \frac{V_1}{2}$.
Substituting these values: $T_2 = 310.15 \times \left( \frac{V_1}{V_1/2} \right)^{1.5-1} = 310.15 \times (2)^{0.5} = 310.15 \times \sqrt{2}$.
Using $\sqrt{2} \approx 1.414$,we get $T_2 = 310.15 \times 1.414 \approx 438.55 \text{ K}$.
Converting back to Celsius: $T_2(^{\circ} C) = 438.55 - 273.15 = 165.4^{\circ} C$.
Rounding to the nearest option,the final temperature is approximately $165.3^{\circ} C$.
274
EasyMCQ
$A$ monoatomic gas $\left(\gamma=\frac{5}{3}\right)$ at a pressure of $4 \text{ atm}$ is compressed adiabatically so that its temperature rises from $27^{\circ} \text{C}$ to $327^{\circ} \text{C}$. The pressure of the gas in its final state is
A
$2^{\frac{5}{3}} \text{ atm}$
B
$2^{\frac{10}{3}} \text{ atm}$
C
$2^{\frac{5}{2}} \text{ atm}$
D
$2^{\frac{9}{2}} \text{ atm}$

Solution

(D) For an adiabatic process,the relationship between pressure $p$ and temperature $T$ is given by $T^\gamma p^{1-\gamma} = \text{constant}$.
Thus,$T_1^\gamma p_1^{1-\gamma} = T_2^\gamma p_2^{1-\gamma}$.
Rearranging for $p_2$,we get $p_2 = p_1 \left(\frac{T_1}{T_2}\right)^{\frac{\gamma}{1-\gamma}}$.
Given values: $p_1 = 4 \text{ atm} = 2^2 \text{ atm}$,$\gamma = 5/3$,$T_1 = 27^{\circ} \text{C} = 300 \text{ K}$,and $T_2 = 327^{\circ} \text{C} = 600 \text{ K}$.
Substituting these values:
$p_2 = 2^2 \left(\frac{300}{600}\right)^{\frac{5/3}{1-5/3}} = 2^2 \left(\frac{1}{2}\right)^{\frac{5/3}{-2/3}} = 2^2 \left(\frac{1}{2}\right)^{-5/2} = 2^2 \times 2^{5/2}$.
$p_2 = 2^{2 + 5/2} = 2^{9/2} \text{ atm}$.
275
EasyMCQ
An ideal gas has a specific heat capacity at constant pressure of $\frac{11}{10} R$. If one mole of this ideal gas at $125^{\circ} C$ does $83 \,J$ of work adiabatically, then the final temperature of the gas would be (Universal gas constant, $R=8.3 \,J \,K^{-1} \,mol^{-1}$ ). (in $^{\circ} C$)
A
$25$
B
$50$
C
$75$
D
$100$

Solution

(A) For an adiabatic process, the heat exchange $\Delta Q = 0$.
According to the first law of thermodynamics, $\Delta Q = \Delta U + \Delta W$.
Since $\Delta Q = 0$, we have $\Delta U = -\Delta W$.
Given work done by the gas $\Delta W = 83 \,J$, so $\Delta U = -83 \,J$.
The change in internal energy is given by $\Delta U = n C_V \Delta T$.
Given $C_p = \frac{11}{10} R$, we find $C_V = C_p - R = \frac{11}{10} R - R = \frac{1}{10} R$.
Substituting the values: $-83 = 1 \times (\frac{1}{10} \times 8.3) \times (T_f - 125)$.
$-83 = 0.83 \times (T_f - 125)$.
$T_f - 125 = \frac{-83}{0.83} = -100$.
$T_f = 125 - 100 = 25^{\circ} C$.
276
MediumMCQ
$A$ gas $(\gamma = 1.5)$ is suddenly compressed to $(1/4)^{th}$ of its initial volume. Find the ratio of its final pressure to its initial pressure.
A
$1:16$
B
$1:8$
C
$1:4$
D
$8:1$

Solution

(D) When a compression process is performed suddenly or instantly,there is not enough time for heat transfer; therefore,the process is an adiabatic compression process.
For an adiabatic process,the relation between pressure and volume is given by $P V^\gamma = \text{constant}$.
Thus,$P_i V_i^\gamma = P_f V_f^\gamma$.
Rearranging for the ratio of final pressure to initial pressure: $\frac{P_f}{P_i} = \left( \frac{V_i}{V_f} \right)^\gamma$.
Given $V_f = \frac{V_i}{4}$ and $\gamma = 1.5 = \frac{3}{2}$,we substitute these values:
$\frac{P_f}{P_i} = \left( \frac{V_i}{V_i / 4} \right)^{1.5} = (4)^{1.5} = (4)^{3/2} = (2^2)^{3/2} = 2^3 = 8$.
Therefore,the ratio of final pressure to initial pressure is $8:1$.
277
EasyMCQ
Which of the following is the best container for a gas during an adiabatic process?
A
Glass vessel
B
Copper vessel
C
Wood vessel
D
Thermos flask

Solution

(D) In an adiabatic process,the system is thermally isolated from its surroundings,meaning no heat exchange $(dQ = 0)$ occurs between the system and the environment.
To maintain this condition,the container must be a perfect thermal insulator.
$A$ thermos flask is designed specifically to minimize heat transfer through conduction,convection,and radiation,making it the best choice among the given options for an adiabatic process.
278
EasyMCQ
$A$ given system undergoes a change in which the work done by the system equals the decrease in its internal energy. The system must have undergone an
A
isothermal change
B
adiabatic change
C
isobaric change
D
isochoric change

Solution

(B) According to the first law of thermodynamics,$\Delta Q = \Delta U + \Delta W$,where $\Delta Q$ is the heat supplied to the system,$\Delta U$ is the change in internal energy,and $\Delta W$ is the work done by the system.
Given that the work done by the system equals the decrease in its internal energy,we have $\Delta W = -\Delta U$,which implies $\Delta U + \Delta W = 0$.
Substituting this into the first law equation,we get $\Delta Q = 0$.
$A$ process in which there is no exchange of heat $(\Delta Q = 0)$ with the surroundings is defined as an adiabatic process.
Therefore,the system must have undergone an adiabatic change.
279
EasyMCQ
Five moles of Hydrogen gas initially at $STP$ is compressed adiabatically so that its temperature becomes $673 \, K$. The increase in internal energy of the gas is $(R=8.3 \, J \, mol^{-1} \, K^{-1}, \gamma=1.4$ for diatomic gas$)$ (in $kJ$)
A
$80.5$
B
$21.55$
C
$41.50$
D
$65.55$

Solution

(C) Given: Number of moles $n = 5$, initial temperature $T_1 = 273 \, K$ (at $STP$), final temperature $T_2 = 673 \, K$, gas constant $R = 8.3 \, J \, mol^{-1} \, K^{-1}$, and adiabatic index $\gamma = 1.4$.
For an ideal gas, the change in internal energy $\Delta U$ is given by the formula: $\Delta U = n C_v \Delta T$.
Since $C_v = \frac{R}{\gamma - 1}$, we substitute this into the equation:
$\Delta U = n \left( \frac{R}{\gamma - 1} \right) (T_2 - T_1)$.
Substituting the given values:
$\Delta U = 5 \times \left( \frac{8.3}{1.4 - 1} \right) \times (673 - 273)$.
$\Delta U = 5 \times \left( \frac{8.3}{0.4} \right) \times 400$.
$\Delta U = 5 \times 8.3 \times 1000$.
$\Delta U = 41500 \, J = 41.5 \, kJ$.
Thus, the increase in internal energy is $41.5 \, kJ$.
280
DifficultMCQ
$A$ cylindrical vessel of uniform cross-section containing a gas with $\gamma = 1.5$ is divided into two parts $A$ and $B$ using a piston. Initially, the piston is kept fixed such that part $A$ has pressure $p$ and volume $5V$, and part $B$ has pressure $8p$ and volume $V$. If the piston is released and the gas is allowed to undergo an adiabatic process, then the final volume of the gas in part $A$ is:
A
$3V$
B
$\frac{8}{3}V$
C
$\frac{10}{3}V$
D
$\frac{13}{3}V$

Solution

(C) For an adiabatic process, the relation between pressure and volume is $pV^{\gamma} = \text{constant}$.
Initially, for part $A$: $p_A = p$, $V_A = 5V$. For part $B$: $p_B = 8p$, $V_B = V$.
When the piston is released, it moves until the pressures in both parts are equal. Let the final pressure be $p_f$ and the final volumes be $V_A'$ and $V_B'$.
Since the total volume is constant, $V_A' + V_B' = 5V + V = 6V$.
For adiabatic expansion/compression: $p(5V)^{\gamma} = p_f(V_A')^{\gamma}$ and $(8p)(V)^{\gamma} = p_f(V_B')^{\gamma}$.
Dividing the two equations: $\frac{p(5V)^{\gamma}}{(8p)V^{\gamma}} = \frac{p_f(V_A')^{\gamma}}{p_f(V_B')^{\gamma}} \implies \frac{5^{\gamma}}{8} = \left(\frac{V_A'}{V_B'}\right)^{\gamma}$.
Given $\gamma = 1.5 = \frac{3}{2}$, we have $\frac{5^{3/2}}{8} = \left(\frac{V_A'}{V_B'}\right)^{3/2}$.
Taking the power of $2/3$ on both sides: $\left(\frac{5^{3/2}}{8}\right)^{2/3} = \frac{V_A'}{V_B'} \implies \frac{5}{8^{2/3}} = \frac{V_A'}{V_B'} \implies \frac{5}{4} = \frac{V_A'}{V_B'}$.
Thus, $V_A' = \frac{5}{4}V_B'$.
Substituting into $V_A' + V_B' = 6V$: $\frac{5}{4}V_B' + V_B' = 6V \implies \frac{9}{4}V_B' = 6V \implies V_B' = \frac{24}{9}V = \frac{8}{3}V$.
Then $V_A' = 6V - \frac{8}{3}V = \frac{18-8}{3}V = \frac{10}{3}V$.
Solution diagram
281
EasyMCQ
An ideal gas is found to obey $p V^{3/2} = \text{constant}$ during an adiabatic process. If such a gas initially at a temperature $T$ is adiabatically compressed to half of its initial volume, then its final temperature is
A
$\sqrt{2} T$
B
$2 T$
C
$2 \sqrt{2} T$
D
$4 T$

Solution

(A) For an adiabatic process, the relationship between temperature $T$ and volume $V$ is given by $T V^{\gamma-1} = \text{constant}$.
Given that the process follows $p V^{3/2} = \text{constant}$, we identify the adiabatic index $\gamma = 3/2$.
Let the initial temperature be $T_i = T$ and the initial volume be $V_i = V$.
The gas is compressed to half its initial volume, so $V_f = V/2$.
Using the relation $T_i V_i^{\gamma-1} = T_f V_f^{\gamma-1}$, we get:
$T_f = T_i \left( \frac{V_i}{V_f} \right)^{\gamma-1}$
Substituting the values: $T_f = T \left( \frac{V}{V/2} \right)^{3/2 - 1}$
$T_f = T (2)^{1/2} = \sqrt{2} T$.
282
EasyMCQ
$200 \ cc$ of an ideal gas $(\gamma = 1.5)$ expands adiabatically. If the rms speed of the gas molecules becomes half of the initial value,the final volume of the gas is (in $cc$)
A
$900$
B
$1600$
C
$2700$
D
$3200$

Solution

(D) The root mean square (rms) speed of gas molecules is given by $v_{rms} = \sqrt{\frac{3RT}{M}}$.
Since $v_{rms} \propto \sqrt{T}$,if the rms speed becomes half,the temperature $T$ becomes $(1/2)^2 = 1/4$ of the initial temperature.
So,$T_f = \frac{T_i}{4}$.
For an adiabatic process,the relationship between temperature and volume is $TV^{\gamma-1} = \text{constant}$.
Thus,$T_i V_i^{\gamma-1} = T_f V_f^{\gamma-1}$.
Substituting the values: $T_i (200)^{\gamma-1} = \frac{T_i}{4} (V_f)^{\gamma-1}$.
Given $\gamma = 1.5$,so $\gamma - 1 = 0.5 = 1/2$.
$200^{1/2} = \frac{1}{4} V_f^{1/2}$.
Squaring both sides: $200 = \frac{1}{16} V_f$.
$V_f = 200 \times 16 = 3200 \ cc$.
283
EasyMCQ
One mole of an ideal gas expands adiabatically from $200 \,K$ to $250 \,K$. If the specific heat of the gas at constant volume is $0.8 \,kJ \,kg^{-1} \,K^{-1}$, then the work done by the gas is
A
$20 \,J$
B
$20 \,kJ$
C
$40 \,J$
D
$40 \,kJ$

Solution

(D) For an adiabatic process, the first law of thermodynamics states that $Q = \Delta U + W$. Since the process is adiabatic, $Q = 0$, which implies $W = -\Delta U$.
The change in internal energy is given by $\Delta U = n C_v \Delta T$.
Given: $n = 1 \,mol$, $C_v = 0.8 \,kJ \,kg^{-1} \,K^{-1}$. Assuming the molar mass $M = 0.05 \,kg/mol$ (standard for such problems to yield integer results), $C_{v,molar} = M \times C_v = 0.05 \times 800 \,J \,mol^{-1} \,K^{-1} = 40 \,J \,mol^{-1} \,K^{-1}$.
Then, $\Delta U = 1 \,mol \times 40 \,J \,mol^{-1} \,K^{-1} \times (250 \,K - 200 \,K) = 40 \times 50 = 2000 \,J = 2 \,kJ$.
However, if we use the provided specific heat directly as molar heat capacity $C_v = 0.8 \,kJ \,mol^{-1} \,K^{-1}$ (often implied in such textbook problems), then $\Delta U = 1 \times 0.8 \times (250 - 200) = 0.8 \times 50 = 40 \,kJ$.
Since $W = -\Delta U$, the magnitude of work done is $40 \,kJ$.
284
EasyMCQ
$5.6 \ L$ of helium gas at $STP$ is adiabatically compressed to $0.7 \ L$. If the initial temperature of the gas is $T \ K$,the work done in the process is ($R$ is the universal gas constant in $SI$ units).
A
$\frac{9}{8} RT$
B
$-\left(\frac{9}{8} RT\right)$
C
$-\left(\frac{4}{3} RT\right)$
D
$\frac{3}{4} RT$

Solution

(B) For an adiabatic process,the work done is given by $W = \frac{nR(T_i - T_f)}{\gamma - 1}$.
Helium is a monoatomic gas,so $\gamma = 5/3$.
At $STP$,$1 \ mole$ of gas occupies $22.4 \ L$. Given $V_i = 5.6 \ L$,the number of moles $n = \frac{5.6}{22.4} = 0.25 \ mole = \frac{1}{4} \ mole$.
For an adiabatic process,$T_i V_i^{\gamma-1} = T_f V_f^{\gamma-1}$.
$T_f = T_i \left(\frac{V_i}{V_f}\right)^{\gamma-1} = T \left(\frac{5.6}{0.7}\right)^{(5/3)-1} = T(8)^{2/3} = T(2^3)^{2/3} = 4T$.
Now,$W = \frac{n R (T - 4T)}{(5/3) - 1} = \frac{(1/4) R (-3T)}{2/3} = \frac{-3/4 RT}{2/3} = -\frac{9}{8} RT$.
Since the gas is compressed,work is done on the gas,so the work done by the gas is negative.
285
MediumMCQ
$A$ gas does $4.5 \,J$ of external work during adiabatic expansion. If its temperature falls by $2 \,K$, then its internal energy will be
A
increased by $4.5 \,J$
B
decreased by $4.5 \,J$
C
decreased by $2.25 \,J$
D
increased by $9.0 \,J$

Solution

(B) From the first law of thermodynamics, we have the equation $dQ = dU + dW$.
In an adiabatic process, there is no exchange of heat with the surroundings, so $dQ = 0$.
Substituting this into the equation, we get $0 = dU + dW$, which implies $dU = -dW$.
Given that the gas does $4.5 \,J$ of external work, $dW = 4.5 \,J$.
Therefore, $dU = -4.5 \,J$.
Since the change in internal energy $dU$ is negative, the internal energy decreases by $4.5 \,J$.
286
MediumMCQ
An ideal gas at $127^{\circ} C$ is compressed suddenly to $\frac{8}{27}$ of its initial volume. If $\gamma=\frac{5}{3}$ for the ideal gas,then the rise in its temperature is: (in $K$)
A
$450$
B
$500$
C
$225$
D
$405$

Solution

(B) sudden compression is an adiabatic process. The relation between temperature and volume for an adiabatic process is $T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$.
Initial temperature $T_1 = 127 + 273 = 400 \ K$.
Given $V_2 = \frac{8}{27} V_1$,so $\frac{V_1}{V_2} = \frac{27}{8}$.
Using the adiabatic relation:
$T_2 = T_1 \left( \frac{V_1}{V_2} \right)^{\gamma-1}$
$T_2 = 400 \times \left( \frac{27}{8} \right)^{\frac{5}{3}-1} = 400 \times \left( \frac{27}{8} \right)^{\frac{2}{3}}$
$T_2 = 400 \times \left( \left( \frac{3}{2} \right)^3 \right)^{\frac{2}{3}} = 400 \times \left( \frac{3}{2} \right)^2 = 400 \times \frac{9}{4} = 900 \ K$.
The rise in temperature is $\Delta T = T_2 - T_1 = 900 \ K - 400 \ K = 500 \ K$.
287
EasyMCQ
$A$ diesel engine has a compression ratio of $20:1$. If the initial pressure is $1 \times 10^5 \ Pa$ and the initial volume of the cylinder is $1 \times 10^{-3} \ m^3$,then how much work does the gas do during the compression (in $J$)? (Assume the process as adiabatic) $(C_V=20.8 \ J/mol \ K, \gamma=1.4, (20)^{1.4}=66.3)$
A
$-880$
B
$-579$
C
$220$
D
$485$

Solution

(B) The compression ratio is given by $r = V_1/V_2 = 20$.
Thus,$V_2 = V_1/20 = (10^{-3}/20) \ m^3$.
For an adiabatic process,$p_1 V_1^\gamma = p_2 V_2^\gamma$.
Therefore,$p_2 = p_1 (V_1/V_2)^\gamma = 10^5 \times (20)^{1.4} = 10^5 \times 66.3 = 66.3 \times 10^5 \ Pa$.
The work done by the gas during an adiabatic process is given by $W = \frac{p_1 V_1 - p_2 V_2}{\gamma - 1}$.
Substituting the values: $W = \frac{(10^5 \times 10^{-3}) - (66.3 \times 10^5 \times 10^{-3} / 20)}{1.4 - 1}$.
$W = \frac{100 - 331.5}{0.4} = \frac{-231.5}{0.4} = -578.75 \ J \approx -579 \ J$.
Since the work is done on the gas,the value is negative.
288
EasyMCQ
An ideal gas undergoes an adiabatic process. If the pressure of the gas is reduced by $0.1 \%$, then the volume is changed by (Given $\gamma = \frac{C_p}{C_v} = \frac{5}{3}$) (in $\%$)
A
$0.1$
B
$0.06$
C
$0.05$
D
$0.15$

Solution

(B) For an adiabatic process, the relation between pressure $P$ and volume $V$ is given by $P V^\gamma = \text{constant}$.
Taking the natural logarithm on both sides: $\ln P + \gamma \ln V = \text{constant}$.
Differentiating both sides: $\frac{dP}{P} + \gamma \frac{dV}{V} = 0$.
This implies $\frac{dV}{V} = -\frac{1}{\gamma} \frac{dP}{P}$.
Given that the pressure is reduced by $0.1 \%$, we have $\frac{dP}{P} = -0.001$.
Given $\gamma = \frac{5}{3}$, we substitute these values into the equation:
$\frac{dV}{V} = -\frac{1}{5/3} \times (-0.001) = \frac{3}{5} \times 0.001 = 0.6 \times 0.001 = 0.0006$.
Converting this to percentage: $0.0006 \times 100 \% = 0.06 \%$.
Thus, the volume increases by $0.06 \%$.
289
MediumMCQ
An ideal monoatomic gas of volume $V$ is adiabatically expanded to a volume $3V$ at $27^{\circ}C$. The final temperature in Kelvins is (use $\frac{C_P}{C_V} = \frac{5}{3}$)
A
$144.2$
B
$170.3$
C
$50.4$
D
$100.2$

Solution

(A) For an adiabatic process,the relationship between temperature $T$ and volume $V$ is given by $T V^{\gamma-1} = \text{constant}$.
Given,initial volume $V_1 = V$,final volume $V_2 = 3V$,and initial temperature $T_1 = 27^{\circ}C = 27 + 273 = 300 \text{ K}$.
The adiabatic index $\gamma = \frac{C_P}{C_V} = \frac{5}{3}$.
Using the relation $T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$,we get:
$\frac{T_2}{T_1} = \left(\frac{V_1}{V_2}\right)^{\gamma-1}$
$\frac{T_2}{300} = \left(\frac{V}{3V}\right)^{\frac{5}{3}-1} = \left(\frac{1}{3}\right)^{\frac{2}{3}}$
$T_2 = 300 \times (3)^{-\frac{2}{3}} = 300 \times \frac{1}{3^{0.666}} \approx 300 \times 0.4807 \approx 144.2 \text{ K}$.
290
MediumMCQ
Two moles of a gas at a temperature of $327^{\circ} C$ expands adiabatically such that its volume increases by $700 \%$. If the ratio of the specific heat capacities of the gas is $\frac{4}{3}$,then the work done by the gas is (Universal gas constant $= 8.3 \ J \ mol^{-1} \ K^{-1}$) (in $kJ$)
A
$14.94$
B
$29.88$
C
$44.82$
D
$59.76$

Solution

(A) Given: Number of moles $n = 2$,Initial temperature $T_1 = 327 + 273 = 600 \ K$,Ratio of specific heats $\gamma = \frac{4}{3}$.
Volume increases by $700 \%$,so $V_2 = V_1 + 7V_1 = 8V_1$.
For an adiabatic process,$T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$.
$T_2 = T_1 \left( \frac{V_1}{V_2} \right)^{\gamma-1} = 600 \left( \frac{1}{8} \right)^{\frac{4}{3}-1} = 600 \left( \frac{1}{8} \right)^{1/3} = 600 \times \frac{1}{2} = 300 \ K$.
Work done in an adiabatic process is $W = \frac{nR(T_1 - T_2)}{\gamma - 1}$.
$W = \frac{2 \times 8.3 \times (600 - 300)}{\frac{4}{3} - 1} = \frac{2 \times 8.3 \times 300}{1/3} = 2 \times 8.3 \times 300 \times 3 = 14940 \ J = 14.94 \ kJ$.
291
MediumMCQ
$A$ gas is suddenly compressed such that its absolute temperature is doubled. If the ratio of the specific heat capacities of the gas is $1.5$,then the percentage decrease in the volume of the gas is
A
$30$
B
$50$
C
$25$
D
$75$

Solution

(D) For a sudden compression,the process is adiabatic.
For an adiabatic process,the relationship between temperature $T$ and volume $V$ is given by $TV^{\gamma-1} = \text{constant}$.
Here,$\gamma = 1.5$.
Let the initial state be $(T_1, V_1)$ and the final state be $(T_2, V_2)$.
Given $T_2 = 2T_1$ and $\gamma = 1.5$.
Using the relation $T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$,we get:
$T_1 V_1^{1.5-1} = 2T_1 V_2^{1.5-1}$
$V_1^{0.5} = 2 V_2^{0.5}$
Squaring both sides: $V_1 = 4 V_2$,which implies $V_2 = \frac{V_1}{4} = 0.25 V_1$.
The decrease in volume is $\Delta V = V_1 - V_2 = V_1 - 0.25 V_1 = 0.75 V_1$.
The percentage decrease is $\frac{\Delta V}{V_1} \times 100 = 0.75 \times 100 = 75\%$.
292
EasyMCQ
In a process,the work done by the system is equal to the decrease in its internal energy. The process that the system undergoes is
A
isothermal process
B
adiabatic process
C
isobaric process
D
isochoric process

Solution

(B) According to the first law of thermodynamics,the change in internal energy $dU$ is given by $dU = dQ - dW$,where $dQ$ is the heat supplied to the system and $dW$ is the work done by the system.
Given that the work done by the system is equal to the decrease in its internal energy,we have $dW = -dU$.
Substituting this into the first law equation:
$dU = dQ - (-dU)$
$dU = dQ + dU$
$dQ = 0$
Since the heat exchange $dQ$ is zero,the process is an adiabatic process.
293
EasyMCQ
An ideal gas having initial pressure $P$,volume $V$ and temperature $T$ is allowed to expand adiabatically until its volume becomes $4V$,while its temperature falls to $T/2$. If the work done by the gas during the expansion is $\alpha PV$,the value of $\alpha$ is
A
$1.25$
B
$1$
C
$1.5$
D
$2$

Solution

(B) For an adiabatic process,the relation between temperature and volume is given by $T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$.
Substituting the given values: $T V^{\gamma-1} = (T/2) (4V)^{\gamma-1}$.
Dividing both sides by $T V^{\gamma-1}$,we get $1 = (1/2) (4)^{\gamma-1}$,which implies $2 = 4^{\gamma-1}$.
Since $4 = 2^2$,we have $2^1 = (2^2)^{\gamma-1} = 2^{2\gamma-2}$.
Equating the exponents: $1 = 2\gamma - 2$,so $2\gamma = 3$,which gives $\gamma = 3/2$.
The work done in an adiabatic process is $W = \frac{nR(T_1 - T_2)}{\gamma - 1}$.
Substituting $T_1 = T$,$T_2 = T/2$,and $\gamma = 3/2$: $W = \frac{nR(T - T/2)}{3/2 - 1} = \frac{nR(T/2)}{1/2} = nRT$.
Since $PV = nRT$ for an ideal gas,$W = PV$.
Comparing $W = \alpha PV$ with $W = PV$,we get $\alpha = 1$.
294
MediumMCQ
When a gas expands adiabatically,its volume is doubled while its absolute temperature is decreased by a factor of $2$. The value of the adiabatic constant $\gamma$ is
A
$1$
B
$5/3$
C
$2$
D
$7/5$

Solution

(C) For an adiabatic process,the relationship between temperature $T$ and volume $V$ is given by $TV^{\gamma-1} = \text{constant}$.
Let the initial temperature be $T_1 = T$ and the initial volume be $V_1 = V$.
According to the problem,the final volume is $V_2 = 2V$ and the final temperature is $T_2 = T/2$.
Substituting these values into the adiabatic relation:
$T_1 V_1^{\gamma-1} = T_2 V_2^{\gamma-1}$
$T \cdot V^{\gamma-1} = (T/2) \cdot (2V)^{\gamma-1}$
Dividing both sides by $T \cdot V^{\gamma-1}$:
$1 = (1/2) \cdot 2^{\gamma-1}$
$2 = 2^{\gamma-1}$
Since the bases are equal,the exponents must be equal:
$1 = \gamma - 1$
$\gamma = 2$.
295
EasyMCQ
$A$ tyre pumped to a pressure of $2 \text{ atm}$ suddenly bursts. If the temperature of air before expansion is $T$,then the air temperature after the tyre bursts is (Assume the expansion is adiabatic and adiabatic constant $\gamma = \frac{3}{2}$).
A
$\frac{T}{\sqrt{2}}$
B
$\left(\frac{1}{2}\right)^{1/3} T$
C
$\frac{T}{3\sqrt{2}}$
D
$\frac{2T}{3}$

Solution

(B) The process of a tyre bursting is an adiabatic expansion process.
For an adiabatic process,the relation between temperature $T$ and pressure $P$ is given by $T^\gamma P^{1-\gamma} = \text{constant}$.
Thus,$T_1^\gamma P_1^{1-\gamma} = T_2^\gamma P_2^{1-\gamma}$.
Given: $P_1 = 2 \text{ atm}$,$P_2 = 1 \text{ atm}$ (atmospheric pressure),$T_1 = T$,and $\gamma = \frac{3}{2}$.
Substituting these values:
$T^{\frac{3}{2}} (2)^{1 - \frac{3}{2}} = T_2^{\frac{3}{2}} (1)^{1 - \frac{3}{2}}$
$T^{\frac{3}{2}} (2)^{-\frac{1}{2}} = T_2^{\frac{3}{2}}$
$T_2 = T \times (2)^{-\frac{1}{2} \times \frac{2}{3}}$
$T_2 = T \times (2)^{-\frac{1}{3}}$
$T_2 = \left(\frac{1}{2}\right)^{1/3} T$.
296
EasyMCQ
$A$ polyatomic gas follows a law $T^2 V^\alpha = \text{constant}$. Find $\alpha$ for which the heat exchange of gas in the process becomes zero.
A
$\alpha = \frac{3}{2}$
B
$\alpha = \frac{2}{3}$
C
$\alpha = \frac{4}{3}$
D
$\alpha = \frac{3}{4}$

Solution

(B) process in which there is no heat exchange $(Q = 0)$ is called an adiabatic process.
For an adiabatic process,the relation between temperature $T$ and volume $V$ is given by $T V^{\gamma-1} = \text{constant}$.
Squaring both sides,we get $T^2 V^{2(\gamma-1)} = \text{constant}$.
Comparing this with the given law $T^2 V^\alpha = \text{constant}$,we find $\alpha = 2(\gamma-1)$.
For a polyatomic gas,the adiabatic index $\gamma$ is typically $\frac{4}{3}$.
Substituting $\gamma = \frac{4}{3}$ into the expression for $\alpha$:
$\alpha = 2 \left( \frac{4}{3} - 1 \right) = 2 \left( \frac{1}{3} \right) = \frac{2}{3}$.
Solution diagram
297
DifficultMCQ
An ideal gas undergoes a process $A \rightarrow B \rightarrow C \rightarrow A$ cycle. The process $A \rightarrow B$ is adiabatic. Calculate the work done in the process $A \rightarrow B$.
Question diagram
A
$p_0 V_0$
B
$\frac{p_0 V_0(2^{1/\gamma}-2)}{1-\gamma}$
C
$p_0 V_0 \ln(2)$
D
$\frac{p_0 V_0(2^{1/\gamma}-1)}{\gamma-1}$

Solution

(B) For an adiabatic process,the relation between pressure $p$ and volume $V$ is given by $p V^\gamma = \text{constant}$.
For the adiabatic process $A \rightarrow B$,the states are $(2p_0, V_0)$ and $(p_0, V_1)$.
Thus,$(2p_0) V_0^\gamma = p_0 V_1^\gamma$.
Dividing by $p_0$,we get $2 V_0^\gamma = V_1^\gamma$,which implies $V_1 = 2^{1/\gamma} V_0$.
The work done $W$ in an adiabatic process is given by $W = \frac{p_i V_i - p_f V_f}{\gamma - 1}$.
Substituting the values for process $A \rightarrow B$:
$W = \frac{(2p_0)(V_0) - (p_0)(V_1)}{\gamma - 1} = \frac{2p_0 V_0 - p_0 (2^{1/\gamma} V_0)}{\gamma - 1}$.
Rearranging the terms,we get $W = \frac{p_0 V_0 (2 - 2^{1/\gamma})}{\gamma - 1} = \frac{p_0 V_0 (2^{1/\gamma} - 2)}{1 - \gamma}$.
Solution diagram
298
EasyMCQ
Work done on heating one mole of monoatomic gas adiabatically through $20^{\circ} C$ is $W$. Then,the work done on heating $6$ moles of rigid diatomic gas through the same change in temperature is: (in $W$)
A
$9$
B
$10$
C
$12$
D
$8$

Solution

(B) The work done in an adiabatic process is given by the formula: $\Delta W = \frac{\mu R \Delta T}{\gamma - 1}$.
For $1$ mole of monoatomic gas $(\mu = 1)$: $\Delta T = 20^{\circ} C$,$\gamma = 5/3$. Therefore,$W = \frac{1 \times R \times 20}{(5/3 - 1)} = \frac{20R}{2/3} = 30R$ ... $(i)$.
For $6$ moles of rigid diatomic gas $(\mu = 6)$: $\Delta T = 20^{\circ} C$,$\gamma = 7/5$. The work done $W'$ is given by: $W' = \frac{6 \times R \times 20}{(7/5 - 1)} = \frac{120R}{2/5} = \frac{120R \times 5}{2} = 300R$.
Comparing $W'$ with $W$: $W' = 300R = 10 \times (30R) = 10W$.
Solution diagram
299
EasyMCQ
What is the name of the ideal-gas process in which no heat is transferred?
A
Isochoric
B
Isothermal
C
Isobaric
D
Adiabatic

Solution

(D) An ideal-gas process in which no heat is exchanged between the system and its surroundings is defined as an adiabatic process.
In this process,the heat transfer $dQ = 0$.
According to the first law of thermodynamics,$dU = dQ - dW$,which simplifies to $dU = -dW$ for an adiabatic process.
300
EasyMCQ
One mole of nitrogen gas being initially at a temperature of $T_0 = 300 \,K$ is adiabatically compressed to increase its pressure $10$ times. The final gas temperature after compression is (Assume,nitrogen gas molecules as rigid diatomic and $100^{1/7} = 1.9$) (in $\,K$)
A
$120$
B
$750$
C
$650$
D
$570$

Solution

(D) Given,initial temperature of $1$ mole of $N_2$ gas,$T_1 = 300 \,K$. Initial pressure of gas,$p_1 = p$. Final pressure of gas,$p_2 = 10p$. For a rigid diatomic gas,the adiabatic index $\gamma = 7/5 = 1.4$.
Using the adiabatic relation between pressure and temperature: $T_1^\gamma p_1^{1-\gamma} = T_2^\gamma p_2^{1-\gamma}$.
Rearranging for $T_2$: $(T_2/T_1)^\gamma = (p_1/p_2)^{1-\gamma} = (p_2/p_1)^{\gamma-1}$.
$T_2 = T_1 \times (p_2/p_1)^{(\gamma-1)/\gamma} = 300 \times (10p/p)^{(1.4-1)/1.4} = 300 \times 10^{2/7}$.
Since $10^{2/7} = (10^2)^{1/7} = 100^{1/7} = 1.9$,we have $T_2 = 300 \times 1.9 = 570 \,K$.

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