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Isobaric and Isochoric Processes Questions in English

Class 11 Physics · Thermodynamics · Isobaric and Isochoric Processes

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Showing 49 of 125 questions in English

1
EasyMCQ
In a given process for an ideal gas,$dW = 0$ and $dQ < 0$. Then for the gas:
A
The temperature will decrease
B
The volume will increase
C
The pressure will remain constant
D
The temperature will increase

Solution

(A) According to the First Law of Thermodynamics $(FLOT)$,the change in internal energy $dU$ is given by $dU = dQ - dW$.
Given that $dW = 0$ (isochoric process) and $dQ < 0$ (heat is released by the system).
Substituting these values,we get $dU = dQ - 0 = dQ$.
Since $dQ < 0$,it follows that $dU < 0$.
For an ideal gas,internal energy $U$ is a function of temperature $T$ only $(U \propto T)$.
Therefore,a decrease in internal energy $(dU < 0)$ implies a decrease in temperature $(dT < 0)$.
2
EasyMCQ
$A$ gas expands under constant pressure $P$ from volume $V_1$ to $V_2$. The work done by the gas is
A
$P(V_2 - V_1)$
B
$P(V_1 - V_2)$
C
$P(V_1^\gamma - V_2^\gamma)$
D
$P\frac{V_1 V_2}{V_2 - V_1}$

Solution

(A) The work done $W$ by a gas during an expansion or compression process is given by the integral of pressure with respect to volume: $W = \int_{V_1}^{V_2} P \, dV$.
Since the pressure $P$ is constant (isobaric process),it can be taken out of the integral: $W = P \int_{V_1}^{V_2} dV$.
Evaluating the integral,we get: $W = P [V]_{V_1}^{V_2} = P(V_2 - V_1)$.
Therefore,the work done by the gas is $P(V_2 - V_1)$.
3
EasyMCQ
When heat is given to a gas in an isobaric process,then
A
The work is done by the gas
B
Internal energy of the gas increases
C
Both $(a)$ and $(b)$
D
None from $(a)$ and $(b)$

Solution

(C) In an isobaric process,the pressure remains constant. According to the first law of thermodynamics,$\Delta Q = \Delta U + \Delta W$.
When heat $\Delta Q$ is supplied to the gas,it expands against the external pressure,so work $\Delta W = P \Delta V$ is done by the gas.
Simultaneously,the temperature of the gas increases,which leads to an increase in the internal energy $\Delta U$ of the gas.
Therefore,both work is done by the gas and the internal energy of the gas increases.
4
EasyMCQ
Work done by air when it expands from $50 \, L$ to $150 \, L$ at a constant pressure of $2 \, atm$ is:
A
$2 \times 10^4 \, J$
B
$2 \times 100 \, J$
C
$2 \times 10^5 \times 100 \, J$
D
$2 \times 10^{-5} \times 100 \, J$

Solution

(A) The work done during an isobaric expansion is given by the formula $W = P \Delta V$.
Given:
Pressure $P = 2 \, atm = 2 \times 1.013 \times 10^5 \, Pa \approx 2 \times 10^5 \, Pa$.
Initial volume $V_1 = 50 \, L = 50 \times 10^{-3} \, m^3$.
Final volume $V_2 = 150 \, L = 150 \times 10^{-3} \, m^3$.
Change in volume $\Delta V = V_2 - V_1 = (150 - 50) \times 10^{-3} \, m^3 = 100 \times 10^{-3} \, m^3 = 0.1 \, m^3$.
Substituting the values into the work formula:
$W = (2 \times 10^5 \, Pa) \times (0.1 \, m^3) = 2 \times 10^4 \, J$.
Thus,the correct option is $A$.
5
EasyMCQ
Work done by $0.1$ mole of a gas at $27^{\circ}C$ to double its volume at constant pressure is ....... $cal$ $(R = 2 \, cal \, mol^{-1} \, K^{-1})$.
A
$54$
B
$600$
C
$60$
D
$546$

Solution

(C) For an isobaric process,the work done is given by $W = P \Delta V$.
Using the ideal gas equation $PV = nRT$,we can write $W = nR \Delta T$.
Given:
Number of moles $n = 0.1 \, mol$.
Initial temperature $T_1 = 27^{\circ}C = 27 + 273 = 300 \, K$.
Since the pressure is constant,by Charles's Law,$V \propto T$. If the volume doubles $(V_2 = 2V_1)$,the temperature must also double $(T_2 = 2T_1 = 600 \, K)$.
Change in temperature $\Delta T = T_2 - T_1 = 600 - 300 = 300 \, K$.
Substituting the values into the work formula:
$W = 0.1 \times 2 \times 300 = 60 \, cal$.
6
EasyMCQ
$A$ gas expands by $0.25 \, m^3$ at a constant pressure of $10^3 \, N/m^2$. The work done is:
A
$2.5 \, ergs$
B
$250 \, J$
C
$250 \, W$
D
$250 \, N$

Solution

(B) The work done $W$ by a gas during an isobaric expansion is given by the formula: $W = P \Delta V$.
Given:
Pressure $P = 10^3 \, N/m^2$
Change in volume $\Delta V = 0.25 \, m^3$
Substituting the values into the formula:
$W = 10^3 \, N/m^2 \times 0.25 \, m^3$
$W = 1000 \times 0.25 \, J$
$W = 250 \, J$
Therefore,the work done is $250 \, J$.
7
MediumMCQ
Two kg of water is converted into steam by boiling at atmospheric pressure. The volume changes from $2 \times 10^{-3} \ m^3$ to $3.34 \ m^3$. The work done by the system is about ....... $kJ$.
A
$-340$
B
$-170$
C
$170$
D
$340$

Solution

(D) The work done by a system at constant pressure (isobaric process) is given by the formula $W = P \Delta V$.
Here,the pressure $P$ is atmospheric pressure,which is $1.013 \times 10^5 \ Pa$.
The change in volume $\Delta V = V_f - V_i = 3.34 \ m^3 - 2 \times 10^{-3} \ m^3 = 3.34 - 0.002 = 3.338 \ m^3$.
Substituting these values into the work formula:
$W = (1.013 \times 10^5 \ Pa) \times (3.338 \ m^3)$
$W \approx 338,137 \ J$
Converting to kilojoules $(kJ)$:
$W \approx 338 \ kJ \approx 340 \ kJ$.
Therefore,the work done by the system is approximately $340 \ kJ$.
8
MediumMCQ
$A$ sample of gas expands from volume ${V_1}$ to ${V_2}$. The amount of work done by the gas is greatest when the expansion is
A
Isothermal
B
Isobaric
C
Adiabatic
D
Equal in all cases

Solution

(B) In a thermodynamic process,the work done by a gas is equal to the area under the $PV$ curve with respect to the volume axis.
For a given expansion from volume ${V_1}$ to ${V_2}$,the pressure $P$ remains constant in an isobaric process,whereas it decreases in both isothermal and adiabatic processes.
Since the isobaric process maintains the highest pressure throughout the expansion,the area under the $PV$ curve is the largest.
Therefore,the work done follows the order: ${W_{adiabatic}} < {W_{isothermal}} < {W_{isobaric}}$.
Thus,the work done is greatest for an isobaric expansion.
9
EasyMCQ
How much work is to be done in decreasing the volume of an ideal gas by an amount of $2.4 \times 10^{-4} \ m^3$ at normal temperature and constant normal pressure of $1 \times 10^5 \ N/m^2$ (in $J$)?
A
$28$
B
$27$
C
$25$
D
$24$

Solution

(D) The work done in a thermodynamic process at constant pressure is given by the formula $W = P \Delta V$.
Here,the change in volume is $\Delta V = 2.4 \times 10^{-4} \ m^3$.
The constant pressure is $P = 1 \times 10^5 \ N/m^2$.
Substituting these values into the formula:
$W = (1 \times 10^5 \ N/m^2) \times (2.4 \times 10^{-4} \ m^3)$
$W = 2.4 \times 10^{5-4} \ J$
$W = 2.4 \times 10^1 \ J$
$W = 24 \ J$.
Therefore,the work done is $24 \ J$.
10
EasyMCQ
$A$ container having $1$ mole of a gas at a temperature $27^{\circ}C$ has a movable piston which maintains a constant pressure in the container of $1 \, atm$. The gas is compressed until the temperature becomes $127^{\circ}C$. The work done is ........ $J$ ($C_P$ for the gas is $7.03 \, cal/mol-K$).
A
$703$
B
$814$
C
$121$
D
$2035$

Solution

(B) For an isobaric process (constant pressure),the work done is given by $W = P \Delta V$.
Using the ideal gas equation $PV = nRT$,for a constant pressure process,$P \Delta V = nR \Delta T$.
Given: $n = 1 \, mol$,$T_1 = 27^{\circ}C = 300 \, K$,$T_2 = 127^{\circ}C = 400 \, K$.
Change in temperature $\Delta T = T_2 - T_1 = 400 - 300 = 100 \, K$.
Using $R = 1.987 \, cal/mol-K$ (since $C_P$ is given in $cal$),$W = nR \Delta T = 1 \times 1.987 \times 100 = 198.7 \, cal$.
To convert work from calories to Joules,we use $1 \, cal \approx 4.184 \, J$.
$W = 198.7 \times 4.184 \approx 831.4 \, J$.
Comparing with the given options,the closest value is $814 \, J$.
11
EasyMCQ
In a reversible isochoric change,
A
$\Delta W = 0$
B
$\Delta Q = 0$
C
$\Delta T = 0$
D
$\Delta U = 0$

Solution

(A) An isochoric process is defined as a thermodynamic process in which the volume of the system remains constant,meaning $\Delta V = 0$.
Since the work done in a reversible process is given by $W = \int P \, dV$,if $\Delta V = 0$,then the work done $\Delta W = 0$.
Therefore,the correct option is $(a)$.
12
EasyMCQ
The work done in which of the following processes is zero?
A
Isothermal process
B
Adiabatic process
C
Isochoric process
D
None of these

Solution

(C) The work done by a gas during a thermodynamic process is given by the formula $W = \int P \, dV$.
In an isochoric process,the volume of the system remains constant,which means the change in volume $\Delta V = 0$.
Since the work done $W = P \Delta V$,substituting $\Delta V = 0$ gives $W = P \times 0 = 0$.
Therefore,the work done in an isochoric process is zero.
13
EasyMCQ
In which thermodynamic process,volume remains same?
A
Isobaric
B
Isothermal
C
Adiabatic
D
Isochoric

Solution

(D) An isochoric process is a thermodynamic process in which the volume of the system remains constant $(dV = 0)$.
Since the work done by the system is given by $W = \int P dV$,if the volume is constant,the work done is $zero$.
According to the first law of thermodynamics,$\Delta Q = \Delta U + W$. Since $W = 0$,all heat energy transferred to the system is absorbed as internal energy $(\Delta Q = \Delta U)$.
14
EasyMCQ
In an isochoric process,if ${T_1} = {27^o}C$ and ${T_2} = {127^o}C$,then ${P_1}/{P_2}$ will be equal to:
A
$9/59$
B
$2/3$
C
$4/3$
D
$3/4$

Solution

(D) In an isochoric process,the volume $V$ remains constant.
According to Gay-Lussac's Law,for a fixed mass of an ideal gas at constant volume,the pressure $P$ is directly proportional to the absolute temperature $T$ $(P \propto T)$.
Therefore,$\frac{P_1}{P_2} = \frac{T_1}{T_2}$.
First,convert the temperatures from Celsius to Kelvin:
$T_1 = 27^o C = 27 + 273 = 300 \ K$
$T_2 = 127^o C = 127 + 273 = 400 \ K$
Now,substitute these values into the ratio:
$\frac{P_1}{P_2} = \frac{300}{400} = \frac{3}{4}$.
15
MediumMCQ
$A$ monoatomic gas is supplied heat $Q$ very slowly while keeping the pressure constant. The work done by the gas will be
A
$\frac{2}{3}Q$
B
$\frac{3}{5}Q$
C
$\frac{2}{5}Q$
D
$\frac{1}{5}Q$

Solution

(C) For an isobaric process,the heat supplied is given by $\Delta Q_P = n C_P \Delta T = Q$.
The change in internal energy is $\Delta U = n C_V \Delta T$.
According to the first law of thermodynamics,$\Delta Q = \Delta U + \Delta W$,so $\Delta W = \Delta Q_P - \Delta U$.
Substituting the expressions,$\Delta W = n C_P \Delta T - n C_V \Delta T = n \Delta T (C_P - C_V) = n R \Delta T$.
Since $\Delta Q_P = n C_P \Delta T = Q$,we have $n \Delta T = \frac{Q}{C_P}$.
Thus,$\Delta W = \frac{R}{C_P} Q$.
For a monoatomic gas,$C_V = \frac{3}{2}R$ and $C_P = \frac{5}{2}R$.
Therefore,$\Delta W = \frac{R}{5/2 R} Q = \frac{2}{5} Q$.
16
MediumMCQ
When an ideal gas $(gamma = 5/3)$ is heated under constant pressure,what percentage of the given heat energy will be utilized in doing external work (in $\%$)?
A
$40$
B
$30$
C
$60$
D
$20$

Solution

(A) For an ideal gas undergoing an isobaric process (constant pressure),the first law of thermodynamics is given by $\Delta Q = \Delta U + \Delta W$.
Here,$\Delta Q = n C_P \Delta T$ is the heat supplied,$\Delta U = n C_V \Delta T$ is the change in internal energy,and $\Delta W$ is the external work done.
The fraction of heat utilized for external work is $\frac{\Delta W}{\Delta Q} = \frac{\Delta Q - \Delta U}{\Delta Q} = 1 - \frac{\Delta U}{\Delta Q}$.
Substituting the expressions,we get $\frac{\Delta W}{\Delta Q} = 1 - \frac{n C_V \Delta T}{n C_P \Delta T} = 1 - \frac{C_V}{C_P} = 1 - \frac{1}{\gamma}$.
Given $\gamma = 5/3$,the fraction is $1 - \frac{1}{5/3} = 1 - 3/5 = 2/5$.
Converting to percentage: $(2/5) \times 100\% = 40\%$.
17
MediumMCQ
When $1 \, mol$ of nitrogen gas is heated at constant pressure at room temperature,it is supplied with $1163.4 \, J$ of energy. The increase in temperature is ....... $K$ $(R = 8.31 \, J \, mol^{-1} \, K^{-1})$.
A
$54$
B
$28$
C
$65$
D
$40$

Solution

(D) For an isobaric process,the heat supplied is given by the formula: $dQ = \mu C_P dT$.
Here,$\mu = 1 \, mol$,$dQ = 1163.4 \, J$,and $R = 8.31 \, J \, mol^{-1} \, K^{-1}$.
For a diatomic gas like nitrogen,the molar heat capacity at constant pressure is $C_P = \frac{7}{2} R$.
Substituting the values into the equation: $dT = \frac{dQ}{\mu C_P}$.
$dT = \frac{1163.4}{1 \times (\frac{7}{2} \times 8.31)}$.
$dT = \frac{1163.4 \times 2}{7 \times 8.31} = \frac{2326.8}{58.17} \approx 40 \, K$.
Therefore,the increase in temperature is $40 \, K$.
18
MediumMCQ
The state of a thermodynamic system changes from $(1)$ $(P_1, V)$ to $(2P_1, V)$ and $(2)$ $(P, V_1)$ to $(P, 2V_1)$. The work done during these two processes is respectively:
A
Zero,Zero
B
Zero,$PV_1$
C
$PV_1$,Zero
D
$PV_1$,$P_1V_1$

Solution

(B) For process $(1)$,the volume $V$ is constant. Since the change in volume $dV = 0$,the work done $W = \int P dV = 0$.
For process $(2)$,the pressure $P$ is constant. The work done is given by $W = \int_{V_1}^{2V_1} P dV = P [V]_{V_1}^{2V_1} = P(2V_1 - V_1) = PV_1$.
Therefore,the work done in the two processes is $0$ and $PV_1$ respectively.
19
EasyMCQ
When the temperature of $1 \, \text{mole}$ of an ideal gas is increased from $0^{\circ}C$ to $100^{\circ}C$ at constant pressure, what is the work done during the process? $(R = 8.3 \, J/mol \cdot K)$
A
$8.3 \times 10^{-3} \, J$
B
$8.3 \times 10^{-2} \, J$
C
$8.3 \times 10^{2} \, J$
D
$8.3 \times 10^{3} \, J$

Solution

(C) For an isobaric process (constant pressure), the work done $W$ is given by the formula $W = P \Delta V$.
Using the ideal gas equation $PV = nRT$, for a constant pressure process, $P \Delta V = nR \Delta T$.
Given:
Number of moles $n = 1 \, \text{mol}$.
Change in temperature $\Delta T = 100^{\circ}C - 0^{\circ}C = 100 \, K$.
Gas constant $R = 8.3 \, J/mol \cdot K$.
Substituting these values into the formula:
$W = nR \Delta T = 1 \times 8.3 \times 100$.
$W = 830 \, J = 8.3 \times 10^{2} \, J$.
20
MediumMCQ
The work done to double the volume of $0.1 \ mol$ of a gas at constant pressure and a temperature of $27 \ ^\circ C$ is equal to ..... $cal$.
A
$54$
B
$600$
C
$60$
D
$546$

Solution

(C) At constant pressure,according to Charles's Law,$\frac{V_1}{T_1} = \frac{V_2}{T_2}$.
Given $V_1 = V$,$V_2 = 2V$,and $T_1 = 27 + 273 = 300 \ K$.
Substituting the values: $\frac{V}{300} = \frac{2V}{T_2}$,which gives $T_2 = 600 \ K$.
The change in temperature is $\Delta T = T_2 - T_1 = 600 - 300 = 300 \ K$.
The work done in an isobaric process is given by $W = P \Delta V = \mu R \Delta T$.
Using $\mu = 0.1 \ mol$ and $R \approx 2 \ cal/(mol \cdot K)$:
$W = (0.1) \times (2) \times (300) = 60 \ cal$.
21
EasyMCQ
For an isochoric process,if $T_1 = 27 \, ^\circ C$ and $T_2 = 127 \, ^\circ C$,then $P_1 / P_2 = \dots$
A
$9/59$
B
$2/3$
C
$3/4$
D
None of these

Solution

(C) For an isochoric process,the volume $V$ remains constant.
According to Gay-Lussac's Law,$P \propto T$ or $P_1 / T_1 = P_2 / T_2$.
First,convert the temperatures from Celsius to Kelvin:
$T_1 = 27 + 273 = 300 \, K$
$T_2 = 127 + 273 = 400 \, K$
Now,calculate the ratio:
$P_1 / P_2 = T_1 / T_2 = 300 / 400 = 3/4$.
22
EasyMCQ
Which relation is correct for an isochoric process?
A
$\Delta Q = \Delta U$
B
$\Delta W = \Delta U$
C
$\Delta Q = \Delta W$
D
None of these

Solution

(A) For an isochoric process,the volume remains constant,so $\Delta V = 0$.
Since the work done is given by $\Delta W = P \Delta V$,it follows that $\Delta W = 0$.
According to the first law of thermodynamics,$\Delta Q = \Delta U + \Delta W$.
Substituting $\Delta W = 0$,we get $\Delta Q = \Delta U$.
23
MediumMCQ
Which of the following relations is correct for an isochoric process?
A
$\Delta Q = \Delta U$
B
$\Delta W = \Delta U$
C
$\Delta Q = \Delta W$
D
None of these.

Solution

(A) For an isochoric process,the volume remains constant,so $\Delta V = 0$.
From the first law of thermodynamics,$\Delta Q = \Delta U + \Delta W$.
Since $\Delta W = P \Delta V$,and $\Delta V = 0$,we have $\Delta W = 0$.
Therefore,$\Delta Q = \Delta U + 0$,which gives $\Delta Q = \Delta U$.
24
EasyMCQ
$A$ gas expands by $0.25 \ m^{3}$ at a constant pressure of $10^{3} \ N/m^{2}$. Calculate the work done.
A
$2.5 \ \text{ergs}$
B
$250 \ J$
C
$250 \ W$
D
$250 \ N$

Solution

(B) The work done by a gas during expansion at constant pressure is given by the formula: $\Delta W = P \Delta V$.
Given:
Pressure $P = 10^{3} \ N/m^{2}$
Change in volume $\Delta V = 0.25 \ m^{3}$
Substituting the values:
$\Delta W = 10^{3} \times 0.25 = 250 \ J$.
Therefore,the work done is $250 \ J$.
25
DifficultMCQ
If $\gamma$ is the ratio of molar specific heat at constant pressure to molar specific heat at constant volume for a gas,find the change in internal energy of $1 \, mol$ of the gas when its volume changes from $V$ to $2V$ at constant pressure $P$.
A
$\frac{PV}{(\gamma - 1)}$
B
$PV$
C
$\frac{R}{(\gamma - 1)}$
D
$\frac{\gamma PV}{(\gamma - 1)}$

Solution

(A) Given that $C_P/C_V = \gamma$.
We know that $C_P - C_V = R$,so $C_V = \frac{R}{\gamma - 1}$.
The change in internal energy for any process is given by $\Delta U = n C_V \Delta T$.
Since $n = 1$,$\Delta U = C_V \Delta T = \frac{R \Delta T}{\gamma - 1}$.
From the ideal gas equation $PV = nRT$,at constant pressure $P$,$P \Delta V = nR \Delta T$. Since $n = 1$,$P \Delta V = R \Delta T$.
Substituting this into the internal energy equation: $\Delta U = \frac{P \Delta V}{\gamma - 1}$.
Given $\Delta V = 2V - V = V$,we get $\Delta U = \frac{PV}{\gamma - 1}$.
26
DifficultMCQ
Two containers $A$ and $B$ with pistons contain a diatomic gas at $300\, K$. The piston in container $A$ is free to move,while the piston in container $B$ is fixed. Both containers are supplied with the same amount of heat. If the temperature in container $A$ increases by $30\, K$,by how many $K$ will the temperature in container $B$ increase? Both containers contain the same gas.
A
$30$
B
$18$
C
$50$
D
$42$

Solution

(D) For container $A$ (isobaric process),the heat supplied is given by $\Delta Q_A = \mu C_p \Delta T_A$.
For container $B$ (isochoric process),the heat supplied is given by $\Delta Q_B = \mu C_v \Delta T_B$.
Since the heat supplied is the same,$\Delta Q_A = \Delta Q_B$.
Therefore,$\mu C_p \Delta T_A = \mu C_v \Delta T_B$.
This simplifies to $\Delta T_B = \frac{C_p}{C_v} \Delta T_A = \gamma \Delta T_A$.
For a diatomic gas,the adiabatic index $\gamma = \frac{7}{5} = 1.4$.
Given $\Delta T_A = 30\, K$,we have $\Delta T_B = 1.4 \times 30 = 42\, K$.
27
DifficultMCQ
When heat is supplied to a monoatomic gas,it expands at constant pressure. What fraction of the heat supplied is converted into work?
A
$\frac{1}{\gamma}$
B
$\left(1 - \frac{1}{\gamma}\right)$
C
$\gamma - 1$
D
$\left(1 - \frac{1}{\gamma^2}\right)$

Solution

(B) For a gas expanding at constant pressure,the heat supplied is $dQ = n C_p dT$.
The work done is $dW = P dV = n R dT$.
The fraction of heat converted into work is $\eta = \frac{dW}{dQ} = \frac{n R dT}{n C_p dT} = \frac{R}{C_p}$.
Since $C_p = \frac{\gamma R}{\gamma - 1}$,we have $\eta = \frac{R}{\frac{\gamma R}{\gamma - 1}} = \frac{\gamma - 1}{\gamma} = 1 - \frac{1}{\gamma}$.
28
MediumMCQ
$A$ monoatomic gas $(\gamma = 5/3)$ expands at constant pressure when heat is supplied. What percentage of the heat supplied is converted into internal energy and work, respectively?
A
$60\%, 40\%$
B
$40\%, 60\%$
C
$75\%, 25\%$
D
$25\%, 75\%$

Solution

(A) For an isobaric process, the heat supplied is $dQ = nC_p dT$.
The change in internal energy is $dU = nC_v dT$.
The work done is $dW = PdV = nR dT$.
The fraction of heat converted to internal energy is $\frac{dU}{dQ} = \frac{nC_v dT}{nC_p dT} = \frac{C_v}{C_p} = \frac{1}{\gamma}$.
Given $\gamma = 5/3$, the fraction is $\frac{1}{5/3} = 3/5 = 0.6$ or $60\%$.
The fraction of heat converted to work is $\frac{dW}{dQ} = \frac{nR dT}{nC_p dT} = \frac{R}{C_p} = \frac{C_p - C_v}{C_p} = 1 - \frac{1}{\gamma} = 1 - 0.6 = 0.4$ or $40\%$.
Thus, the percentages are $60\%$ and $40\%$ respectively.
29
MediumMCQ
Find the heat energy required to increase the temperature of $2$ moles of a monoatomic gas from $273 \ K$ to $373 \ K$. (No work is done in the process.) (in $R$)
A
$100$
B
$150$
C
$300$
D
$500$

Solution

(C) Since no work is done in the process,the process is isochoric (constant volume).
For an isochoric process,the heat supplied is equal to the change in internal energy: $\Delta Q = \Delta U$.
The formula for internal energy change is $\Delta U = n \, C_v \, \Delta T$.
For a monoatomic gas,the molar heat capacity at constant volume is $C_v = \frac{3}{2} R$.
Given: $n = 2 \ moles$,$\Delta T = 373 \ K - 273 \ K = 100 \ K$.
Substituting the values: $\Delta Q = 2 \times \frac{3}{2} R \times 100 = 300 \ R$.
30
DifficultMCQ
Calculate the work done when the volume of a gas increases by $0.25\,m^3$ at a constant pressure of $10^3\,N/m^2$.
A
$2.5\,ergs$
B
$250\,J$
C
$250\,W$
D
$250\,N$

Solution

(B) The work done $W$ by a gas at constant pressure $P$ is given by the formula $W = P \Delta V$.
Given:
Pressure $P = 10^3\,N/m^2$
Change in volume $\Delta V = 0.25\,m^3$
Substituting the values into the formula:
$W = 10^3\,N/m^2 \times 0.25\,m^3$
$W = 1000 \times 0.25\,J$
$W = 250\,J$
Therefore,the work done is $250\,J$.
31
MediumMCQ
The volume $(V)$ of a monatomic gas varies with its temperature $(T)$,as shown in the graph. The ratio of work done by the gas to the heat absorbed by it,when it undergoes a change from state $A$ to state $B$,is
Question diagram
A
$\frac{2}{5}$
B
$\frac{2}{3}$
C
$\frac{2}{7}$
D
$\frac{1}{3}$

Solution

(A) From the graph,$V \propto T$,which implies the process is isobaric (constant pressure).
For an isobaric process,the heat absorbed is given by $dQ = nC_p dT$,where $C_p$ is the molar heat capacity at constant pressure.
For a monatomic gas,the degrees of freedom $f = 3$. Thus,$C_p = \frac{f+2}{2}R = \frac{3+2}{2}R = \frac{5}{2}R$.
Therefore,$dQ = n \left( \frac{5}{2}R \right) dT$.
The work done by the gas is $dW = PdV$. Since $PV = nRT$,for a constant pressure process,$PdV = nRdT$.
The required ratio is $\frac{dW}{dQ} = \frac{nRdT}{n(\frac{5}{2}R)dT} = \frac{1}{5/2} = \frac{2}{5}$.
32
MediumMCQ
$A$ cylinder of fixed capacity $44.8 \ L$ contains a monatomic gas at standard temperature and pressure $(STP)$. The amount of heat required to raise the temperature of the gas in the cylinder by $10^{\circ}C$ is: ($R =$ universal gas constant)
A
$R$
B
$10R$
C
$20R$
D
$30R$

Solution

(D) As we know,$1 \ mol$ of any ideal gas at $STP$ occupies a volume of $22.4 \ L$.
Hence,the number of moles of gas $\mu = \frac{44.8}{22.4} = 2 \ mol$.
Since the volume of the cylinder is fixed,the process is isochoric (constant volume).
The heat required at constant volume is given by $\Delta Q_V = \mu C_V \Delta T$.
For a monatomic gas,the molar heat capacity at constant volume is $C_V = \frac{3}{2}R$.
Substituting the values: $\Delta Q_V = 2 \times \frac{3}{2}R \times 10 = 30R$.
33
MediumMCQ
Volume versus temperature graph of two moles of helium gas is as shown in figure. The ratio of heat absorbed and the work done by the gas in process $1-2$ is
Question diagram
A
$3$
B
$2.5$
C
$1.67$
D
$3.5$

Solution

(B) The $V-T$ graph is a straight line passing through the origin. According to the ideal gas law $PV = nRT$,if $V \propto T$,then the pressure $P$ remains constant. Thus,the process $1-2$ is an isobaric process.
For an isobaric process,the heat absorbed is given by $\Delta Q = nC_P \Delta T$.
The work done by the gas is given by $\Delta W = P \Delta V = nR \Delta T$.
Using the relation $C_P - C_V = R$,we can write $\Delta W = n(C_P - C_V) \Delta T$.
The ratio of heat absorbed to the work done is:
$\frac{\Delta Q}{\Delta W} = \frac{nC_P \Delta T}{n(C_P - C_V) \Delta T} = \frac{C_P}{C_P - C_V} = \frac{1}{1 - \frac{C_V}{C_P}} = \frac{1}{1 - \frac{1}{\gamma}}$
For a monoatomic gas like helium,the adiabatic index $\gamma = \frac{C_P}{C_V} = \frac{5}{3}$.
Therefore,the ratio is:
$\frac{\Delta Q}{\Delta W} = \frac{1}{1 - \frac{3}{5}} = \frac{1}{\frac{2}{5}} = \frac{5}{2} = 2.5$.
34
DifficultMCQ
An ideal gas undergoes the process $1 \rightarrow 2$ as shown in the figure. The heat supplied and work done in the process are $\Delta Q$ and $\Delta W$ respectively. The ratio $\Delta Q : \Delta W$ is
Question diagram
A
$\gamma : \gamma - 1$
B
$\gamma$
C
$\gamma - 1$
D
$\frac{\gamma}{\gamma - 1}$

Solution

(D) From the graph, the process $1 \rightarrow 2$ is an isobaric process because $V \propto T$ (i.e., $V/T = \text{constant}$), which implies pressure $P$ is constant.
For an isobaric process, the heat supplied is given by $\Delta Q = n C_P \Delta T$.
The work done by the gas is $\Delta W = P \Delta V = n R \Delta T$.
We know that for an ideal gas, $C_P = \frac{\gamma R}{\gamma - 1}$.
Substituting this into the expression for $\Delta Q$, we get $\Delta Q = n \left( \frac{\gamma R}{\gamma - 1} \right) \Delta T$.
Now, the ratio $\Delta Q : \Delta W$ is:
$\frac{\Delta Q}{\Delta W} = \frac{n \left( \frac{\gamma R}{\gamma - 1} \right) \Delta T}{n R \Delta T} = \frac{\gamma}{\gamma - 1}$.
Solution diagram
35
MediumMCQ
One mole of an ideal gas is contained within a cylinder by a frictionless piston and is initially at temperature $T$. The pressure of the gas is kept constant while it is heated and its volume doubles. If $R$ is the molar gas constant,the work done by the gas in increasing its volume is
A
$RT \ln 2$
B
$1/2 \, RT$
C
$RT$
D
$3/2 \, RT$

Solution

(C) The work done by a gas at constant pressure is given by the formula:
$W = P \Delta V$
Given that the pressure $P$ is constant and the volume doubles from an initial volume $V$ to $2V$,the change in volume is $\Delta V = 2V - V = V$.
Substituting this into the work formula:
$W = P(V) = PV$
According to the ideal gas equation,$PV = nRT$. For $n = 1$ mole of gas at temperature $T$,we have:
$PV = RT$
Therefore,the work done by the gas is:
$W = RT$
36
AdvancedMCQ
In an isobaric expansion of an ideal gas,which of the following is zero?
A
work done
B
$\Delta Q$
C
$\Delta U$
D
$d^2V / dT^2$

Solution

(D) For an isobaric expansion of an ideal gas,the pressure $P$ is constant.
Since the gas expands,the volume increases,so $\Delta V > 0$.
Work done $W = P \Delta V$,which is not zero.
From the ideal gas equation $PV = nRT$,since $P$ is constant,$V \propto T$. As volume increases,temperature $T$ also increases,so $\Delta U \neq 0$.
From the first law of thermodynamics,$\Delta Q = \Delta U + W$,so $\Delta Q \neq 0$.
For an isobaric process,$V = (nR/P)T$.
Differentiating with respect to $T$,we get $dV/dT = nR/P$,which is a constant.
Differentiating again with respect to $T$,we get $d^2V/dT^2 = 0$.
37
DifficultMCQ
$A$ vessel contains an ideal monoatomic gas which expands at constant pressure,when heat $Q$ is given to it. Then the work done in expansion is:
A
$Q$
B
$\frac{3}{5} Q$
C
$\frac{2}{5} Q$
D
$\frac{2}{3} Q$

Solution

(C) For an ideal gas at constant pressure,the work done is given by $W = P \Delta V$.
From the ideal gas equation,$PV = nRT$,so for constant pressure,$P \Delta V = nR \Delta T$.
Heat supplied at constant pressure is $Q = n C_p \Delta T$.
For a monoatomic gas,the molar heat capacity at constant pressure is $C_p = \frac{5}{2} R$.
Substituting this into the heat equation,we get $Q = n \left( \frac{5}{2} R \right) \Delta T$.
Since $nR \Delta T = W$,we can write $Q = \frac{5}{2} W$.
Rearranging for work done,we get $W = \frac{2}{5} Q$.
38
MediumMCQ
Two moles of helium gas are taken over the cycle $ABCDA$,as shown in the $P-T$ diagram. Assuming the gas to be ideal,the work done on the gas in taking it from $A$ to $B$ is ...... $R$.
Question diagram
A
$300$
B
$400$
C
$600$
D
$200$

Solution

(B) The process from $A$ to $B$ is an isobaric process because the pressure $P$ remains constant at $2 \times 10^5 \text{ Pa}$.
For an ideal gas,the work done by the gas is given by $W_{\text{by}} = nR\Delta T$.
Here,$n = 2 \text{ moles}$,$T_A = 300 \text{ K}$,and $T_B = 500 \text{ K}$.
$W_{\text{by}} = 2 \times R \times (500 - 300) = 400R$.
The question asks for the work done $ON$ the gas.
Work done on the gas is $W_{\text{on}} = -W_{\text{by}} = -400R$.
However,in the context of such physics problems,the magnitude is often expected. Given the options,the magnitude $400R$ is the correct answer.
39
MediumMCQ
Air is pumped into a balloon,of initial volume $V$,until its diameter has doubled. If the atmospheric pressure is $p$,what is the work done against the atmosphere?
A
$pV$
B
$3pV$
C
$4pV$
D
$7pV$

Solution

(D) The volume of a sphere is given by $V = \frac{4}{3} \pi R^3$,which implies $V \propto R^3$.
When the diameter doubles,the radius $R$ also doubles $(R \rightarrow 2R)$.
Therefore,the new volume $V'$ becomes $V' = \frac{4}{3} \pi (2R)^3 = 8 \times (\frac{4}{3} \pi R^3) = 8V$.
The work done against the constant atmospheric pressure $p$ during an isobaric expansion is given by $W = p \Delta V$.
Substituting the values,$W = p(V' - V) = p(8V - V) = 7pV$.
40
MediumMCQ
In the figure shown,the amount of heat supplied to one mole of an ideal gas is plotted on the horizontal axis and the amount of work done by the gas is plotted on the vertical axis. Assuming the process to be isobaric,identify the gas.
Question diagram
A
$He$
B
$CO$
C
$CO_2$
D
$NH_3$

Solution

(A) For an isobaric process,the work done $W$ is given by $W = nR\Delta T$ and the heat supplied $Q$ is given by $Q = nC_p\Delta T$.
From the graph,at $Q = 80 \ J$,$W = 32 \ J$.
Therefore,the ratio $\frac{W}{Q} = \frac{nR\Delta T}{nC_p\Delta T} = \frac{R}{C_p} = \frac{32}{80} = \frac{2}{5}$.
This implies $5R = 2C_p$.
Since $C_p = C_v + R$,we have $5R = 2(C_v + R) = 2C_v + 2R$.
$3R = 2C_v \implies C_v = \frac{3}{2}R$.
$A$ gas with $C_v = \frac{3}{2}R$ is a monoatomic gas. Among the given options,$He$ (Helium) is a monoatomic gas. Thus,the correct option is $A$.
41
MediumMCQ
$A$ monoatomic ideal gas expands at constant pressure,with heat $Q$ supplied. The fraction of $Q$ which goes as work done by the gas is
A
$1$
B
$0.67$
C
$0.6$
D
$0.4$

Solution

(D) For an ideal gas,the heat supplied at constant pressure is given by $\Delta Q = n C_p \Delta T$.
The change in internal energy is given by $\Delta U = n C_v \Delta T$.
According to the first law of thermodynamics,$\Delta Q = \Delta U + \Delta W$,so the work done is $\Delta W = \Delta Q - \Delta U$.
Substituting the expressions,$\Delta W = n C_p \Delta T - n C_v \Delta T = n(C_p - C_v) \Delta T = n R \Delta T$.
The fraction of heat used as work is $\frac{\Delta W}{\Delta Q} = \frac{n R \Delta T}{n C_p \Delta T} = \frac{R}{C_p}$.
For a monoatomic gas,$C_p = \frac{5}{2} R$.
Therefore,the fraction is $\frac{R}{\frac{5}{2} R} = \frac{2}{5} = 0.4$.
42
MediumMCQ
$A$ container having $1 \ mol$ of a gas at a temperature of $27 \ ^oC$ has a movable piston which maintains a constant pressure of $1 \ atm$ in the container. The gas is compressed until the temperature becomes $127 \ ^oC$. The work done is ........ $J$ $(R = 8.31 \ J/mol-K)$.
A
$703$
B
$831$
C
$121$
D
$2035$

Solution

(B) For an ideal gas at constant pressure,the work done $W$ is given by the formula $W = P \Delta V$.
Using the ideal gas equation $PV = nRT$,at constant pressure $P$,we have $P \Delta V = nR \Delta T$.
Given:
Number of moles $n = 1 \ mol$.
Initial temperature $T_1 = 27 \ ^oC = 27 + 273 = 300 \ K$.
Final temperature $T_2 = 127 \ ^oC = 127 + 273 = 400 \ K$.
Change in temperature $\Delta T = T_2 - T_1 = 400 \ K - 300 \ K = 100 \ K$.
Universal gas constant $R = 8.31 \ J/mol-K$.
Substituting the values into the work formula:
$W = nR \Delta T = 1 \times 8.31 \times 100 = 831 \ J$.
Thus,the work done is $831 \ J$.
43
MediumMCQ
Given the relation $V = K \left( \frac{P}{T} \right)^{0.33}$,where $K$ is a constant. Identify the type of thermodynamic process.
A
isothermal process
B
adiabatic process
C
isochoric process
D
isobaric process

Solution

(C) From the ideal gas law,we know that $PV = nRT$,which implies $\frac{P}{T} = \frac{nR}{V}$.
Substituting this into the given equation $V = K \left( \frac{P}{T} \right)^{0.33}$,we get:
$V = K \left( \frac{nR}{V} \right)^{0.33}$
$V = K (nR)^{0.33} \cdot V^{-0.33}$
Multiplying both sides by $V^{0.33}$,we get:
$V^{1.33} = K' \text{ (where } K' = K(nR)^{0.33} \text{ is a constant)}$
Since $V^{1.33}$ is constant,it implies that $V$ must be a constant.
$A$ process in which the volume remains constant is known as an isochoric process.
44
MediumMCQ
$A$ diatomic ideal gas undergoes a thermodynamic change according to the $P-V$ diagram shown in the figure. The heat given to the gas in process $AB$ is: (in $, P_0 V_0$)
Question diagram
A
$2.5$
B
$1.4$
C
$3.9$
D
$1.1$

Solution

(A) For process $AB$,the volume is constant $(V = V_0)$,so it is an isochoric process.
Work done $W_{AB} = 0$.
The heat given is $\Delta Q = \Delta U = n C_v \Delta T$.
For a diatomic gas,$C_v = \frac{5}{2} R$.
Using the ideal gas law $PV = nRT$,we have $n \Delta T = \frac{\Delta(PV)}{R} = \frac{P_B V_B - P_A V_A}{R}$.
Given $P_A = P_0, V_A = V_0$ and $P_B = 2P_0, V_B = V_0$.
So,$n \Delta T = \frac{(2P_0)(V_0) - (P_0)(V_0)}{R} = \frac{P_0 V_0}{R}$.
Substituting this into the heat equation:
$\Delta Q = n \left( \frac{5}{2} R \right) \Delta T = \frac{5}{2} R (n \Delta T) = \frac{5}{2} R \left( \frac{P_0 V_0}{R} \right) = 2.5 P_0 V_0$.
45
MediumMCQ
At constant pressure,what fraction of the heat supplied to an ideal gas is converted into mechanical work?
A
$\frac{\gamma - 1}{\gamma}$
B
$\frac{\gamma}{\gamma - 1}$
C
$\gamma - 1$
D
$\frac{\gamma}{\gamma + 1}$

Solution

(A) According to the First Law of Thermodynamics,$Q = \Delta U + W$.
For an isobaric process (constant pressure),the heat supplied is $Q = n C_p \Delta T$.
The work done is $W = P \Delta V = n R \Delta T$.
The change in internal energy is $\Delta U = n C_v \Delta T$.
The fraction of heat converted into work is $\frac{W}{Q} = \frac{n R \Delta T}{n C_p \Delta T} = \frac{R}{C_p}$.
Since $C_p = \frac{\gamma R}{\gamma - 1}$,we substitute this into the expression:
$\frac{W}{Q} = \frac{R}{\frac{\gamma R}{\gamma - 1}} = \frac{\gamma - 1}{\gamma}$.
46
MediumMCQ
$A$ cyclic process is shown in the figure. The work done during the isobaric expansion is ...... $J$.
Question diagram
A
$150$
B
$100$
C
$400$
D
$200$

Solution

(C) In a $P-V$ diagram,the work done is equal to the area under the curve.
An isobaric process is one where the pressure remains constant.
In the given figure,the process $AB$ represents an isobaric expansion because the pressure is constant at $P = 2 \times 10^2 \, N/m^2$ while the volume increases from $V_1 = 1 \, m^3$ to $V_2 = 3 \, m^3$.
The work done during an isobaric process is given by $W = P \Delta V$.
Here,$P = 2 \times 10^2 \, N/m^2$ and $\Delta V = V_2 - V_1 = 3 - 1 = 2 \, m^3$.
Therefore,$W = (2 \times 10^2) \times 2 = 400 \, J$.
47
MediumMCQ
The ratio of work done by an ideal monoatomic gas to the heat supplied to it in an isobaric process is
A
$0.4$
B
$1.5$
C
$0.6$
D
$0.67$

Solution

(A) In an isobaric process,the work done $W$ by an ideal gas is given by $W = P \Delta V = nR \Delta T$.
The heat supplied $Q$ at constant pressure is given by $Q = n C_P \Delta T$.
For a monoatomic gas,the molar heat capacity at constant pressure is $C_P = \frac{5}{2}R$.
The ratio of work done to the heat supplied is $\frac{W}{Q} = \frac{nR \Delta T}{n C_P \Delta T} = \frac{R}{C_P}$.
Substituting the value of $C_P$,we get $\frac{W}{Q} = \frac{R}{\frac{5}{2}R} = \frac{2}{5} = 0.4$.
48
DifficultMCQ
The equation of state for a gas is given by $PV = nRT + \alpha V$,where $n$ is the number of moles and $\alpha$ is a positive constant. The initial temperature and pressure of one mole of the gas contained in a cylinder are $T_0$ and $P_0$ respectively. The work done by the gas when its temperature doubles isobarically will be
A
$\frac{P_0 T_0 R}{P_0 - \alpha}$
B
$\frac{P_0 T_0 R}{P_0 + \alpha}$
C
$P_0 T_0 R \ln 2$
D
$P_0 T_0 R$

Solution

(A) Given the equation of state: $PV = nRT + \alpha V$.
For $n = 1$ mole,$PV = RT + \alpha V$,which can be rewritten as $V(P - \alpha) = RT$,or $V = \frac{RT}{P - \alpha}$.
Initially,at $T = T_0$ and $P = P_0$,the volume is $V_0 = \frac{RT_0}{P_0 - \alpha}$.
Since the process is isobaric,$P$ remains constant at $P_0$. When the temperature doubles,$T_f = 2T_0$.
The final volume is $V_f = \frac{R(2T_0)}{P_0 - \alpha} = 2V_0$.
The work done in an isobaric process is $W = P_0(V_f - V_0)$.
Substituting the values: $W = P_0(2V_0 - V_0) = P_0 V_0$.
Since $V_0 = \frac{RT_0}{P_0 - \alpha}$,we have $W = P_0 \left( \frac{RT_0}{P_0 - \alpha} \right) = \frac{P_0 T_0 R}{P_0 - \alpha}$.
49
EasyMCQ
Half a mole of an ideal monoatomic gas is heated at a constant pressure of $1\, atm$ from $20\,^oC$ to $90\,^oC$. The work done by the gas is close to ..... $J$ (Gas constant $R = 8.31\, J/mol\cdot K$).
A
$581$
B
$291$
C
$146$
D
$73$

Solution

(B) For an ideal gas undergoing a process at constant pressure (isobaric process),the work done $W$ is given by the formula:
$W = P\Delta V$
Using the ideal gas equation $PV = nRT$,for a constant pressure process,we have $P\Delta V = nR\Delta T$.
Given:
Number of moles $n = 0.5\, mol$
Gas constant $R = 8.31\, J/mol\cdot K$
Change in temperature $\Delta T = 90\,^oC - 20\,^oC = 70\, K$
Substituting the values into the formula:
$W = nR\Delta T$
$W = 0.5 \times 8.31 \times 70$
$W = 290.85\, J$
Rounding to the nearest integer,the work done is approximately $291\, J$.

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