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Thermodynamics MCQs for NEET — Physics Questions with Answers

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In a thermodynamic process, pressure of a fixed mass of a gas is changed in such a manner that the gas molecules absorb 30 J of heat and 10 J of work is done by the gas. If the initial internal energy of the gas was 40 J, then the final internal energy will be -

30 J

20 J

60 J

40 J

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Explanation

$Δ Q = Δ U + Δ W = (U_{f} - U_{i}) + Δ W$

⇒ $30 = (U_{f} - 40) + 10$ ⇒ $U_{f} = 60 J$

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First law of thermodynamics is a special case of

Newton's law

Law of conservation of energy

Charle's law

Law of heat exchange

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Explanation

Heat supplied to a gas raise its internal energy and does some work against expansion, so it is a special case of law of conservation of energy.

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If the ratio of specific heat of a gas at constant pressure to that at constant volume is γ, the change in internal energy of a mass of gas, when the volume changes from V to 2V constant pressure p, is

$R / (γ - 1)$

$p V / (γ - 1)$

$γ p V / (γ - 1)$

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Explanation

$Δ U = μ C_{V} Δ T = n (\frac{R}{γ - 1}) Δ T$

$\Rightarrow Δ U = \frac{P Δ V}{(γ - 1)} = \frac{P (2 V - V)}{γ - 1} = \frac{P V}{(γ - 1)}$

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If heat given to a system is 6 kcal and work done is 6 kJ. Then the change in internal energy is :

19.1 kJ

12.5 kJ

25 kJ

Zero

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Explanation

$Δ Q = Δ U + Δ W$ ⇒ $Δ U = Δ Q - Δ W$

$= 6 \times 4 .18 - 6 = 19 .08 k J$

$\approx 19 .1 k J$

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A monoatomic gas of n-moles is heated from temperature T₁ to T₂ under two different conditions (i) at constant volume and (ii) at constant pressure. The change in internal energy of the gas is

More for (i)

More for (ii)

Same in both cases

Independent of number of moles

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Explanation

Change in internal energy $Δ U = μ C_{V} Δ T$

it doesn’t depend upon type of process. Actually it is a state function

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Can two isothermal curves cut each other

Never

Yes

They will cut when temperature is 0°C

Yes, when the pressure is critical pressure

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Explanation

If isothermal curves cut each other then at equilibrium two temperature will be there which is impossible.

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A vessel containing 5 litres of a gas at 0.8 m pressure is connected to an evacuated vessel of volume 3 litres. The resultant pressure inside will be (assuming whole system to be isolated)

4/3 m

0.5 m

2.0 m

3/4 m

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Explanation

$0 .8 \times 5 = P \times (3 + 5) \Rightarrow P = 0 .5 m$

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For an isothermal expansion of a perfect gas, the value of $\frac{Δ P}{P}$ is equal -

$- γ^{1 / 2} \frac{Δ V}{V}$

$- \frac{Δ V}{V}$

$- γ \frac{Δ V}{V}$

$- γ^{2} \frac{Δ V}{V}$

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Explanation

Differentiate PV = constant w.r.t V

⇒ $P Δ V + V Δ P = 0$ ⇒ $\frac{Δ P}{P} = - \frac{Δ V}{V}$

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If a gas is heated at constant pressure, its isothermal compressibility

Remains constant

Increases linearly with temperature

Decreases linearly with temperature

Decreases inversely with temperature

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Explanation

$E_{θ} = P$ , if P = constant, $E_{θ} =$ constant

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In isothermal expansion, the pressure is determined by -

Temperature only

Compressibility only

Both temperature and compressibility

None of these

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Explanation

For such a case, pressure $= \frac{1}{Compressibility}$

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