The internal energy of a monoatomic gas is 3nRT. One mole of heli… - Solution | JEE Physics

MCQMediumJEE 2026First Law & Internal Energy

The internal energy of a monoatomic gas is $3nRT$ . One mole of helium... heated slowly by supplying $126 \, \text{J}$ heat... piston will move _____ cm.

A
$15.5$
B
$1.55$
C
$1.45$
D
$14.5$

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: Internal energy is given as $U = 3nRT$ . One mole of helium is heated slowly, so the process is isobaric. Heat supplied is $Q = 126 \, \text{J}$ .

Find: The displacement of the piston.

From

U = 3nRT

we get

C_v = 3R

Therefore,

C_p = C_v + R = 4R

Using the extracted solution working

For an isobaric process,

Q = n C_p \Delta T = n(3R+R)\Delta T

So the extracted working gives

Q = 4W

which implies

W = \frac{Q}{4} = \frac{126}{4} = 31.5 \, \text{J}

Option discrepancy noted in source solution

The source solution then explicitly switches to

W = \frac{126}{5} = 25.2 \, \text{J}

and uses

W = PA\Delta x

with

P = 10^5 \, \text{Pa}, \quad A = 17 \times 10^{-4} \, \text{m}^2

Thus,

25.2 = 10^5 \times 17 \times 10^{-4} \times \Delta x

\Delta x = \frac{25.2}{170} = 0.148 \, \text{m} = 14.8 \, \text{cm}

Hence the closest option is $14.5$ , so the correct option is D.

Note: The extracted solution contains an internal inconsistency: it first obtains $W = 31.5 \, \text{J}$ from $C_p = 4R$ , but then uses $W = 25.2 \, \text{J}$ to match the options. Since the source solution concludes with option D, the answer is taken as D.

Common mistakes

Using only $Q = \Delta U$ here is incorrect because the piston moves, so work is also done. In an isobaric process, use the heat relation and the work relation together.
Taking $C_v$ and $C_p$ interchangeably is wrong. First infer $C_v$ from $U = 3nRT$ , then use $C_p = C_v + R$ .
Forgetting unit conversion from metre to centimetre gives a wrong final value. After finding $\Delta x$ in metres, convert it to centimetres before matching options.

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Step-by-step solution

Common mistakes

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