JEE MAIN - Physics (2024 - 6th April Evening Shift - No. 15)

A total of $$48 \mathrm{~J}$$ heat is given to one mole of helium kept in a cylinder. The temperature of helium increases by $$2^{\circ} \mathrm{C}$$. The work done by the gas is: Given, $$\mathrm{R}=8.3 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$$.
23.1 J
48 J
24.9 J
72.9 J

Explanation

To solve this problem, we can use the first law of thermodynamics, which states:

$$\Delta Q = \Delta U + W$$

where:

  • $$\Delta Q$$ is the heat transferred to the system, in this case, $$48 \mathrm{~J}$$.
  • $$\Delta U$$ is the change in internal energy of the system.
  • $$W$$ is the work done by the system.

For one mole of an ideal gas, the change in internal energy can be calculated using the equation:

$$\Delta U = nC_{v}\Delta T$$

where:

  • $$n$$ is the number of moles, which is $$1$$ in this case.
  • $$C_{v}$$ is the molar specific heat capacity at constant volume.
  • $$\Delta T$$ is the change in temperature, in Kelvins.

Given that we are dealing with helium, a monatomic ideal gas, we know that:

$$C_{v} = \frac{3}{2}R$$

Substituting the given values, we get:

$$\Delta U = (1) \frac{3}{2}(8.3 \mathrm{~J} \mathrm{~mol}^{-1} \mathrm{~K}^{-1})(2^{\circ} \mathrm{C})$$

Since a change in temperature of $$1^{\circ} \mathrm{C}$$ is equivalent to a change in temperature of $$1 \mathrm{~K}$$, we can directly substitute $$2^{\circ} \mathrm{C}$$ as a $$2 \mathrm{~K}$$ change without needing to convert Celsius to Kelvin as both scales have the same magnitude of degree.

Therefore,

$$\Delta U = 1 \times \frac{3}{2} \times 8.3 \times 2 = 24.9 \mathrm{~J}$$

Now, substituting $$\Delta U$$ and $$\Delta Q$$ into the first law of thermodynamics:

$$48 = 24.9 + W$$

Solving for $$W$$ we find:

$$W = 48 - 24.9 = 23.1 \mathrm{~J}$$

Thus, the work done by the gas is $$23.1 \mathrm{~J}$$, which corresponds to Option A.

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