Question

A gas absorbs heat equal to 100 calorie. The gas expands against a constant pressure of 1 atm and the volume of the gas increases by 20 litre, calculate the change in internal energy of the gas

Answer 1

Set the units right. 1 calorie = 4.18 J

Use first law of thermodynamics (ΔQ=ΔU+ΔW) .

Note that according to sign conventions in physics, heat given to the system as well as work done by the system are taken as positive.

Answer: 268 J

Answer 2

The change in internal energy of the gas will be 1926.6 J.

Explanation:

The first law of thermodynamics is given as

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

here, $\Delta U$ is the change in internal energy, $\Delta Q$ is the heat absorbed to the system and W is the work done.

We know,

$\Delta W=-P_e_x_t(\Delta V)$

here, $\Delta V$ is the change in volume.

Given P_ext = 1 atm and $\Delta V=20 L$.

So,

$\Delta W=-1atm\times20L$

$\Delta W=-20atmL$

1 L atm = 101.33 joule

so,

$\Delta W=-20\times101.33 J=-2026.6J$

From first law of thermodynamics equation, we get

$\Delta U=100 J-2026.6J=1926.6J$

Thus, the change in internal energy of the gas will be 1926.6 J.

#Learn More: first law of thermodynamics.

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