A gas absorbs 500 J of heat and does 200 J of work. Find the change in internal energy — Physics problems | PhysLabo
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Change in internal energy of a gas

Easy 10 grade Molecular Physics and Thermodynamics / Thermodynamics
Problem statement

A gas absorbs a quantity of heat $Q = 500$ J and at the same time does work $A = 200$ J. Find the change in internal energy of the gas.

Given
  • $Q = 500$ J
  • $A = 200$ J
  • $\Delta U = ?$
Visual demonstration
Hints
Hint 1 / 2
The first law of thermodynamics: $\Delta U = Q - A$.
Hint 2 / 2
$A$ is the work done by the gas itself (not on the gas): the problem says the gas did the work.
Solution

The first law of thermodynamics: $Q = \Delta U + A$, hence

$$\Delta U = Q - A = 500 - 200 = 300 \text{ J}$$

The internal energy of the gas increased by 300 J.

Answer

$\Delta U = 300$ J

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