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Reviewed CSCA Physics question · Standard

A gas in a cylinder with a movable piston absorbs 400 J of heat from a reservoir. During the process, the gas expands and does 150 J of work on the piston. What is the change in internal energy of the gas?

  1. 250 J
  2. 550 J
  3. -250 J
  4. 150 J
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Correct answer

A. 250 J

Principle or equation

The first law of thermodynamics: ΔU = Q - W, where Q is heat added to the system and W is work done by the system.

Why this answer is correct

Here Q = +400 J (heat absorbed) and W = +150 J (work done by gas). Thus ΔU = 400 - 150 = 250 J.

Example

If a gas absorbs 400 J and does 150 J of work, its internal energy increases by 250 J.

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