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Reviewed CSCA Physics question · Standard

A gas in a cylinder is compressed by a piston. During the compression, the gas loses 300 J of heat to the surroundings, and the work done on the gas by the piston is 500 J. What is the change in internal energy of the gas?

  1. -800 J
  2. -200 J
  3. 200 J
  4. 800 J
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Correct answer

C. 200 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 on the system.

Why this answer is correct

Here Q = -300 J (heat lost) and W = +500 J (work done on gas). So ΔU = -300 J + 500 J = 200 J.

Example

If Q = -100 J and W = 300 J, ΔU = 200 J.

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