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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 200 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. +300 J
  2. -300 J
  3. +700 J
  4. -700 J
Show the answer and explanation

Correct answer

A. +300 J

Principle or equation

The first law of thermodynamics: ΔU = Q + W, where Q is heat added to the system (positive if added, negative if lost) and W is work done on the system (positive if done on the gas).

Why this answer is correct

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

Example

If a gas loses 100 J of heat and 300 J of work is done on it, ΔU = -100 + 300 = +200 J.

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