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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 250 J of heat to the surroundings, and the work done on the gas by the piston is 400 J. What is the change in internal energy of the gas?

  1. -650 J
  2. -150 J
  3. +150 J
  4. +650 J
Show the answer and explanation

Correct answer

C. +150 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 = -250 J (heat lost by gas), W = +400 J (work done on gas). Thus ΔU = -250 + 400 = +150 J.

Example

If a gas absorbs 300 J of heat and does 100 J of work, its internal energy increases by 200 J.

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