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Reviewed CSCA Physics question · Easy

A gas in a cylinder absorbs 800 J of heat from its surroundings while the gas expands and does 300 J of work on the piston. What is the change in internal energy of the gas?

  1. -500 J
  2. 300 J
  3. 500 J
  4. 1100 J
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Correct answer

C. 500 J

Principle or equation

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 = +800 J (heat absorbed) and W = +300 J (work done by the gas). Thus ΔU = 800 J - 300 J = 500 J.

Example

If a gas absorbs 800 J of heat and does 300 J of work, its internal energy increases by 500 J.

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