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Reviewed CSCA Physics question · Easy

A gas in a cylinder absorbs 750 J of heat from its surroundings while the gas expands and does 450 J of work on the piston. What is the change in internal energy of the gas?

  1. 300 J
  2. 1200 J
  3. -300 J
  4. 750 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 and W is work done by the system.

Why this answer is correct

Here, Q = +750 J (heat absorbed) and W = +450 J (work done by the gas on the piston). Therefore, ΔU = 750 J - 450 J = 300 J. The internal energy increases by 300 J.

Example

If a gas absorbs 500 J of heat and does 200 J of work, its internal energy increases by 300 J.

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