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Applying the first law to a flow process requires a sign convention. The common chemical engineering choice takes

  • A.heat out of the system and work done on the system as positive
  • B.both heat and work as positive when they enter
  • C.both heat and work as positive when they leave
  • D.heat into the system and work done by the system as positive

Answer: D

This convention makes a heat engine produce positive work from positive heat input, which is why it became standard. Textbooks that take work into the system as positive change the sign in the equation rather than the physics.

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