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I use the sign convention:

  • Heat absorbed by the system = $q+$ (positive)
  • Heat evolved by the system = $q-$ (negative)
  • Work done on the system = $w +$ (positive)
  • Work done by the system = $w -$ (negative)

Could anyone please tell me, that volume increased in system does positive or negative work?

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3 Answers 3

up vote 1 down vote accepted

Volume increase in the system is due to work done by the system. Therefore W is negative using your notation. Think of it this way, work done on the system would push the system inwards, decreasing volume. Therefore a volume increase is work done by the system.

Alternatively you could reason using the formula: $dU = Q - dW$ (using your notation conventions, were $U$ is internal energy, $W$ is work and $Q$ is heat added to the system)

$dW = PdV.$

Therefore

$dU = Q - PdV$.

Therefore if $dV$ (change in volume) is positive, $dU$ (change in internal energy) is negative.

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Thank you Chris –  Amhar Sep 21 '12 at 7:32

I think the confusion arises due to difference in equations from Physics and Chemistry text books.

The physics text writes $U=q-w$ while chemistry says $U=q+w$.
From the physics text its mentioned that work done by the system is positive means $U=q-(+w)$ and work done on the system is negative means $U=q-(-w)$.

THEREFORE, both are correct in their own way. Hope I've figured it out correctly.

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Hi Ubi. Welcome to Physics.SE. This site uses an unique TeX markup style called MathJax. This markup is very useful for understanding math equations and parameters. Please have a look here for an intro or our FAQ for more info. For example, $\theta$ results $\theta$, $\omega$ inserts $\omega$, etc. It's quite interesting. You can revise your post if you can ;-) –  Waffle's Crazy Peanut May 19 '13 at 17:08

to solve this sign conflict between physics and chemistry, you may, in the two cases, only refer to the decrease of the internal energy, U, as an indication of work done by the system, and refer to the increase of the internal energy, U, as an indication of work done on the system.

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protected by Qmechanic Nov 17 at 1:31

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