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October 3rd, 2013, 07:10 AM   #1
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when work be - and +


How are you

I have a question how we can know the work if - or +

also heat energy

please could see this question why in answer
take work as -
and heat as +

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October 11th, 2013, 06:50 AM   #2
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Re: when work be - and +

If you talk about work, you always have to point out (or at least it has to be clear from the context) by whom/by what the work is done. If you have two interacting systems, then the work is done by system-number-one has the opposite sign as the system-number-2. It is quite similar to the action-reaction law of Newton (third law). It is not by accident, on the contrary, it is the straight consequence of the defintion of work.

which is in fact means:

So if a ball is (system1) originally in rest and you (system2) accelerate it, then the force is parallel to displacement, so W is positive, the formula says you did positive work (let's say +100J) on the ball. From aspect of the ball, the ball did negative work on you. The ball energy increased by +100J, your energy increased by -100J, that is your energy decreased by +100J.

In the particular example, there are two systems, the inner system (we can think gas container with movable wall) and the surrounding system. The text words clearly, the inner system is who done the +400J work, so it's internal energy must decrease by 400J. For example the gas extends, that is the gas exerts force on the wall and the wall moves in that direction, so the gas does positive work.
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