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### q= -w

Posted: Thu Feb 07, 2019 8:14 pm
What exactly is meant by q=-w or -q=w ?

### Re: q= -w

Posted: Thu Feb 07, 2019 10:11 pm
The only way for a system to gain internal energy is for the surroundings to do work on it or for the system to absorb heat. It loses energy when it does work on its surroundings or releases heat. Thus, we have the delta U (internal energy) = q (heat) + w (work).

In an isothermal, reversible system of an ideal gas, there is no change in internal energy. 0 = q + w. Thus q must equal -w or w = -q

Conceptually, this makes sense because if a system loses energy by doing some amount of work, it must gain an equivalent value of heat for the internal energy not to change (or the other way around). In this "perfect" system, there is 100% efficiency in transforming heat to work or work to heat. In real life, this isn't possible but this allows us to know the maximum amount of work that could potentially happen in a best case scenario.

### Re: q= -w

Posted: Thu Feb 07, 2019 10:17 pm
If a system has a net internal energy change of zero, then the energy transferred by work and heat are equal and opposite of each other.

### Re: q= -w

Posted: Thu Feb 07, 2019 10:46 pm
Isothermal systems have a net energy change of zero so the work must be equal to the opposite of the amount of heat.

### Re: q= -w

Posted: Fri Feb 08, 2019 3:02 pm
It is essentially saying that q and w are equal but opposite

### Re: q= -w

Posted: Fri Feb 08, 2019 5:00 pm
This is because of the equation delta U= q+ w. If there is no change in internal energy, then consequently the opposite of w will equal q.

### Re: q= -w

Posted: Sat Feb 09, 2019 6:39 pm
If the problem states there is no change in internal energy, then delta U is equal to 0. Therefore, you can either subtract q or w and get q= -w or w=-q.

### Re: q= -w

Posted: Sat Feb 09, 2019 8:39 pm
This is the case when the change in internal energy is 0

### Re: q= -w

Posted: Sat Feb 09, 2019 9:06 pm
This means that in isothermal systems, the heat lost is equal to the work done, but a negative sign is added.