# Which form of the first law of thermodynamics should I use?

I usually wonder which thermodynamics first law is better to use ?

The one given by physics : $\Delta U=Q-\Delta W$

or the one by chemistry : $\Delta U=Q+\Delta W$

In other words, should I take the gas as my system and take every parameter in its terms?

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As a student it is the greatest doubt for me. And in exams (having all subjects like IIT-JEE simulators ) it is a big thing to see before marking an answer that the question belongs to physics or chemistry. –  Mr.ØØ7 Apr 5 at 10:43
Its not a bad question at all! I am an engineer and I never realized that chemists used a $\Delta U = \delta Q + \delta W$! I always use a $-\delta W$... –  drN Apr 5 at 11:59
@drN: What if I told you that chemists look at dipoles the wrong way as well? :) –  Manishearth Apr 5 at 12:22
That was also a confusion but now i have just got it . In chemistry the direction is towards the more electro-negative element, ie. -ve charge. :) –  Mr.ØØ7 Apr 5 at 12:39

There's no "better" or worse here. It's just that "work" in physics is defined differently than in chemistry.

In chemistry, all quantities follow this sign convention: They are positive if their effect is on the system. So, basically,

• $\Delta U$ is energy imparted to the system by the surroundings
• $\Delta Q$ is the heat passed to the system from the surroundings
• $\Delta W$ is the work done on the system by the surroundings

In physics, the sign convention of $W$ is the opposite

• $\Delta U$ is energy imparted to the system by the surroundings
• $\Delta Q$ is the heat passed to the system from the surroundings
• $\Delta W$ is the work done by the system on the surroundings

which means that $\Delta U_C = \Delta Q_C + \Delta W_C$ ($C$ means chemistry) becomes $\Delta U_P = \Delta Q_P - \Delta W_P$

Try to keep these conventions separate in your mind. Don't use the physics FLT for a chemistry problem and vice versa, many times problems specify values of $W,Q,U$ and expect you to know the sign convention.

Note that these are the IUPAC/IUPAP conventions. Some books (as @dmckee mentions, Feynman's Lectures is one of them) use different conventions. In such cases, just make note of the convention and remember that the FLT is just a statement of conservation of energy.

Here's the menemonic I used to remember it. It's not a great one, but it works:

Chemists are interested in supplying hear/energy/pressure to a reaction to make it occur. Thus, action done by the surroundings on the system is "good" or "positive.

Physicists are more interested in supplying heat/energy to a system and making it do work. So, supplying heat/energy is "good", and getting work out is "good".

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+1 for the lines after <hr>. :) –  Mr.ØØ7 Apr 5 at 11:55
Are you in IIT Bombay? –  Mr.ØØ7 Apr 5 at 12:00
@exploringnet: Yes, how did you guess? (Lets move this to Physics Chat, irrelevant here.) –  Manishearth Apr 5 at 12:04
The Chemists/Physicists is not perfect. The Feynman Lectures takes $W$ to be the work done on the system. One must simply ask what conventions are in use and recall that this is a way of expressing the conservation of energy. –  dmckee Apr 5 at 13:40
@dmckee: Right, edited, thanks :) –  Manishearth Apr 5 at 13:43