QS) Derive first law of thermodynamics and write the equation for it.
Answers
The first law of thermodynamics states that the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system.
In equation form, the first law of thermodynamics is ΔU = Q − W.
Here ΔU is the change in internal energy U of the system. Q is the net heat transferred into the system—that is, Q is the sum of all heat transfer into and out of the system. W is the net work done by the system—that is, W is the sum of all work done on or by the system. We use the following sign conventions: if Q is positive, then there is a net heat transfer into the system; if W is positive, then there is net work done by the system. So positive Q adds energy to the system and positive W takes energy from the system. Thus ΔU = Q − W. Note also that if more heat transfer into the system occurs than work done, the difference is stored as internal energy. Heat engines are a good example of this—heat transfer into them takes place so that they can do work.We will now examine Q, W, and ΔU further.