# Homework Help: Why is entropy not conserved in 1st law of thermodynamics?

1. Oct 16, 2011

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1. The problem statement, all variables and given/known data
The first law of thermodynamics is about the conservation of internal energy. Why is
entropy not conserved?

2. Relevant equations

dU = dq + dwexp + dw, where dwe is work in addition (e for ‘extra’) to the expansion work, dwexp.

3. The attempt at a solution
In thermodynamics, the total energy of a system is called its internal energy, U. The
internal energy is the total kinetic and potential energy of the molecules in the system.
We denote by ∆U the change in internal energy when a system changes from an initial
state i with internal energy Ui to a ﬁnal state f of internal energy U. A molecule has a certain number of motional degrees of freedom, such as the ability
to translate (the motion of its centre of mass through space), rotate around its centre
of mass, or vibrate (as its bond lengths and angles change, leaving its centre of mass
unmoved). Entropy is also related to "freedom" but i couldn't understand the relationship between entropy and 1st law of thermodynamics.

Last edited: Oct 16, 2011