# Measurement of entropy

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1. Mar 14, 2017

### Lafith

Can we measure 'absolute entropy' of a system.
Also can entropy become zero at 0 kelvin.is it possible only theoretically?

2. Mar 16, 2017

### jambaugh

Entropy is not a system observable, we don't measure it the same way we measure momentum or even temperature. So "no" literally to your question however we can define absolute entropy so that ...
Theoretically yes you can have a zero entropy system (any single particle which has been maximally observed) but in practice for complex systems zero entropy is an asymptotic limit like zero temperature and indeed you can't have non-zero temperature without some amount of entropy.

3. Mar 19, 2017

### jim hardy

For me entropy is a navigation aid to help one find where he is in the steam tables.
It's a number, just as on a channel marker, that is also written on your navigation chart to show where you are .
It's a tremendously handy tool for practical work with Moliere charts where it's the horizontal axis..

https://ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=th&chap_sec=06.2&page=theory

In a turbine, expansion moves straight down the chart and moisture extraction moves diagonally up along a constant temperature line.

There's no entropy gage or measuring instrument that i know of.

I dont understand entropy. But then , one doesn't have to understand momentum and impulse to drive a nail.
Use it to solve homework problems and confidence in your skill will grow .

If "The light comes on" , would you kindly enlighten me ?

old jim

Last edited: Mar 19, 2017
4. Mar 20, 2017

### DrDu

In principle, it can be done. We know the entropy of all elements in its standard state, so the measurement boils down to measuring entropy differences between the pure elements and the compound or mixture at some given temperature and pressure (or whatever variables define the equilibrium state in question). Now you can set up a reference process in the course of which a given amount of entropy is created. Now you can test out how much of this reference process you need minimally to convert your pure elements into the compound in the given state. If the entropy for this process is negative, you have to reverse your standard process. So in principle, you can test how much of a standard entropy process you need just like you test how much of different weights you need to bring a balance into balance when you determine mass.
While this is theoretically sound, in practice, it is usually completely imperformable, but this should not come as a surprise. It is also completely impracticable to determine almost any absolute temperature using a reversible Carnot engine, nevertheless, it could be done in principle, and this is all what counts.