# Entropy VS Quality

1. Jun 9, 2007

### sd_barry

I read that the higher the temperature of a substance (for example gas) the higher the quality to extract work from it.

But for the same substance, at same temperature, it would have high entropy as well which accounts for disorderness and hence less availibility of energy.

2. Jun 9, 2007

### Cyrus

Ehhhhhhhh, dont say Entropy vs. Quality. Quality has a reserved meaning for a mixture and is independent of Entropy.

3. Jun 9, 2007

### sd_barry

but this is still not clear, both are related to second law of thermodynamics and there must b a relation b/w them. if neone can guide me thru ths misery it would b appreciated

4. Jun 10, 2007

### Cyrus

I dont know what you're even asking.

5. Jun 10, 2007

### sd_barry

look lets consider air at 1000 K

at 1000 K it has a high entropy due to high disorderness, correct? thus it is very difficult to use it for energy generation, for example to rotate a turbine.

Where as it has a high quality also due to HIGH temperature which claims that v can use it easily to generate energy.

My question is: How can we compare entropy and quality?

6. Jun 10, 2007

### Cyrus

"cyrus', what do you mean by "quality"?

Are you asking me, or is that to barry?

7. Jun 10, 2007

### ZapperZ

Staff Emeritus
Room temperature is at a higher temperature than liquid helium temperature. According to your hypothesis, it has a higher "quality" of energy. Now, can you please extract work out of a volume of room temperature air?

This is a hint on why what you state isn't an "absolute" parameter.

Zz.

8. Mar 6, 2011

### anchorage

'Quality' can be defined loosely as 'the propensity of energy to be converted to other forms of energy'. For a gas in a particular state, the quality would be the ratio of incremental energy (energy added or lost) per incremental entropy change, provided the energy change took place under conditions of constant volume and constant (molecular or atomic) composition.

To put it in more simpler terms, yes, a higher temperature gas has more entropy, but it also has more energy. The ratio of energy to entropy is the quality.

To put it in simple thermodynamic terms, under a constant-volume, constant-composition change, the incremental change of energy is given by

dE = T dS (first law of thermodynamics)

Hence,

T = dE / dS

and this is the measure of quality for this system. ie, the measure of quality is the temperature.

9. Apr 6, 2011

### sd_barry

Hey thanks... I now get a clearer picture :)