# Compressability of gases

1. Jun 16, 2006

### Mattius_

Is there a unit to measure the compressability of a gas? What gases are the hardest to compress? The easiest? Any links to a better understanding?

2. Jun 16, 2006

### Bystander

No.

Compressibility is measured in reciprocal pressure.

Ordinarily, I don't get too picky about spelling --- this is a thermodynamic term --- as such, it makes me picky.

3. Jun 16, 2006

### Cyrus

Sure, its called the Bulk Modulus and is defined as:

$$E_v = \frac{dp}{d \rho / \rho} = - \frac{dp}{dV / V}$$

Basically, it is the negative of the ratio of a change in pressure, p, to the ratio of the change in volume V, to the origional volume, V. (Or instead of volume V, density $\rho$ <but no minus sign>)

You lost me with this statement.

Last edited: Jun 16, 2006
4. Jun 16, 2006

### Bystander

dV/V ? Unitless. 1/dP ? Reciprocal pressure --- unit(s) of compressibility? Reciprocal pressure.

5. Jun 16, 2006

### Cyrus

The units of the Bulk modulus are pressure, $FL^{-2}$ not the reciprocal. You still have me confused with what your trying to say.

6. Jun 16, 2006

### Bystander

Compressibility is the reciprocal of the bulk modulus. Do the dimensional analysis.

7. Jun 16, 2006

### Cyrus

Oh, my book says the "bulk modulus" but does not say the inverse is called the compressiblity. I just looked it up on wiki.

Then yes, now you make sense.

In that case, take the inverse of everything I have said! (or read your screen upside down)

Last edited: Jun 16, 2006
8. Jun 16, 2006

### Bystander

Thermo for mechs vs. thermo for eggheads vs. thermo for chemists, and all the different approaches taken by authors? Don't let it bug you.