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# Ideal Gas

In this article, we will discuss Ideal Gas. So, let’s get started.

Ideal Gas

The limit as the density of the gas approaches zero of a dilute gas.

We will call the thermometer a constant volume ”ideal gas thermometer”, and the parameter is the pressure of the gas. Using the references of the Celsius scale, we find that the pressure of the gas would be zero at around −273°C. It will be useful to choose the value of temperature when the pressure of an ideal gas is zero to be zero degrees. With this choice we have:

T ∝ P (1)

So, temperature is defined to be proportional to the pressure of an ideal gas. The constant of proportionality is chosen such that the change of one degree is the same as the change of one degree Celsius. That is, the difference in temperature between boiling water and the triple point is 100°. For this choice, the proportionality constant becomes 273.16° at the triple point of water.

T = P/Ptp 273.16 (2)

where Ptp is the pressure of the gas at the triple point of water. It is important to note that both pressures must be measured at the same volume of the gas (Constant Volume Thermometer).

The unit of this definition of temperature is the Kelvin (K). To measure temperature in the lab or in our homes we can’t carry around a dilute gas and water at the triple point. Commercial thermometer are calibrated against this standard thermometer and have a range of validity.