Ideal gas
Thermal Physics - OCR A-Level Physics
Key Definition
Ideal gas
A theoretical gas that perfectly obeys the equation of state pV = nRT at all pressures and temperatures. Real gases approximate this at low pressures and high temperatures.
A theoretical gas that perfectly obeys the equation of state pV = nRT at all pressures and temperatures. Real gases approximate this at low pressures and high temperatures.
$$\begin{aligned}
pV &= nRT \\
&= NkT
\end{aligned}$$
- The gas contains a very large number of molecules in random motion.
- The volume of the molecules is negligible compared to the volume of the container.
- Molecular collisions (with each other and the walls) are perfectly elastic (no energyThe capacity to do work. Measured in joules (J). lost).
- There are no intermolecular forces between molecules except during collisions.
- The time of each collision is negligible compared to the time between collisions.
- Molecules obey Newton's laws of motion.
Common Mistake
MEDIUM
Wrong: Saying 'molecules don't collide with each other' as an ideal gasA theoretical gas composed of many randomly moving point particles that do not interact except during brief elastic collisions. assumption.
Right: Molecules DO collide with each other and with the walls. The assumption is that collisions are perfectly ELASTIC and the TIME of each collision is negligible.
Right: Molecules DO collide with each other and with the walls. The assumption is that collisions are perfectly ELASTIC and the TIME of each collision is negligible.
Examiner Tips and Tricks
- This is a 6-mark question favourite.
- Learn all the assumptions precisely.
- Examiners look for: large number, random motion, negligible volume, elastic collisions, no intermolecular forces, negligible collision time.