5.18-5.19 - Temperature and average molecular kinetic energy
Temperature is not just a thermometer reading. For a gas, it tells you about the random motion of its molecules. In this lesson, the key link is: higher temperature means greater average molecular kinetic energy, so the gas molecules move faster on average.
Temperature and molecular speed
Gas molecules are in constant random motion. They travel in different directions and change direction when they collide with other molecules or with the walls of their container.
The molecules do not all have the same speed. Some are moving faster than others at any instant, so Physics uses an average speed for the whole sample of gas.
Average molecular speed
Average molecular speed is the mean speed of the gas molecules in a sample.
When the temperature of a gas increases, energy is transferred to the random motion of its molecules. Their average kinetic energy increases, and a molecule with more kinetic energy is moving faster. This is why an increase in temperature results in an increase in the average speed of gas molecules.
[DIAGRAM: asset_name: Temperature and average molecular kinetic energy - diagram 01; asset_slug: p47_temperature_and_average_molecular_kinetic_energy__diagram_01; recommended_method: image_gen; description: White 16:9 NovaLearn pale-palette diagram with grey outlines and dark-grey labels. Two neutral gas panels contain identical particles at the same positions, with longer clay motion arrows in the higher-temperature panel.]
The arrows in the diagram are not all pointing the same way because gas molecular motion is random. The important change is the average arrow length: in the hotter gas, the molecules have a greater average speed. The diagram keeps each molecule in the same position and direction in both panels so that the arrow lengths are easy to compare. In a real gas, the positions and directions would differ, and the molecules would still have a range of speeds.
Why average matters
The word average is doing important work. A gas contains a very large number of molecules. They collide, exchange energy and keep changing speed and direction.
So a hotter gas does not mean:
- every molecule has exactly the same speed
- every molecule speeds up by the same amount
- all molecules start moving in the same direction
It means the mean speed of the molecules is greater. There can still be some slower molecules in a hotter gas and some faster molecules in a cooler gas, but the overall average has shifted upwards.
This also explains why a gas can look still while its molecules are moving rapidly. The molecules move randomly in all directions, so their individual motions do not usually produce a steady movement of the whole gas in one direction.
| Weak wording | Stronger wording |
|---|---|
| "The molecules all move fast." | "The molecules have a greater average speed." |
| "The gas moves faster to the right." | "The molecules move randomly, but their average speed is higher." |
| "Temperature is the speed of one molecule." | "Temperature is linked to the average kinetic energy of many molecules." |
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