1. How Temperature Affects Molecular Motion
Temperature tells us how fast the molecules of a gas are moving on average. When temperature increases, the molecules gain energy and move faster. When temperature decreases, the molecules slow down.
This increase or decrease in molecular motion changes how a gas behaves.
2. Temperature and Average Kinetic Energy
The kinetic energy of gas molecules is directly proportional to temperature. If the temperature doubles (in Kelvin), the average kinetic energy doubles.
2.1. Key Formula
\( KE_{avg} = \dfrac{3}{2} k_B T \)
Higher temperature → higher kinetic energy → faster molecules.
2.2. Meaning
Temperature is not just a measure of hotness—it is a direct measure of the average molecular energy.
3. Effect of Temperature on Pressure
When a gas is heated, its molecules hit the container walls more frequently and with greater force. This increases the pressure if volume is fixed.
3.1. Everyday Example
A sealed aerosol can should not be heated because rising temperature increases the molecular motion, which increases the pressure inside the can.
4. Effect of Temperature on Volume
If the pressure is kept constant, increasing temperature makes gas molecules move faster and spread out more, increasing the volume.
4.1. Why This Happens
Faster molecules push the container walls outward, making the gas occupy more space. This explains why hot air rises—it expands, becomes lighter, and moves upward.
5. Effect on Speed Distribution
At any temperature, gas molecules have a range of speeds—some slow, some fast. When temperature rises, the whole speed distribution shifts toward higher values.
5.1. Key Idea
Average, most probable, and RMS speeds all increase when temperature increases.
6. Examples Linking Temperature and Motion
- A balloon shrinks in cold weather because molecules move slower and exert less pressure.
- A hot air balloon rises because warmer air inside moves faster and becomes less dense.
- Car tyres become harder after long driving because friction heats the air inside, increasing molecular motion and pressure.