Q.Air pressure in a car tyre increases during driving. Explain.

Concept: Thermal Radiation Properties – The tyre heats up due to friction and deformation, increasing the average kinetic energy of air molecules inside.

1. During driving, the tyre flexes repeatedly and rubs against the road. This mechanical work converts into heat, raising the temperature of the tyre and the air inside it. …

The question is about a real, everyday observation — but the physics behind it is pure kinetic theory and the ideal gas law. When you drive, the tyre is not just rolling; it is constantly deforming. The rubber flexes, the tread grips the road, and the sidewalls bend. All of this mechanical work turns into heat. That heat flows into the air inside the tyre, raising its temperature.

Now, the tyre is a nearly fixed volume container. The air inside cannot expand much because the tyre walls are stiff. So you have a fixed mass of gas trapped in a constant volume. For an ideal gas (and air is close enough), the relation is:

This is a direct consequence of the ideal gas law $PV = nRT$. If $V$ and $n$ are constant, then $P$ and $T$ are proportional. Raise $T$, and $P$ must rise.

Let’s walk through the chain of reasoning step by step.

1. Where does the heat come from?

   As the tyre rolls, the rubber is repeatedly compressed and released. This hysteresis (internal friction in the rubber) generates heat. Also, friction between the tyre and the road surface adds more heat. The tyre gets noticeably warm to the touch after a long drive — that warmth is conducted to the air inside.

2. What happens to the air?

   The air molecules gain kinetic energy from the heat. Their average speed increases. In kinetic theory, temperature is a measure of the average kinetic energy of the molecules. So the temperature of the air inside the tyre rises.

3. Why does pressure increase?

   Pressure is caused by molecules colliding with the tyre walls. Faster molecules hit the walls harder and more often. Even though the volume hasn’t changed, the number of collisions per second and the force per collision both increase. The result is a higher pressure.

   Mathematically, from $PV = nRT$:

Since $n$, $R$, and $V$ are all constant, $P$ is directly proportional to $T$. A rise in $T$ means a rise in $P$.

4. Is there any other factor? …

Switch from the macroscopic ideal-gas-law view to the microscopic kinetic-theory picture for extra intuition: driving flexes and grips the tyre, pumping mechanical energy into the rubber and then into the trapped air molecules, so they move faster on average. Faster molecules strike the (essentially fixed) tyre walls both harder and more often per second, and pressure is nothing but the rate and force of these collisions — so pressure must rise even though no ex …