2.27. Amontons’ Law http://www.ck12.org
2.27 Amontons’ Law
- State Amontons’ law.
- Explain why the pressure of a fixed volume of gas increases as the gas heats up.
Lawrence checked the air pressure in one of his tires before driving to school on a cold winter day. The tire pressure
gauge registered 32 pounds of force per square inch. After making the 10-mile trip to school, Lawrence checked the
air pressure again. This time the gauge registered 34 pounds of force per square inch. Lawrence didn’t add any air
to the tire in between the two measurements, so why did air pressure in the tire increase?
Keep on Rolling
There was no additional air in the tire the second time Lawrence checked the air pressure, but something did change
between the two measurements. The tires had rolled over 10 miles of road on the trip to school. Any time one
surface moves over another, it causes friction. Friction is a force that opposes the motion of two surfaces that are
touching, and friction between two surfaces always generates heat. Quickly rub your hands together and you’ll feel
the heat generated by the friction between them. As the tires moved over the road, friction between the tires and
road generated heat. In short, the tires got warmer and so did the air inside them.
It’s the Law!
The space inside a car tire is more-or-less fixed, so it has a constant volume. What happens when the volume of air
is constant and its temperature increases? Lawrence found out the answer to that question when he measured the air
pressure in his tire. Increasing the temperature of a gas such as air, while holding its volume constant, increases the