Hot Tires and Cold Drinks
“That tire looks a little flat. But I just checked the pressure yesterday when we got back from our trip and it was fine.” Tire pressure will change depending on the temperature. By understanding how temperature affects pressure (and how pressure affects temperature), we can better understand our tires, our air conditioners, and a variety of other pressurized devices we may have around the house.
Why It Matters
- The label on the aerosol can says “Do not leave on stove or expose to heat.” The warning label goes on to point out that the can could explode under such conditions. The label gives another clue when it says “Do not puncture this can”. Something is under pressure inside the can and puncturing it could lead to a rapid release of pressure, creating a hazardous situation. In the same way, heating the gas increases the molecular motion of the propellant molecules, producing an increase in pressure.
- Tire pressures are sensitive to temperature. Pressures will differ in the summer as compared to winter and will be affected by how much driving is being done. In competition racing (such as NASCAR), the tire pressure is a crucial component of performance. These tires are usually filled with nitrogen, which contains less moisture than air. The moisture would expand as the tire heats up, causing changes in handling. By using nitrogen, this factor is minimized.
- Air conditioners and refrigerators operate on the basis of gas expansion and temperature changes. Compression of a coolant gas causes it to liquefy and increase in temperature. Moving this liquid through a condenser allows the material to vent thermal energy to the outside of the building. The liquefied material then goes through a pinhole into the evaporator, where it expands rapidly back into a gas and absorbs heat from the surroundings.
- Watch a video at the link below that illustrates Gay-Lussac’s Law: https://www.youtube.com/watch?v=zSL6RT1DEHI
Can You Apply It?
Use the links below to learn more about Gay-Lussac’s law. Then answer the following questions.
- What are common propellants used in aerosol cans?
- What type of products would use dimethyl ether as a propellant?
- Approximately how many psi will your tire pressure change with a 10°F change in temperature?
- What was the pressure change for the tire stored in the shade? In the sun?
- What is the role of the metal fins on an air conditioner?