As I started my car to run some errands this weekend, I noticed that my tire pressure was low. Instead of the recommended 30 psi, my tires were around 28 psi. This phenomenon happens every winter when the temperature drops below freezing. This drop in tire pressure with the drop in temperature occurs due to Gay-Lussac’s law. This law explains that if the number of moles and volume of the gas within the tire remain constant, then a change in temperature is directly linked to a change in pressure within the tire. Because temperature is a measurement of kinetic energy of the gas particles within the tire, when the temperature drops, the kinetic energy also drops. A drop in kinetic energy means gas particle collide with the tire’s exterior walls less often and with less force, thus causing a lower overall tire pressure. 
Therefore, a drop in temperature by 20 ºC (68 ºF) from the summer to the winter caused about a 2 psi pressure drop in my tires, thus, causing the low tire pressure light on my car to turn on.
Image:
http://www.scpr.org/news/2016/06/21/61825/extreme-heat-leads-to-rise-in-flat-tires-spent-car/
I think there is more going on than just the simple PT relationship. The weight of the car is constant, so the net force exerted by the tire against the ground must remain constant. Since the tire is flexible, one would think the volume would contract (tire goes flat) only to the extent that the pressure is restored. A rigid tire would exhibit a faithful PT relationship. But real world observation disagrees with my comment, so something else must be at play. As the tire contracts, not only does it's volume get smaller, but it's contact area with the ground increases. Thus for a constant weight, and increased ground contact, a lower pressure is needed to support the fixed weight vehicle. I know this is an old thread but I hope I generate some comments.
ReplyDeleteIf Volume stays constant, then the formula is more or less correct, asssuming ideal gas. But tire volume is not really constant because it is like a balloon and contracts as temperature decreases
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