Very good question!
Well, temperature of a
gas is related to how fast the gas molecules are
moving around, and how hard they crash into the
walls of the container. Gas molecules move faster
as the temperature of the gas increases, and when
they move faster, they have more MOMENTUM, so the
result is that they hit harder on the walls of the
tires. Since the tires are made of rubber, they
can expand - but only up to a certain limit.
So, the hotter you make the air in the tires, the
faster the air molecules move around inside, and
then they hit the walls of the tires harder. And
since they are moving faster, they also crash into
the tire walls more frequently (that is, more
crashes per second), and the result is they push
out the tires and make them expand.
rubber can only expand so far - so the pressure
If you could heat up the
air in a different container - one with walls that
could expand without any resistance, then the
pressure would not increase, if the volume of the
container could increase as the temperature got
hotter. But since there is a limit to the
stretching of the tires, the pressure inside will
increase as the air gets hotter.
That is why
you should fill your tires when they are cold, and
leave a little room for expansion, 'cause the air
inside gets hotter when you drive.
Cold air is more dense. Meaning that there are you
can pack more molecules of oxygen into a smaller
space. If you have more molecules in a smaller
space you have a greater pressure on that space.
However, the air as a result becomes more
compact. So, if you drive your car from the desert
to the cold mountains and do not add any air to
your tires you will get a decreased pressure in
that space due to the molecules becoming more compact.
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