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For a science project i am doing tennis balls bounce and if temperature effects it. The bit about the bounce was good but i need to know how thermal diffusion works in it. Does the expansion of the rubber allow the molecules to escape easier or does the heat speed up the energy state of the air inside enough to allow them to escape quicker?
- Andrew Piper (age 13)
St Leos, Sidney, NSW, AUS
Good question. You're right that the air gradually
leaks out by thermal diffusion. That's the little random motions that
the molecules have. Since the hotter they are, the more of that motion
they have, you're right that the balls should leak more at high
temperature. I'm not sure, but I'd bet your right that the rubber also
gets leakier when it's hotter, because its molecules shake more and
make it easier for the air to go through.
Just to make sure
no one misunderstands, the immediate change you see in bounciness when
you change the temperature just comes from changing the air pressure
inside the ball. When it's hotter, it has higher pressure. The changes
from the air leaking out by thermal diffusion happen very gradually.
For information about how heating up / cooling off a ball works, check out the answer to the question Hot and Cold Basketballs
-Tamara & Mike
(republished on 07/19/06)
Follow-up on this answer.