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# Physics/Sound

Question
QUESTION: Why does sound travels faster in higher temperature in the same medium ? According to me it should travel slower because when a medium is heated the particles move apart so the distance increases this should reduce the speed of sound............?

ANSWER: The distance increases if you allow the density to decrease, yes, but so do the velocities of the scattering molecules.  Therefore, while the density term in the speed of sound will decrease, you still have to allow for the faster molecular collision rate due to the increase in molecular speed.

---------- FOLLOW-UP ----------

QUESTION: Sir can u tell me what is the molecular speed & why does it increases when density decreases?

No, no, the average molecular speed (which is just what it sounds like, the average speed of the molecules as they fly around in the gas).  It doesn't depend on the density, it depends only the temperature.  The density also depends on the temperature and the pressure.  If the temperature increases, the average speed of the molecules increases.  I was just noting the density decrease if you allow the pressure to remain the same, as that will change the speed of sound in a far more complex way. If you just heat up the gas in a closed container like a gas cylinder, the speed of sound inside it will increase because of the higher molecular speeds...which will cause more frequent collisions between the molecules.  The collisions drive the pressure wave that we call sound.

Physics

Volunteer

#### Dr. Stephen O. Nelson

##### Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

##### Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/Credentials
Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.