www.bbc.co.uk/news/magazine-21744543
"When it is still we are mostly losing heat by convection. But when a breeze hits the face the initial heat loss is from conduction."
Convection is irrelevant without conduction, surely... What am I missing here?
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Convection is what makes boiling water freeze faster than cold water ;-)
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AFAIUI convection occurs in liquids or gases and is the movement of less dense warmed material and its replacement by denser cool material. The gas or liquid itself moves around in the form of currents. Ocean currents or the morning breezes as sun warms land/air unevenly are cases in point.
Conduction involves the passage of heat by warmer molecules passing heat to cooler molecules. Unlike convection it can occur in solids.
science.yourdictionary.com/convection
EDIT - I tihnk the message on BBC may be wrong. I dont think warm blooded animals can loose heat by convection. The body heats the surrounding air by conduction. Loss of heat may then give rise to convection currents in the air around the body.
Last edited by: Bromptonaut on Wed 13 Mar 13 at 13:06
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Surely a breeze is just forced convection? A car radiator will lose heat just by standing, but it will be much faster if you blow air through it with a fan. But the heat still has to be conducted from the hotter body (radiator, human) to the air before the warmed air can be moved onwards to be replaced by cooler.
Last edited by: Cliff Pope on Wed 13 Mar 13 at 13:23
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As a (lapsed) physicist I put it like this:
The face loses heat to cold air through conduction.
If there is no wind, that warmed up air will disperse slowly and rise being replaced by cold air - but slowly. (a convection current)
When there is wind, the warmed air is replaced quickly so more cold air is heated - by conduction - so heat loss is greater. (a bigger convection current).
there is also heat loss through radiation - which varies with the skin temperature :-)
Last edited by: madf on Wed 13 Mar 13 at 13:37
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what about wind chill factor? thats not been discussed for ages..... il get my coat
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This item says that some rabbits use their ears as a means of cooling by radiation; this may well be the case but I would have thought convection came into it too.
www.ehow.com/facts_5640376_rabbit_s-ears-warm_.html
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My head acts as a solar panel..
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if you put a tuning fork in a vacuum , where does the sound go?
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There is no sound but the tuning fork will vibrate very slightly longer as there will be no air resistance.
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so the sound energy doesnt escape..where does it go?
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>> so the sound energy doesnt escape..where does it go?
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Sound is (only) in the ear of the beholder?
There is no sound energy in the vacuum - which is why it will vibrate for a little longer in the vacuum.
Last edited by: pmh on Thu 14 Mar 13 at 17:57
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It would have to be attached to the side of whatever the vacuum is contained in, so the sound would leak up the stem and the container would vibrate.
A tuning fork only makes a sound when you touch it on something rigid.
My wife once referred to a tuning fork as a pitch fork - quite witty I thought, if unintentional.
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Yes, that's right. Sound has to have a medium to travel through.
There's a cheap trick you can do: you pluck two prongs of a table fork and say you have the sound in your hand and will drop it into a glass. As you open your fingers over the glass, you surreptitiously touch the end of the 'four candle' to the table. The resulting sound astonishes your victims.
There are people who just don't understand that vacuum doesn't transmit sound. Science fiction films often have sound when a spaceship passes in, er, space. Really gets up my nose that. Similarly, a very intelligent late friend of mine simply couldn't get his head round the truth that in space, in free fall, there is no up or down. He figured down was past your feet and up was above your head. Intelligence has its limits.
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"There are people who just don't understand that vacuum doesn't transmit sound."
Are you trying to tell me that there isn't an orchestra up their playing "The Blue Danube'.
Come on, I saw the film.
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>> Are you trying to tell me that there isn't an orchestra up their playing "The Blue Danube'.
That isn't sound, it's the intrusive music that so often messes movies up (but not in the case you mention).
I'm talking about the sort of roar you get in run-of-the-mill space movies. Goddam crap.
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Broadly speaking neither stage and TV dramas, films nor TV documentaries need much music, especially when people are speaking. Beeb kindly note!
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For the benefit of anyone who missed my reference to 2001.
www.youtube.com/watch?v=q3oHmVhviO8
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so to answer my original question.. where does the sound energy go in a vacuum?
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"where does the sound energy go in a vacuum?"
Surely there is no such such thing as "sound energy". Sound is only the means by which the energy of a vibrating object ,say a tuning fork is dissipated into a surrounding gas liquid or solid. If there is no means for the energy to be dissipated in that manner i.e the object is vibrating is in a vacuum the energy cannot escape by that means. The vibrating object will of course get warmer and the energy will be radiated in the form of heat.
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does the energy stay in perpetuam as we say.. im still lost for an answer
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"does the energy stay in perpetuam as we say."
Yes, energy can neither be created nor destroyed. It is just converted from one form into another.
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Exactly. 'Sound' is the 'good vibrations' of the air. In said vacuum there is nothing to vibrate, ergo, no sound.
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The tuning fork loses it's kinetic energy by heating up rather than transmitting sound.
Some of this energy may of course be transmitted as electromagnetic radiation in the infra-red spectrum.
Like what CGN sez :-)
Last edited by: Lygonos on Thu 14 Mar 13 at 23:58
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A tuning fork is just a kind of spring really, but designed to resonate at an audible frequency, and more so if held against a sounding board to amplify it.
You could compress a car spring and push it out of the spaceship window, and it would drift off into space boinging away silently to itself until it lost it's energy through heating and radiation.
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NASA points out that although humans can't hear it, there is sound in space.
"Q. Is there really sound in space?
A. Actually...yes!!
What is sound? It is a pressure wave. So long as you have some kind of gaseous medium, you will have the possibility of forming pressure waves in it by "shocking" it in some way. In space, the interplanetary medium is a very dilute gas at a density of about 10 atoms per cubic centimeter, and the speed of sound in this medium is about 300 kilometers per second. Typical disturbances due to solar storms and "magneto-sonic turbulence" at the Earth's magnetopause have scales of hundreds of kilometers, so the acoustic wavelengths are enormous. Human ears would never hear them, but we can technologically detect these pressure changes and play them back for our ears to hear by electronically compressing them."
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That's interesting - thank you NIL.
We should keep listening. One day we might hear an extra-terrestial with a very deep voice saying something to us.
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i saw a documentary a while back.. and apparently outer space smells of barbequed beef according to space suit sniffers at NASA
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Convection doesn't involve conduction? - Lygonos
