Science Buddies: "Ask an Expert"

I want to do a science fair project on the air you blow out. For example... When your hands are cold you blow on them to warm them up. The air coming from inside your mouth is usually warm. However, when your food is hot you blow on it to cool it off. Depending on how you hold your mouth and lips the temparture of the air you blow out changes. I want to explain how and why, I just don't know how to go about it.

Hey Md,
Think of the relative differences for the two examples you gave. You warm your hands with your breath because the air from inside your body is warmer than your hands that have been loosing heat to your environment.
When you cool your food with your breath, the food is much warmer than your breath but that may not have much to do with it, in that case the heat of the food creates and isulating layer around it which you are disrupting with your breath, thereby causing the food to cool faster.

Explaining these two effects could make a decent project and would fall under the subject of Thermodynamics.

Here is a link with some of the basic physics of heat transfer. You might also look up a concept known as "entropy".

http://hyperphysics.phy-astr.gsu.edu/hb ... ra.html#c1
Jim

The difference isn't in your breath or how you blow, but in the temperature of the objects you are blowing on.

Your hands are cold. Your breath (which is internal body temperature) is warmer than your hands, it will feel warm to your hands.

Your food is hot. Your breath (same temperature as when you blew on yoru hands) is colder than the food, it will cool the food a little.

Heat transfer can be a good topic, but the kind of heat transfer you do when blowing on something is air convection, probably the hardest kind to work with in a simple way. Plus extraordinarily hard to measure and keep consistent, if you actually want to use human breath.

In addition to the air temperature discussion by Bradley, your breath has very high humidity. (That could be an experiment too.) Ever notice how your cold hands feel damp after you blow on them? They are condensing water from all the steam you are blowing off.

When conducting this experiment, you need to take the humidity into consideration. Remember how cool objects get fogged over when you breathe on them?

In terms of cooling, evaporating water out of your lungs is a significant source of cooling for the body, in addition of skin evaporation. For a dog who is panting, the breath is about the only cooling mechanism. When an air condition engineer is determining how much cooling a building will require, each person inside is rated at 600 BTUs per hour. Most of this is related to the humidity increase from our breathing.


Robert Reavis

As with the other 3 replies above, my first reaction was one of "it's not HOW you're blowing, but relative temperature" as well.

But then I actually did what the original poster said. I blew on my hands as if I was blowing on hot food, and then blew on them as if I was warming them up when it's cold. And what do you know, the latter feels significantly warmer.

Try it yourself!

Just based on that, I think that's a GREAT idea for a project. It's counter-intutive, at least to us science geeks, and there's something a bit subtle going on. But it's something everyone can do for themselves.

I suspect what's going on is a few things. First, the speed at which you blow; the cooling blow is faster. But then I tried to account for that, and still my cooling blow felt cooler than my warming blow. There's a difference is the lips and cheeks, so I suspect that there's something going on with the how the air is travelling and the warming blow has a higher humidity -- the warming blow does feel "wetter" than the cooling blow.

So, some things that I suspect you'll want to measure would be:
-- speed of the blown air (X inches per second)
-- the rate of the blown air (X cubic inches per second)
-- temperature of the air being blown
-- humidity of the air being blown

For the first two, you might be able to get a device to measure it from your doctor, since something like that would be used for asthma testing. You or a friend might also have one. It's called a Peak Flow Meter, as I recall.

For the temperature, you'll need a device that measures temperature quickly -- a normal thermometer won't work. I'm thinking something digital, but I can't recall having come across anything that doesn't involve infrared, which won't work with air. Anyone else have an idea?

For the humidity, I'm also at a loss since the only humidity devices I'm familiar with are for using with the weather, so they wouldn't suit you well. Again, something digital is probably called for.

I very much agree with Taed's post. I tried mdgibson's experiment whilst reading the post (yes, I am that easily led! ) and had the same observations.

What makes it interesting is that the sensation of cooling when the air is blown quickly across the skin may well be because the breath is reducing the heat at the site - thus giving the sensation of cooling, just like on a breezy day. Would this cooling phenomenon be observed if the same breath was blown across a (digital) thermometer? If the thermometer was at body temperature, I would imagine that it would. If the thermometer was at ambient temperature, I'm not so sure. This is what makes it interesting!

I also absolutely agree with the posts made by Jim, Bradley & Robert, in that the temperature of the object is what is important, as well as the effect of humidity. By the way, there are commercial humidity measuring devices on the market, but you may have to contact your local college/university to see if you can gain access to one.

So, there are a variety of different factors that you can measure - you just need to focus on your experimental design, and what hypothesis you are trying to test.

Please let us know your thoughts!
Geoff.