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Q & A: thermal expansion

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Most recent answer: 09/30/2013
What happen to the density of a body when temperature increases and decreases?
- angel (age 22)
The most common effect is for bodies to expand when heated, giving lower density. At most temperatures, for example, water expands when heated. In the range from 0C to 4C it contracts when heated, as do some other materials.
"Heating" means letting in thermal energy, allowing the atoms or molecules to get to higher-energy states. Usually, those states have molecules spaced a little farther apart than the low-energy states. Sometimes, there's a cluster of low-energy states which have the molecules spaced a little farther apart (like the states in ice, which has more volume than liquid water) and in those cases heating can lead to contraction.

Mike W.

(published on 03/07/2008)

Follow-Up #1: heat or cool to remove ring from finger?

I've heard that if you have a ring stuck on your finger, run cold water over it to make it easier to remove. Wouldn't that cause the ring (and opening) to contract, making it more difficult to remove? Shouldn't I run hot water over it to make it expand? What is the best way to remove the ring?
- Sami Steach (age 17)
Normal, IL, USA

That's an interesting question. As far as the ring goes, you're right that you want to heat it up to get it to expand. The complication comes from biology, not physics. When you warm up your skin, the small blood vessels (capillaries) tend to open up, making your finger swell. Cooling has the opposite effect. (There's a Seinfeld episode on a related topic.) 

The best way to test these effects of temperature would be with experiments, which you could do without having the ring actually on the finger, so as to avoid unnecessary pain. How much do the dimensions of the ring change as temperature is changed? WHat about your finger? Is there a difference in the time-dependence of the direct physical effect on the ring and the complicated physiological effect on the finger?

One possibility for the real case might be to soak your finger in cold water for a long time to reduce the blood flow, then to quickly heat up just the ring and remove it before the blood flow recovered. Quick heating might be done by heating some other piece of metal and contacting just the ring with it.

Mike W.

Try putting some butter on your finger.   LeeH

(published on 09/30/2013)

Follow-up on this answer.