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Cold Drinks, Faster (Heat Conduction 101)

July 28, 2009

The problem: You volunteered bring beer to a picnic, you’re running late, and the beer is warm. What do you do? Whatever you decide, do NOT put them in the refrigerator! It will take well over an hour. There is a simple solution, but first let’s learn a little about heat conduction:

1. Everything has heat
Let’s talk about heat – solids, liquids, and gases are made up of molecules. Molecules vibrate and interact with each other, and this is known as thermal energy. Thermal energy also manifests as heat – a glass of hot water has more thermal energy than a glass of cool water.

2. Heat passes between any two substances 
Heat (or thermal energy) is transmitted between substances, where thermal energy from the warmer object always passes to the cooler object. If you touch a hot iron, thermal energy from the iron passes into your hand, and I have a scar to prove it. Conversely, if you hold a cold soda, thermal energy passes from your hand to the soda. The process of heat (thermal energy) moving from one source to another is called conduction (details, with math here).

3. But, heat does not pass evenly between substances
However, thermal energy does not pass through all substances evenly. Metals, like copper and aluminum conduct thermal energy extremely well. The most important point here is that thermal energy passes through water 30 times faster than through air (geek details here)*! Think about it – the “insulation” in many travel coffee mugs is a trapped layer of air. Heat does not pass through air very well, so this slows down the transmission of heat from your coffee to the outside, keeping it warm and delicious for a long time.
*of course, nothing is this simple. I’m ignoring some other aspects of heat transmission, particularly convection.

4. If you want cold drinks now, do not put them in the refrigerator
So, if you need to chill some beer or wine, by far the fastest way to do this is by submerging your drinks in ice water. This will only take 5-15 minutes!

Heat conduction is neat! Tell me more….
-You can think about this problem in reverse. If you need to thaw something that is frozen, like a piece of chicken, don’t leave it on the counter or in the fridge. Instead, place it in a bowl of warm water – you’ll be amazed at how fast it will thaw.
-When you’re going out in the cold, dress in layers. There will be air pockets between each clothing layer. As you now know, air is a poor conductor of thermal energy, so the layers of clothing will help you retain your body heat better then a single, thicker sweater.
-Otters have thick fur coats that keep air bubbles next to the otters’ skin, which keep the otters warm while they’re diving. Smart move, otters!

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