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Hot and Cold

Do you look around the natural world and wonder?  Wonder why things work the way they do, why things are the way they are?  If you do, then there’s a bit of physicist inside of you!  Physics is all around us.

Let’s wonder together about temperature, and how and why certain objects are either hot or cold to the touch. Your immediate reaction to the topic might be simply that hot things feel hot and cold things feel cold. Not necessarily! For example, you’ve probably had the experience of walking in bare feet on a tile floor on a cold winter’s morning, then walking on a rug on top of that floor. The tiles felt much colder, yet because the tiles and rug were in the same room they were almost certainly both at the same temperature.

Do you think you could touch something that is, say, 450 degrees Fahrenheit?

One of the ways we measure hotness or coldness is by our sense of touch. Do you think you could touch something that is, say, 450 degrees Fahrenheit? How long could you touch it without feeling intense pain? The answers might surprise you… sure you could, and probably for quite a while. Have you ever, say, reached into a hot oven to nudge a potato while it’s baking? The temperature of air in the oven was perhaps 450 degrees Fahrenheit, and yet you don’t get burned.  

How can this be? How can you touch something, even air, which is so hot and not feel discomfort? There are two bits of physics that some into play. First, when two objects of different temperature are in contact, the rapidly moving molecules in the hotter one smack into the cooler object’s slower moving molecules and make them move rapidly, too. This process of energy transfer, which causes the hotter object to lose energy and therefore get cooler and the cooler object to gain energy and therefore get hotter, is known as conduction.  

When you touch a good conductor, it quickly carries energy away from your hand, making it feel cool to the touch. The opposite is true of a poor conductor. So consider again the tile floor and the rug. Tile is good conductor of heat, so it quickly draws energy from your bare feet. The tiles feel cold. Carpet, made of wool or something similar, is a poor heat conductor. It does not draw energy from your feet as quickly, so the rug feels warm.  

The other reason you can touch the air inside your hot oven without discomfort is that there just aren’t that many air molecules in contact with your hand. So it’s not only how hot the material is that you touch, and not just how well it conducts heat, but also how much of it there is.


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