Wednesday, March 07, 2018

Quick Physics Fix: Why Metal Feels Colder

I want you to try something: Find an object nearby that's made of metal, and something else made of wood or plastic. Put a hand on each. Which one is colder?
Unless one of the objects you picked just came out of the fridge, or is actively producing heat (like your phone might), the answer is "neither"...but you probably wouldn't guess it just by touch. It's something that everyone has noticed on some level, but most people don't take the time to consciously think about. So what's going on here?

The answer has to do with thermal conductivity, rather than the temperatures of the objects themselves. If both objects have been in the same room for any significant length of time, odds are they're truly at the same temperature as one another, and the same temperature as the air around you. That temperature is likely to be lower than your body's temperature, though, which means that  heat will flow from your fingers into the objects they touch.

This sensory trickery results from the fact that, when you touch something, you're not directly sensing the temperature of the thing itself; you're only able to sense the change in your own skin's temperature.

We can only perceive the world as it affects us—and in doing so, we affect it; you can only tell that an object is cold by transferring some heat energy to it.
Image Credit: S. Skolnick (CC0)
This fact combines with a difference in thermal conductivity to dupe our sense of thermoception. A metal object that conducts heat easily can carry thermal energy away from the body much faster than an insulating material like wood or plastic at an equal temperature. In the insulator, your body's heat transfers relatively slowly to the material, and what heat DOES transfer quickly warms up the surface layer, bringing it close to body temperature. The conductor, on the other hand, absorbs heat more rapidly, and wants to share that heat throughout its entire bulk. Since your nerves can only tell you about the rate at which they're losing heat, there's little sensory difference between a cool conductor and a much colder insulator.

This trick works in reverse, too, at the other end of the temperature spectrum—as you've undoubtedly experienced if you've hopped into a car that's been baking in the summer sun. The leather or fabric of the seats might be uncomfortable, but the belt buckle can give you a downright nasty burn if you're not careful.

That's your physics fix for the day—and a quick reminder that we can only trust our senses to the extent that we understand how they work.

—Stephen Skolnick

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