So I was asked by my 7-year-old tonight. No good answer.
Suggestions?
JohnW77707
Because the granite is good at conducting heat away from your hand, so it feels cold.
Some substances conduct heat better than others. Things like granite and steel tend to feel cold because they conduct heat away from your hand better than, say, wood or plastic.
The Museum of Science here in Boston used to have among many other exhibits a block of metal and a block of wood with thermometers stuck in them. The wood was cooled below room temperature, and the metal was heated above room temperature, but the metal always felt colder because of this effect.
Whether something feels cold doesn’t entirely depend on its temperature – that’s why metal and stone feel cold even if they’re at room temperature. The reason has to do with heat conductivity and specific heat capacity. Or so I recall – I remember reading a detailed explanation, but I don’t know where.
Specific heat capacity is the amount of heat required to change the temperature of a substance. It takes significantly more heat to change the temperature of water (about 4200 J to raise the temperature of 1 kg of water by 1°C) than it does for granite (800 J/(kg·°C) or iron (470 J/(kg·°C). Basically, this means that a comparatively large amount of heat (thermal energy) must be transferred from your body (at 37°C) in order to increase the temperature of a granite countertop by a small amount.
Thermal conductivity is a bit easier to understand. Metals are good thermal conductors – they’re very good at transferring heat, and thus they draw heat out of your body quite readily when you touch them. Thermal conductivity is a major reason why metals at room temperature feel cold to the touch, but granite is not a particularly good conductor of heat. Thus, I think the difference in specific heat capacity is more important than the difference in thermal conductivity in explaining why granite countertops feel cold.
I agree. Also, remember that “standard room temperature” is about 30F (or for Europeans, 17C) cooler than body temperature, so that ability to conduct heat away (better than air) can feel substantial. Heat content (specific heat) or the amount of heat per unit mass (or volume) also matters. A hair dryer may emit air at 130-150F without causing pain, but concrete or asphalt at that temperature can burn your feet, and water or steam that is much above 150F can scald you badly.
It.s actually quite a useful lesson. I’m sure you can expand on it for your son, from personal experience.
The best example I’ve heard that your 7-yr old will understand better is the cake in the oven: the pan conducts heat much better and is thus hot enough to burn you while the cake conducts heat less and you can touch it for a short period without feeling much pain. Also, the air in the oven will burn you even less. And all three are the same temperature.
Yeah, you can walk over hot coals, but try that trick over hot metal.
Just wanted to chime in to say that this is a brilliant example. I’m going to use it incessantly. Eventually I will have forgotten where I heard it, and I will come to believe that I just came up with it on my own. My apologies in advance.
I think I read it in Skeptic magazine. It was used, as Philster pointed out, to debunk firewalking.
I remember Mike Lucas did a story on it on the Straight Dope TV show, but I can’t remember if he used the example. He did get a little tiny blister, I seem to remember.