Why is stone cold?

• December 17th, 2012, 01:17 PM
icewendigo
Why is stone cold?
We have a stone statuette that always appears to be cold, always colder than the ambient temperature, and than any other objects(wood, fabric, etc).
1- What us with that, how does that work? I would have thought after a number of days or months it would reach the same temperature as every other object in the room, why not?

An other thought. Ive read somewhere that Fluaescent colors appear brighter because they reflect UV light at a different wavelength that of the color (if I understand correclty if something is orange fluo, you see the typical light reflected by orange objects plus some of the UV that's added to the light of that spectrum).
2- If this is true, my Question is, can something similar occur with infrared, where infrared is reflected as non-infrared light and thus might appear to be less warm?
• December 17th, 2012, 01:31 PM
tk421
Quote:

Originally Posted by icewendigo
We have a stone statuette that always appears to be cold, always colder than the ambient temperature, and than any other objects(wood, fabric, etc).
1- What us with that, how does that work? I would have thought after a number of days or months it would reach the same temperature as every other object in the room, why not?

As you muse above, after "a number of days or months" (and sooner than that) it will have equilibrated with its surroundings. But stone conducts heat well, and a statue, being a large object, will have a large heat capacity as well. So if you touch it, it will do a fine job of moving you to its temperature. If the stone happens to be at a temperature well below your body temperature, you will perceive it to be cold.

The flip side is that, if the statue were left out in the desert, say, you would be asking why stones are hot.

Quote:

An other thought. Ive read somewhere that Fluaescent colors appear brighter because they reflect UV light at a different wavelength that of the color (if I understand correclty if something is orange fluo, you see the typical light reflected by orange objects plus some of the UV that's added to the light of that spectrum).
2- If this is true, my Question is, can something similar occur with infrared, where infrared is reflected as non-infrared light and thus might appear to be less warm?
None of that is relevant if we assume that your first condition -- a long "soak" in the ambient" -- is satisfied. Once the statue is in equilibrium with its surroundings, you don't care how it got there. It is the temperature that it is.
• December 17th, 2012, 07:59 PM
pmb
Quote:

Originally Posted by icewendigo
We have a stone statuette that always appears to be cold, always colder than the ambient temperature, and than any other objects(wood, fabric, etc).
1- What us with that, how does that work? I would have thought after a number of days or months it would reach the same temperature as every other object in the room, why not?

Below I will use the standard terminology that "room temperature" means 70F.

tk421 said " it will have equilibrated with its surroundings". When the stone comes to thermal equilibrium its tempeature will be room temperature and at room temperature stone will always feel cool to the touch.

Stone has what is called a high thermal conductivity which means that it allows heat (i.e. thermal energy in motion) to flow quickly. Suppose that the stone is at room temperature. Your body has a temperature of 98.6F. Since heat flows from hot to cold heat will flow from your body into the stone. Since the thermal conductivity for stone is high the heat will flow quickly. The sense of coolness that you feel happens when there is a large flow of heat out of your body. If instead of stone it was wood then the heat flow would be less (since the thermal conductivity for wood is small) and thus wood feels warmer than stone. This is the same reason that your kitchen floor is cold to walk on. Kitchen floors usuall are covered with a material that has a high thermal conductivity and your body is always warmer than room temperature.
• December 17th, 2012, 08:30 PM
Harold14370
The body's core temperature is about 98.6F but the skin surface temperature is lower. A comfortable temperature is around 91F.
• December 17th, 2012, 09:21 PM
pmb
Quote:

Originally Posted by Harold14370
The body's core temperature is about 98.6F but the skin surface temperature is lower. A comfortable temperature is around 91F.

Thank you.

Note: For purposes of illustration we use things like a uniform temperature over the body in order to simplify the physics in order to explain the nature of the problem. There is no need set use a model with a perfect model of a bodies temperature distribution. Each part of physics is learned through ideal models such as this. When it comes to solving a real life problem and precision is of extreme importance it is then we worry about exact details such as the surface temperatrure of the body at the points of contact of the body with the heat source/sink.