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(I apologize for this elementary question. I don't know much about physics.)

Let's say that I put a metal pot in the refrigerator for several hours.

At this point, I guess, the pot and the air (in the refrigerator) have the same temperature.

Now, I touch this pot. It feels very cold. But when I "touch" the air (that is inside the fridge) it doesn't "feel" as cold. I don't feel the same "ouch!" that I feel when I touch the pot.

Why is that? Why does the metal seem colder than air although they both have the same temperature?

(I know that gas has less particles in it in one unit of volume compared to solids and liquids, but since "temperature" means "the average kinetic energy", these fewer air particles are supposed to hit my hand in a velocity that's going to compensate for their lower number, aren't they?)

A related question, for clarification:

If I use a thermometer to measure the temperature of the pot & air (let's assume it's a thermometer that has a probe that can touch objects), will it show the same reading for both? If so, what makes the thermometer different than my hand? I mean, my hand is sort of a thermometer, so why would it fail whereas a non-human thermometer would work?

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Veritasium has decent video about it, he compares a cake tin with the cake itself and a book and a metal object. He then asks different people on street what they think youtube.com/watch?v=hNGJ0WHXMyE –  SWeer Jun 17 at 17:40
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The interesting thing here is that your hand is not a thermometer; a thermometer measures the average amount of heat energy that is in an object, but that's not what you're measuring with your hand. "Feeling cold" or "feeling warm" with your hand is actually measuring how fast energy is moving between your hand and the object, not the average energy in the object. –  Eric Lippert Jun 17 at 18:51
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@SWeer, I clicked on this question specifically so I could link that Veritasium video. –  daviewales Jun 18 at 2:29
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As also the Veritasium video shows, you don't need a refrigerator for that effect. Compare the perceived temperature of a metal block (or a pot, or a blade) with that of a styrofoam block, both at room temperature. The metal block will feel colder while the styrofoam block might even feel warmer than the air around it, because it is such a good insulator (i.e. bad thermal conductor). –  Dubu Jun 18 at 7:13
    
Don't the hand and the thermometer both measure the temperature of themselves? Isn't it just that the thermometer reacts more quickly because it's made of metal? –  Wossname Jun 19 at 1:22

4 Answers 4

up vote 60 down vote accepted

Short answer:

The thermometer measures actual temperature (which is the same for both), while your hand measures the transfer of energy (heat), which is higher for the pot than the air.

Long answer:

Keyword: Thermal Conductivity

The difference is a material-specific parameter called thermal conductivity. If you are in contact with some material (gas, liquid, solid), heat, which is a form of energy, will flow from the medium with higher temperature to the one with low temperature. The rate at which this happens is determined by a parameter called thermal conductivity. Metals are typically good heat conductors, which is why metal appears colder than air, even though the temperature is the same.

Regarding your second question: the thermometer will show the same temperature. The only difference is the time at which thermal equilibrium is achieved, i.e. when the thermometer shows the correct temperature.

Final remark: the rate at which heat (energy) is drained from your body determines whether you perceive a material as cold or not, even if the temperature is the same.

For reference, here is a table which lists thermal conductivities for several materials:

enter image description here

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I night add that there are two more components - heat capacity of medium and it's density that can influence how cold it seems. It's sometimes analysed in terms of thermal diffusivity. –  Jarosław Komar Jun 17 at 18:02
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A TL;DR version of this answer is: Our skin measures energy transfer rather than temperature. –  Danu Jun 17 at 18:02
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So if I am super hot on a sunny summer day, I should lay on a silver bed in the shade? Sweet! –  David Wilkins Jun 17 at 18:19
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@Danu, I think the receptors actually respond to temperature of the receptors, since temperature is one of the determining factors of speed and intensity of biological processes. When you touch metal, the temperature of the receptors goes quickly down. When you touch air, your body is capable to counteract the loss of heat so that receptors stay near their natural temperature. –  Ján Lalinský Jun 17 at 18:30
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@JarosławKomar makes a very good point - that's why you can grab aluminum foil right out of the oven and suffer no ill effects, despite it's high conductivity. –  Chris White Jun 17 at 20:01

In essence heat transfer is what you're body is measuring. This video really hits the nail on the head of what you are wondering

www.youtube.com/watch?v=vqDbMEdLiCs

Our body senses heat flow from one source to a sink. When the rate of transfer is greater the object feels cooler/hotter. Objects acclimated to their room temperature will feel hotter or colder depending on the thermal conductivity. You can think of temperature as an absolute metric of sorts.

The bigger the temperature difference the hotter or colder an object will feel. But, thermal conductivity serves as a multiplier if you will. A 70 degree object that is sucking the same flux of energy through your fingertips as a 30 degree object would have a higher thermal conductivity. This means delta H would be the same for both objects, even if T is different and delta T.

We don't measure T, or change in T, but only the change in heat.

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Answer that exist only to provide a pointer to a off-site resource are defined as non-answers. What you have done here is slightly better than that because you have offered a phrase summarizing the situations, but most Stack Exchange users are unlikely to thick very highly of an answer like this. Physics SE seeks to be a repository of quality answers to quality questions, not a link farm. –  dmckee Jun 18 at 0:35
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While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. –  Kyle Jun 18 at 0:52
    
I didn't think my explanation would be any better than the video at hand but I'll include it anyways. –  Skyler Jun 19 at 4:29

I disagree with the opinion that your skin can measure heat transfer. It can only measure temperature, or to be more precise: the surface temperature of the body you are touching. Now the thermal diffusivity comes into play: When you touch a cold piece of wood (low thermal diffusivity), you transfer heat to the wood, the boundary layer of the wood warms up and feels warm. If, in contrast, you touch a cold block of steel (high thermal diffusivity), you transfer heat as well, but the heat gets transported quickly to the interior of the metal and thus the boundary layer stays cold.

For the same reason, cold water feels colder than cold air.

Indeed, this is due to a higher heat transfer, but the skin doesn't measure it directly.

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I would say that "measures" should be understood as "reacts more or less extreme to heat transfer depending on the rate". –  Frederic Brünner Jun 18 at 21:05
    
@FredericBrünner That's the definition of the word "measure". And a system (skin or technical sensor) can't react on a heat flux directly, but only on its effect, that is a change in temperature. A heated thermometer would measure a different temperature in water than in air, even when water and air have the same temperature. Does it also measure heat transfer? –  Peter Stock Jun 20 at 10:11

It is related to how fast the material can transfer energy. There's a name for that, thermal conductivity.

A quote from Wikipedia:

Heat transfer occurs at a higher rate across materials of high thermal conductivity than across materials of low thermal conductivity. Correspondingly materials of high thermal conductivity are widely used in heat sink applications and materials of low thermal conductivity are used as thermal insulation. Thermal conductivity of materials is temperature dependent. The reciprocal of thermal conductivity is called thermal resistivity.

Here's some resources for you:

http://en.wikipedia.org/wiki/Thermal_conductivity

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protected by Qmechanic Jun 18 at 16:39

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