Why does cold metal seem colder than cold air?

Question

(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?

Answer 1

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:

Answer 2

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.

Answer 3

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.

Answer 4

It is more complicated than the physics of heat transfers. Our tactile sensations are pretty weird.

One example would be that humans can taste "cold" and "cold" affects other tastes.

There is not enough research done regarding the processes. In the multitude of cutaneous receptors, you have several that relate to temperature.

One type of nocireceptors, which are responsible for "dangerous" stimuli, react to temperature extremes.

The two types of thermoreceptors register hot and cold differences. The cold receptors have been shown to react to warming stimuli as well... They are also located deeper into the dermal layer which suggests warming stimuli should be sensed first.

There are also corpuscle bulbs which is believed to be how you taste "cold".

The thermoreceptors on your tongue could also affect how something tastes relative to its temperature. Taste is even more complicated since it involves at least 3 "separate" senses and the fact that some taste chemicals taste differently at different temperatures. Fructose favors the fructopyranose state over the fructofuranose at lower temperatures and it tastes orders sweeter than other common sweeteners.

A thermometer measures temperature through equilibrium.

I noticed people mentioning conductivity which is probably the best way to explain it for a small range of temperature changes. Once you get to larger gradients or extremes, it would depend on several factors including which one triggers first, second, third, at all. Then you have to consider lateral/temporal inhibition, polarization states, graded potentials, NT gates, etc. Finally, you have to consider if any of these signals propagate to the brain and how the brain interprets the whole jumbled mess...

Answer 5

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

Answer 6

These are papers related to this subject. Thermal effusivity plays a very significant role in transient phenomena such as touching an object during a very short time:

E Marín Thermal physics concepts: The role of thermal effusivity The Physics Teacher 44, 432-434 October 2006

E. Marín Teaching thermal physics by Touching. Latin American Journal of Physics Education 2, 1, 15-17 (2007)