This morning, while preparing beakfast, I picked a plastic box from the fridge and noticed that it felt… not cold to the touch. My first thought was that I may have left the fridge door ajar during the night but then I remembered that I’d had problem opening it because I had my hands already full. So, the door had been shut properly.
I touched various things in the fridge (glass, metal) and they all felt cold but not the plastic ones. They felt somewhat lukewarm.
This reminded me of something that I’ve noticed when I do the dishes: the glasses are almost too hot to handle and perfectly dry when I take them out of the dishwasher. Ditto for forks and knives. However I can grab the plastic bowls and cups comfortably and they are still dripping wet.
Is it due to the physical properties of plastics ?
Wood will also not feel as warm or cold as metal or glass, for the same reason: low heat conductivity. Both plastics and wood are insulators: they transmit heat badly. Objects with low heat conductivity are worse at absorbing or providing heat to your own hand than those with high values; what you’re noticing is not a difference in temperature but in energy flow.
This sums it up nicely. See thermal inertia, the equation for which includes the density, specific heat, and thermal conductivity of a material. Compared to plastics, metals have high densities and high thermal conductivities, so they tend to have much higher thermal inertias: for a given temperature, a piece of steel can dump more heat into your hand more quickly with less decrease in its own temperature than a piece of polyethylene at the same starting temperature (that “less decrease in its own temperature” is what the term “thermal inertia” is describing). Your hand really does get hotter holding the steel than it does holding the plastic.
As both of these posts admirably show, now add cast iron (those kick ass behemoth pans), which hold heat, hold heat hotter, it just occurrs to me, than anything you normally find as kitchen equipment (I’m not sure which of the “iron” indexes in the cite of joema.
High thermal conductivity and low inertia is why copper pans are the bomb–“oh shit the flame is too high…” and lower it: on a copper pan, instant relief; on cast-iron, the pan will stay blasting away for an eternity.
So, in layman’s terms, plastic objects are just as cold/hot as the other items in the fridge/dishwasher, just not as good as the metal/glass ones at transmitting it?
Still, it’s funny how this phenomenon tricks our senses. I would have sworn that the various objects I touched this morning had very different temperatures.
Well, the actual message from the senses is “thing A makes me cold/hot more/quicker than thing B”; it’s the interpretation that jumps from there to “thing A is colder/hotter than thing B”.
So long as we end up avoiding or protecting ourselves from whichever things make us cold/hot too fast, the message has worked. When I take a tray from the oven using oven gloves, the side of the gloves touching the tray gets as hot as the tray itself pretty fast - but my hands don’t. The message works.
Your fingers have two things limiting their temperature rise:
They have their own thermal inertia. A small fleck of very hot metal (e.g. a hot chip from a high-speed drill) will feel hot for a split-second before it cools down, and your finger will never get terribly hot because it’s got plenty of mass throughout which to spread that heat.
your fingers have their own internal cooling system pulling heat away from the skin. It gets moved down into the deeper tissue by direct thermal conductivity, and by the blood convecting heat away as it flows to/from your skin. When you touch something hot for an extended period, your finger will eventually reach an equilibrium temperature where the cooling rate (from your blood/tissue conductivity) matches the heat input rate from the warm object. That equilibrium temp will be higher for contact with an object of high thermal inertia, which is why metal feels hotter than plastic (i.e. your finger really does get hotter), despite the metal and plastic being at the same temp.
If you attach a small thermometer with very low thermal intertia to the surface of a plastic block and another to the surface of a steel block, with both blocks having just come out of the dishwasher, you’ll find that both blocks of material really are at the same temperature, despite what your subjective sense of touch might tell you.
There’s a really nice experiment one can do with this. You have two blocks of identical size in the freezer, take them out and touch them? Block A feels coldest, B barely feels cold to the touch. Okay, which will melt ice faster?
You place an ice cube on each of them and it melts seriously fast on the “cold” block to “everyone’s” surprise. Why, because one is made of aluminium and conducts heat to the ice cube a lot faster than the one made of hard foam plastic which is a great insulator.
I think cast-iron pans are also typically thicker than copper/aluminum pans, i.e. there’s more stored heat just by virtue of there being more stored mass.
While the metal and plastic are initially the same temperature, once they are touched they no longer remain so. The finger almost instantaneously cools down the contact surface of plastic that it touches, and since the plastic is poor conductor, the internal heat of the plastic cannot travel quick enough to balance out the heat being removed by the finger at the surface, so the contact surface of the plastic remains cool to the touch (despite the internal temp of the plastic remaining hot).
If the contact surface of the plastic and the metal were artificially maintained at the same temperature, they would both feel hot.
Well, this is also equivalent to the old standby of a volume’s “total” heat versus measurement of its temperature, which is by definition a measure of some level of molecular energy at some level of near-instantaneous time (hedges so chemical physics and quantum guys can lob their grenades): a drop of hot bathwater, at some temperature, on your skin, vs a whole-body dunk in the whole tub of water at the same temperature.
This. And hot metal feels hot because it moves heat into your body.
By the way, it’s not just the property of the material, but also the size (weight or mass). A spark from a grinder is very hot, and has very high thermal inertia, but very little mass. So it doesn’t have much heat to transfer to your body, and doesn’t feel hot when it lands on your arm. Similarly, if you remove an aluminum foil and an aluminum pot from a freezer, the pot will feel colder, because it can remove much more heat from your hand than an aluminum foil. (The aluminum foil starts removing heat, but very quickly warms up to body temperature, and at that point it can’t remove any more heat from you.)
