Sometimes, it’s the everyday things that are most confusing. Things like, what are those weird dots on your windshield? Why is pink salt more expensive than the regular stuff? Does blowing on your soup actually do anything?
And here’s another one you’ve probably thought about every cold day since you were about six. Why does metal feel so much colder than wood, at least on a cool day? Why does a cake feel less hot than the cake tin it was cooked in? How the heck is this guy holding a red-hot cube with his bare hands??
The answer is pretty simple – but it does require a bit of lateral thinking. Turns out, you’re just not the objective observer you think.
Why does metal feel colder than wood or plastic?
Imagine you’re outside, facing both a tree and a streetlamp. Theoretically, they should be the same temperature, right? They’re both outside, in the same place; neither has only just come out of the oven or anything. And yet we know, almost instinctively, that the streetlamp would feel colder to the touch than the tree. So, what’s going on?
Well, the clue is in the phrasing: it feels colder. But, somewhat counterintuitively, it isn’t literally colder.
“When you touch something, you don’t actually feel temperature,” explained Derek Muller in a 2012 video for his YouTube channel Veritasium. “You feel the rate at which heat is conducted, either towards or away from you.”
In other words: “It’s about thermal conductivity.”
To illustrate the concept, he invited people to compare the temperatures of a book and a hard drive – objects which he confirmed using an infrared thermometer to be the same temperature. As you might expect, everybody he asked decided the book was warmer – with some even accusing him of lying when he revealed the truth.
The science is sound, though. “The hard drive felt colder even though it was at the same temperature roughly as the book,” Muller explained, “and that’s because the aluminum conducts heat away from your hand faster than the book conducts heat away from your hand.”
“[That] makes the hard drive feel colder and the book feel warmer.”
But here’s something you might not expect: the same thing works in reverse. In other words, given two things warmer than your body, the metal will feel hotter even if the materials are the same temperature.
Now, in a way, this is kind of obvious, right? If you’ve ever baked a cake, for example, you’ll know that the cake tin will “feel” hotter than the cake inside – again, this is because the metal is a much better conductor than the cake, so it’s imparting thermal energy into your hand much more efficiently. But it can lead to some unintuitive results – as Muller showed with an experiment using one block of plastic, one of aluminum, and two ice cubes.
“I’ll put an ice cube on both plates. What will we see?” he asked volunteers – all of whom had told him the aluminum felt colder than the plastic. Unsurprisingly, given that, they intuited that the ice on the “colder” aluminum would stay solid, and the block on the “warmer” plastic would melt.
Instead, the exact opposite happened.
Why? “The aluminum block is melting the ice faster than the plastic block because it’s conducting the heat to the ice cube faster,” Muller explained. “Plastic […] is a worse thermal conductor. Heat is being transferred less quickly to the ice block, so it’s staying cold.”
Why is metal such a good conductor of heat?
So, we’ve sorted out why metal can feel so much colder or hotter than other materials of the same temperature – it’s because they’re usually much better heat conductors. But what is it about metal that gives it this property?
To answer these questions, it helps to understand what “thermal conduction” actually entails. See, once you get to a high enough resolution, heat is just another way of saying movement: “When a material absorbs heat energy, that energy is transformed into kinetic energy, causing the atoms to move,” explains Xometry.
“But, as atoms in solids don’t have much room to move, they start vibrating, and the ones directly exposed to the heat start crashing into their neighbors,” the article continues. “This collision excites the neighbors, and they also begin to vibrate. As this happens and continues to move along from the hot to the cold part of a material, the heat begins to move further down too. It’s kind of like a ripple that spreads from a pebble hitting the surface of a pond.”
Now, think about this for a little while, and you start to realize that metal has a few advantages over, say, wood when it comes to thermal conductivity. Metal is going to have its atoms and molecules more densely packed than wood, making it much easier for more particles to bump into each other; in the same vein, wood will literally have holes in it – useful for moving water and nutrients up from roots, but not great for creating an unbroken chain of jiggling molecules.
The fact that wood is a compound also plays a part in its low conductivity – it’s made up of cellulose, hemicellulose, lignin and tannin, which in turn break down into a whole range of elements in different amounts depending on which tree you got it from. When particles encounter these changes, they get scattered, and deflected from their path – essentially, diluting the thermal conduction through the material.
But what really gives metal the edge over, say, plastic – a material which can also have regular and densely-packed molecular structures, and yet simultaneously have relatively low thermal conductivity – is its free electrons.
“Some of the electrons in a piece of metal can leave their atoms and move about in the metal as free electrons,” explains BBC Bitesize. “The parts of the metal atoms left behind are now positively charged and are called metal ions.”
“When the free electrons absorb heat energy, they move much faster,” it continues. “As they move through the metal, free electrons crash into metal ions. Some of the kinetic energy of the free electron is absorbed by the ions and it vibrates faster and with greater amplitude.”
To put this in terms of something easier to imagine – think about a whole bunch of people riding a packed subway. They’re being jostled, sure, but generally speaking, it would take quite a big bump to get them to collide with each other. They’re our material molecules.
Now imagine each one of them is trying to juggle a collection of ping-pong balls.
Immediately, you can see that the number of collisions is going to increase – and the same is true when you introduce free electrons into a system.
The upshot: thermal conductivity in materials with free electrons – i.e. metals – is “very much faster than conduction caused by just passing vibrations from atom to atom,” Bitesize explains. “Hence, conduction in metals is faster than in non-metals.”
Long story short…
In summary, then: why do metals feel colder than non-metals? It’s because you’re not actually feeling the temperature at all – rather, you’re feeling how well the material moves thermal energy away from your body. And metal, thanks to its particular molecular makeup, is really, really good at that.
