Why does metal feel colder than wood? Metal feels colder than wood because metal has a far higher thermal conductivity than wood does. There are two things we need to look at here: how metal conducts heat, and how our hands sense temperature.
Heat is not actually a thing. Heat is a process. It is the transfer of thermal energy between things. If you put a saucepan full of water on top of a gas burner, you are transferring the energy that is in the natural gas you are burning into the metal saucepan and from there into the water you are trying to boil. Natural gas has a lot of energy trapped in it that can be freed when it is burned. Natural gas comes from decomposed and compressed plants and animals, and the energy in it comes from the energy the plants were able to make through photosynthesis. This is stored as high energy methane. This energy is freed when the gas is burned and converted to thermal energy, which is transferred to the saucepan. Thermal energy can only ever be transferred from an area of high thermal energy to an area of low thermal energy.
The saucepan you are using to boil the water is made of metal because of metal’s high thermal conductivity, which is a measure of how quickly thermal energy can pass through the object. When we heat the saucepan, the thermal energy from the burning natural gas reaches the molecules in the outer layer of the saucepan and makes them vibrate. The thermal energy is converted to kinetic energy. The more thermal energy, the more they vibrate. The molecules on the outer edge of the saucepan vibrate so much that they hit the next layer of molecules, and they pass their energy on, making those molecules vibrate as well. And those molecules pass the energy on. The closer together the molecules are, the denser the material is, the more easily they can pass the thermal energy on. The molecules in wood are widely spaced apart, so it is more difficult for thermal energy to transfer through wood. Silver has the highest thermal conductivity of any metal, although diamond is almost 2.5 times higher.
What this means is that when you touch a piece of metal, the thermal energy in your fingers makes the atoms in the metal vibrate, and that thermal energy very quickly gets passed away from your fingers and through the metal. When you touch a piece of wood, that doesn’t happen, and the thermal energy stays in your fingers.
The second thing we need to look at is how our fingers sense hot and cold. We have temperature sensitive nerve endings in our skin, but they don’t work on the temperature of the object we are touching. They work on how much thermal energy is entering the body or leaving the body. These receptors are called Transient Receptor Potential channels. When they receive thermal energy, they open, and when they lose thermal energy, they close. This changes the flow of ions passing through them, and that is converted into an electrical signal that goes to the brain, and the brain judges it as hot or cold. If we are in a hot environment or a cold environment, or if we touch boiling water or ice, that works. However, it doesn’t work if we touch a piece of steel and a piece of wood that are both exactly the same temperature. The steel will conduct more thermal energy out of our fingers, and the sensors in our fingers will react, sending a cold message to the brain, telling us that the metal is colder than the wood, even though it isn’t.
You can prove this if you put an ice cube on a piece of steel and on a piece of wood. Our brains tell us that the steel is colder than the wood, so we would expect the ice cube on the piece of wood to melt faster than the one on the piece of steel, but the exact opposite happens. This is because the steel can conduct thermal energy from the air to the ice cube much faster than the wood can, and this melts the ice cube faster. The wood isn’t as good a conductor and acts as an insulator, preventing the ice cube from melting. That is the main reason why more houses are made of wood than metal. And this is what I learned today.
Sources
https://www.exploratorium.edu/snacks/cold-metal
https://en.wikipedia.org/wiki/Heat
https://www.digikey.jp/ja/articles/thermal-conductivity-what-is-it-and-why-you-should-care
https://www.bbc.co.uk/bitesize/guides/z2gjtv4/revision/2
https://en.wikipedia.org/wiki/List_of_thermal_conductivities
Photo by Archie Binamira: https://www.pexels.com/photo/grey-metal-frame-2463351/