Why are hot metals softer?

Why does metal feel colder than wood?

A closer look at our perception of warmth shows that our skin is by no means suitable as a thermometer. Because the perceived temperature depends on the material that we are touching.

At low temperatures, a metal fence appears much colder than a wooden fence. The fences - regardless of their material - take on the ambient temperature. This phenomenon can also be observed in many other everyday situations: The 21 degrees Celsius warm water in the sea appears significantly cooler than air with the same temperature. And even a wooden floor feels warmer than a tiled floor at room temperature.

So the difference is not made by the temperature, but solely by our perception of warmth. Apparently the skin is not a good thermometer. But what makes the metal fence colder and the wooden fence warmer for us, even though both are at the same temperature? The solution lies in the physical definition of “temperature” and “warmth”, which deviates from everyday understanding. Because there we often equate the two terms.

Water in various states of aggression

In physics, temperature is an objective measure of how warm or cold something is. It is determined by the constant disorderly movement of the atoms or molecules that make up a solid, liquid or gas. In general, the higher the temperature, the more the particles move. In gases, the atoms or molecules whiz around completely detached from one another, in liquids the building blocks of matter are still freely movable, but touch each other almost permanently - and in solid bodies each atom or molecule sits in a fixed place, but can still do a little to and fro, swing and rotate.

The countless particles that are in an iron fence, in a glass full of water or in the air move at different speeds - but their kinetic energy fluctuates around a certain average value. The temperature is a measure of the average speed of the atoms or molecules. However, only the disordered, random movement of the particles makes a contribution to this. If all the atoms in the iron fence were to move in one direction, the entire fence would move - and that would have no effect on the temperature.

Over time and without any additional external energy supply, a thermal equilibrium is established in every system - whether solid, liquid or gas: the temperature or the average speed of the atoms or molecules takes on the same value everywhere. But if you bring two systems with different temperatures into contact, the equilibrium is suddenly disturbed. Because in the body with the higher temperature, the atoms and molecules move more on average. At the contact surface, these particles therefore transfer part of their kinetic energy to the building blocks of matter in the body with the lower temperature.

So energy is transported - and not just on the contact surface: inside the two bodies, the more energetic particles give off their energy to their closest neighbors. As a result, the temperature of the two systems gradually equalizes until it finally has the same value everywhere and a new thermal equilibrium has been established. The transport of energy does not take place at the same speed in all substances - some transmit heat well, some poorly.

In an irregularly structured, porous material such as wood, it is relatively difficult for the atoms to pass on their kinetic energy. Wood therefore has a low thermal conductivity. If, on the other hand, the atoms are regularly arranged in a crystal lattice, the energy transport within the material works much better. In electrically conductive materials such as iron or copper, the freely moving electrons can also be pushed by the vibrating atoms and thus transport the heat even faster through the grid. Metals are therefore good conductors of heat. Iron, for example, conducts heat around 400 to 800 times better than wood.

Thermal conductivity also determines how warm or cold an object feels to us. The skin temperature is around 30 degrees Celsius. If we now touch a cool object, energy is transported from the warm skin to the cooler material. How quickly heat is withdrawn from our skin depends on the thermal conductivity of the material touched. Compared to wood, for example, metal cools the skin much faster due to its higher thermal conductivity.

Special receptors in the skin react to this flow of heat - i.e. register slow or rapid cooling or warming of the skin - but do not record the actual temperature of the touched object. Therefore, the two materials feel differently warm despite the same temperature. Of course, this effect also works the other way round: We burn our fingers on a hot piece of metal, while we can still touch a piece of wood at the same temperature without any problems.

It is very similar when we walk into sea water with an air temperature of 21 degrees Celsius: water conducts heat around twenty times better than air. As a result, our skin dissipates heat to the water much faster than it does to the air - and we freeze until a new state of equilibrium is established. If you leave the water, you freeze again. Another effect plays a role here: the water on the skin evaporates, removing heat from our body and the “perceived temperature” drops. The wind accelerates evaporation and thereby the flow of heat, which makes us tremble even more.

So the thermoreceptors in our skin can really lead us astray. If we want to determine a temperature, we should rather use a thermometer.