Cold doesn’t exist. But how is this possible if, especially in winter, we hear it clearly? The truth is that the cold is not a physical entity, it is not something that moves and comes upon us. In short, cold is not an active and direct phenomenon, but a bodily sensation which, from a purely physical point of view, is associated with measurable quantities such as temperature And heat.
Let’s start with the concept of temperature of a body. As we know, matter is made up of particles – atoms, molecules – and these particles they vibrate and move continuously: that is, they have a certain kinetic energy. Here, temperature is a macroscopic measure of the average kinetic energy that particles possess on a microscopic scale: the more they move and shake, the higher the temperature, and therefore we say that that body is “hot”; the less they agitate, the lower the body temperature will be, which we therefore define as “cold”.
Hot and cold, therefore, are concepts that have to do with our bodily sensation and are always relative to other bodies, while temperature is a quantitative quantity that does not need to be related to something else. It’s a similar discussion to color: colors do not exist, they are perceptions of our body associated with a measurable quantity which is the wavelength of the light that enters our eye.
Now, thermal energy – that is, the average kinetic energy of particles, associated with temperature – can be transferred from one body to another. But how? For example, consider a glass of water into which ice is placed. Clearly, the temperature of the water is higher than that of the ice, and this – from what we have said – means that the particles in the water are agitating and moving more rapidly than those in the ice. When water molecules collide with ice molecules, they tend to “make them move”, that is, they shake them! In scientific terms, the most agitated molecules are transferring part of its energy to the less agitated molecules. Here, this transfer of thermal energy is called in physics heat: the body with a higher temperature is passing energy, i.e. heat, to the body with a lower temperature.
In nature – you will have noticed – heat transfers spontaneously from bodies at a higher temperature to bodies at a lower temperaturewhile the opposite does not happen unless there is an external intervention (the refrigerator, for example, needs a motor and therefore an injection of energy from the outside). Going back to our example of the glass of water, the moment we insert ice into the water, the water will pass heat to the ice. So the water does not cool because the ice “passes the cold” to it, but rather it is the water to lose heatand that’s why it gets cold! This fact is so general and universal that it rises to a physical principle: the second law of thermodynamics.
To give another example, when let’s open the winter window we feel cold. According to the sensations we experience in our body, it may intuitively seem to us that there is this entity called “cold” outside that enters the house through the window, but in reality the opposite happens. There is no so-called “cold force” that transfers from outside to our room: we feel cold because of the environment inside the house – which has a higher temperature – is giving off heat to the colder air that is entering, thus lowering the temperature of the room.
In short, “feeling cold” essentially means that the flow of heat coming out of our body increases because we are in contact with something that wants to decrease our temperature. However, when we feel hot, the opposite happens: and heat, like cold, does not exist in the physical world but is only a bodily sensation. And if you want concrete proof of the difference between temperature and the physical sensation of cold/hot, here’s a little experiment that will make you think!
