Which process in the universe creates cold

Does warmth go out of the window or does cold come in?

In winter, an often-heard request: "Quickly, close the window, it will come in cold". But if cold is the absence of warmth, how can “nothing” come in? A closer look at this question quickly brings you to the core terms of the theory of heat - or may one also say the theory of the absence of cold?

In summer you want it, in winter you don't want it.

With this question, the everyday understanding differs very much from the physical conception. Of course: in winter the window is open, cold air comes in, and warm air will certainly go out too.

We say: “Cold comes in and warmth goes out”.

From a physical point of view, however, cold is the absence of warmth, ie “nothing” - and nothing cannot come in. Just as shadow is absence like light.

By the way, according to Aristotle, day is the absence of night.

What is there now? And what is missing?

Thermal energy is the energy that is stored in the kinetic energy of atoms and molecules. They move in all directions. So heat is the energy of the more or less moving atoms and molecules. In a hot body, the average kinetic energy of the atoms and molecules is high. In a cold body, the average kinetic energy of the atoms and molecules is low.

That means: the warmth of a body / system is the totality of the movements of the molecules of a body, which does not result in any resulting movement, i.e. its internal movement is disordered, the body does not move in a certain direction.

At absolute zero (-273,… degrees Celsius, 0 Kelvin) everything comes to a standstill - ideally, one that is not reached. Because something is still moving in the last decimal places. The absolute zero point can therefore never be completely reached.

Process variable and state variable

There is also another definition of heat: it is the flow of energy. That energy that flows from a body with high internal energy to a body with low energy. Energy is a state variable. Heat is energy in motion, heat is a process variable. When you are alone in the universe, you cannot feel any warmth. Only when energy flows from you into a surrounding system you say “huh, it's cold”, when energy flows to you then you say “warm”. To do this, however, the surrounding system is needed first.

So that's how warmth goes out of the window. “Nothing” (cold) cannot come in. Physically.

In fact, it is like this: Cold (motionless) air molecules come in, and warm (fast-moving) air molecules go out. Everyday feeling.

The answer to this question boils down to a precise definition of heat and a separation of physical definition from everyday understanding.

Because we're there: heat is the flow of energy. Temperature is a measure of the kinetic energy of the randomly moving atoms and molecules.

Even when it's cold, it's still quite physically warm. Around 273 Kelvin. But be careful: strictly speaking, temperature is different from warmth.

3 mechanisms of heat transfer

There are three mechanisms by which heat can be transferred. Heat flows - by itself - always from hot to cold. From a body of high internal energy to a body of low energy.

1) Heat conduction: through direct contact, the kinetic energy of atoms and molecules is passed on through collisions. Exactly: When bodies of different temperatures come into contact, the faster (air) molecules of the warmer body collide with the slower ones of the cooler body. This leads to the transfer of energy, the faster molecules (warm) become slower, the slower molecules (cold) faster, until all are at the same speed. The molecular velocities are equal and the temperature is equalized. This happens first at the interfaces, and through further collisions also inside the bodies of different temperatures or average molecular speeds. Through these molecular collisions, the energy is transferred from the hotter to the colder body. This “overflowing” energy is heat

In our case this would mean: the warmer air, more precisely the faster air molecules, meets the colder air, more precisely the slower air molecules.

Example: hotplate that is in contact with the pot; or on the beach: we stand on the hot sand, the brine absorbs the heat energy until both are equally warm.

2) Convection / heat flow: Molecules are moved as a whole, they transport the heat energy. Example: thermos, central heating, warm air parcels rise; Thermals as well; or ocean currents: The current can come about by itself, due to the differences in density, due to temperature differences. Or be generated artificially by means of a pump or blower.

3) Thermal radiation: The heat transfer takes place here in the form of an electromagnetic wave. For example: light, but also radio waves. The heat transfer can take place over very long distances and by vacuum. Electromagnetic waves also transport thermal energy through empty space.

For example: on the beach - we are warmed up by the sun's rays or food in the microwave absorbs energy in the form of heat; Kalchelofen. This stove heats the things in the room and not the air, which protects the slime today and makes the room cozy.

Keyword greenhouse effect: light goes through glass into the interior of a glass house / greenhouse. It heats the objects and the air there. This reflects back thermal radiation, electromagnetic radiation in the infrared range. However, these frequencies cannot escape through the glass and are opaque to infrared radiation. The energy stays in the greenhouse.

Still interested?

Take a look at the main principles of thermodynamics. They are real foundations. It couldn't be easier or more precise.


Photo by Hannah Tims on Unsplash.