Have you ever wondered why the fridge door always seems harder to open if we try to do it a few seconds after closing it? This is a very common “problem”, which has in no way to do with the quality of the refrigerator, but more with the physics and the shape of the appliance, which favors the escape of cold air when we open it. The cause is, in most cases, thehot air (or rather at room temperature) that enters the fridge every time we open the door, and it tends to cool quickly when we close the refrigerator. Cooling, the air your blood pressure decreases creating a slight “vacuum” that requires a force majeure to open the door, a bit like when you open a vacuum jar for the first time. After a few minutes, thanks to the internal circulation of the air, the pressure normalizes and we can open the refrigerator again.
Air coming out, air coming in: what happens
Whether small units or gigantic “American-style” two-door appliances, i refrigerators they are normally vertical, often with a built-in freezer and an opening on one side. The shape is dictated by the practicality of having all the food and drinks visible and easily reachable, unlike chest freezers where the opening is at the top and the dishes inevitably end up being “buried” under other food.
However, this form favors cold air escapes every time the door is opened and the rapid exchange of air with the outside air. Usually in a fridge the air temperature is around around 4 °C and when we open the door it tends to go downwards because denseri.e. “heavier” (to use a more common term, even if improper in scientific terms). It falls then out of the refrigerator and it is immediately replaced by the decidedly warmer outside air.
If we want, we can compare this phenomenon with that of a hot air balloon landing: to return to the ground after the flight the air in the balloon is cooled, in this way the cold air mass tends to go downwards, precisely because of the higher density
When we open the freezer, which works at even lower temperatures (around –18 °C) the phenomenon is more evident and sometimes even visible, because in high humidity weather conditions it is possible to notice a light mist that tends to fall on the floordue to the water vapor that condenses in contact with the cold air coming out of the freezer.
The ideal gas law and pressure differences
To explain why it’s harder to open the refrigerator a few seconds after closing it, we also need to involve a little physics. In gases, properties such as density, temperature and volume are closely related, as indicated by ideal gas law:
p · V = n · R · T
Spoiler: in reality gases never behave like a perfect gas, but for our purposes it is an adequate approximation.
In this formula Rit is a constant, n it is the number of moles, a unit used in chemistry to count atoms or molecules (in practice it is a measure of the quantity of substance). p represents the pressure, T the temperature e V the volume of the gas. Even though the density is not directly indicated in the formula, it is related to the ratio n/Vi.e. the quantity of substance per given volume. Given that the volume V inside the fridge, as well as the quantity of substance (or rather of substances present in the air such as N2O2CO2), are constant, the only variables will be the temperature T and the pressure p.
We said that the cold air falls out of the fridge, while the hot air enters: once the fridge door is closed we will therefore have a mass of hot air which will tend to cool due to contact with much colder walls and foods. As the temperature decreases of gas (air), also the internal pressure of the fridge will decrease, Why T And p they are directly proportional, as indicated by the ideal gas law.
So the pressure inside the refrigerator will be lower than the outside one and as a consequence, the external air at higher pressure will “push” with more force on the door to keep it closed. For this reason, at least in the first few seconds after closing the door, the effort to reopen it will be considerably greater.
After a few minutes the pressure difference normalizes
At this point, however, it is legitimate to ask ourselves a question: if the air in the fridge goes to lower pressures as it cools, why can I still reopen the door normally after a few more minutes?
The “normalization” of internal pressures occurs above all thanks to internal air circulationwhich is often common between fridge and freezer. This circulation is useful to avoid the formation of warmer areas but also of humid areas and mold inside domestic or industrial refrigerators. Furthermore, even if the seals are in excellent condition, the seal is never perfectly watertighttherefore in the presence of even a small difference in pressure, like the one just described, a minimal quantity of air can be “sucked” into the fridge, favoring the stabilization of the pressures and allowing us to access our dishes again.
In conclusion, the advice if you want to avoid the “vacuum” effect is to simply wait a few minutes to allow the internal air pressure to equilibrate, and at that point the fridge door will open normally.
