When you have just returned from the store and put a bag of groceries in the refrigerator, one of the fundamental processes of physics is triggered, which we rarely notice in everyday life life. Internal energy the contents of the package begin to change rapidly, following the strict laws of thermodynamics. This process is invisible to the eye, but it is what ensures the safety of food and prevents the proliferation of bacteria.
To understand what is happening inside the chamber, it is necessary to consider the behavior of the molecules that make up the products. At room temperature they are in a state of active motion or vibration, possessing a significant supply of kinetic energy. By placing them in a cold environment, we create conditions for heat transfer, as a result of which energy is transferred from a hotter body to a colder one.
In this review, we will analyze in detail the physical mechanisms of this phenomenon, explain why temperature is a measure of average kinetic energy, and answer the question of where exactly it goes heat taken from your lunch. You will learn how aggregate state water affects the rate of cooling and why some foods cool faster than others.
The physical essence of internal energy
Internal energy is the sum of the kinetic and potential energy of all the molecules that make up the body. When we talk about cooling, we are primarily interested kinetic energy in the chaotic movement of molecules. The higher the temperature of an object, the faster its particles move, and the higher its internal energy.
By placing a warm product in a refrigerator, we are actually placing it in an environment with a lower average kinetic energy of air molecules. According to the second law of thermodynamics, heat only flows spontaneously from hot to cold. Therefore heat flow always directed from the product to the refrigerator evaporator.
It is important to note that internal energy depends not only on temperature, but also on the state of the substance. For example, when water freezes (phase transition), the temperature may remain constant, but the internal energy will continue to decrease as the molecules lose the energy needed to maintain a liquid structure.
⚠️ Attention: Do not confuse temperature and internal energy. Temperature is a measure of average energy, and internal energy also depends on body mass. A bucket of warm water has more internal energy than a glass of boiling water, despite the lower temperature.
Heat exchange mechanism in the refrigerator compartment
The process of heat removal from food in the refrigerator occurs mainly in two ways: convection and thermal conductivity. Air, in contact with the surface of a warm product, heats up, becomes lighter and rises, giving way to colder layers. This creates natural circulation of air masses.
If the product lies directly on the shelf, part of the heat is removed through thermal conductivity the shelf material. Metal grids or glass shelves with an aluminum base work as effective heat exchangers, speeding up the process of temperature equalization. That is why it is not recommended to cover shelves with polyethylene or mats that do not allow heat to pass through.
The rate of change in internal energy directly depends on the temperature difference. In the first minutes, when the temperature gradient between the product and the air in the chamber is maximum, the process is most intense. As it cools, the difference decreases and the rate of heat transfer drops.
The influence of the state of aggregation on energy
The most interesting processes occur when the product contains water. As long as the temperature is above freezing, the decrease in internal energy causes the temperature to decrease. However, when water begins to turn into ice, the latent heat of fusion comes into play latent heat of fusion.
During the phase change, the temperature of the product can remain at 0°C for a long time (or slightly lower, depending on the dissolved substances). At this moment, the internal energy decreases, but the kinetic energy of the molecules (temperature) does not change. Energy is spent on breaking the bonds that hold the molecules in the liquid state.
After complete freezing, further cooling again leads to a drop in temperature. Ice has lower thermal conductivity than water, so freezing the center of the product takes much longer than its initial cooling.
| Process stage | Temperature change | Change in internal energy | Physical process |
|---|---|---|---|
| Fluid cooling | Decreases | Decreases | Decreased kinetic energy |
| Crystallization | Constant (0°C) | Decreases | Latent heat release |
| Ice cooling | Decreases | Decreases | Reduced crystal vibrations |
| Evaporation (open container) | Decreases | Decreases sharply | Leave of the most energetic molecules |
The role of humidity and evaporation
The internal energy of products changes not only due to heat transfer, but also due to the evaporation of moisture. The “fastest” water molecules, which have the greatest kinetic energy, can leave the surface of the product, taking with them a significant amount of energy. This process is called evaporative cooling.
In modern refrigerators with the system No Frost the air is very dry, which intensifies evaporation. If the product is not packaged, it will lose not only heat, but also weight. This leads to the so-called “freezer burn” and a change in taste.
Sealed packaging prevents weight loss, but slows down heat transfer, creating an air gap. Air is a poor conductor of heat, so in a closed container the product will cool longer than an open one, but will retain its structure better.
Why does open water cool faster?
Open water loses energy not only through the walls of the container, but also through the evaporation surface. The most energetic molecules evaporate, taking heat with them, which accelerates the overall process of reducing the internal energy of the system.
Practical aspects of food storage
Understanding the physics of the process helps to organize storage correctly. Warm products placed in the refrigerator locally increase the air temperature, which can temporarily disrupt the operating mode compressor and negatively affect already cooled neighbors.
To minimize the load on equipment and maintain the quality of food, it is recommended to cool the dishes to room temperature before sending them to the chamber. However, keeping them out of the cold for too long is also dangerous due to the risk of bacterial growth.
The optimal solution is to use a rapid cooling zone, if your refrigerator model has one. In this zone, fans operate at increased power, providing intense heat exchange and a rapid decrease in the internal energy of loaded products.
☑️ Rules for safe cooling
Calculation of changes in energy and time
The quantitative change in internal energy ($\Delta U$) can be estimated by knowing the mass of the product ($m$) and its specific heat capacity ($c$). The formula looks like $\Delta U = c \cdot m \cdot \Delta T$. The greater the mass and temperature difference, the more energy the refrigerator must remove.
The time required for this process depends on the heat transfer coefficient. Metal utensils conduct heat better than ceramic or glass ones. Therefore, soup in an aluminum pan will cool faster than in a clay pot, even at the same initial temperature.
It is worth considering that the refrigeration unit cannot instantly compensate for the heat influx. A sharp increase in heat load can lead to a short-term increase in temperature in the entire chamber, which is critical for perishable products.
⚠️ Attention: Technical characteristics of refrigerators (freezing power, refrigerant, compressor type) may vary. Read the instructions for your model to find out the maximum permissible thermal load.
Frequently asked questions (FAQ)
Does the internal energy of the air in the refrigerator increase when we put a warm product there?
Yes, at the moment of heat exchange, the air (and the walls of the chamber) receive energy from the product. However, the cooling system (compressor and condenser) works to remove this heat outside into the room. The internal energy of the air in the chamber tends to return to the set level.
Why does the ice in the freezer not get colder indefinitely?
The internal energy of the ice stops decreasing when its temperature is compared with the temperature of the evaporator or the set operating mode of the thermostat. Further energy removal stops or is balanced by the influx of heat from outside.
Does the color of the package affect the rate of change of internal energy?
Inside a closed refrigerator, the influence of color is minimal, since the main mechanism is convection. However, dark surfaces absorb and emit thermal energy better, which theoretically may have a slight effect on radiative heat transfer, but this effect can be neglected.
Can we say that cold “transfers” to the product?
From the point of view of physics, no. Cold is the absence of heat. “Cold” does not pass into the product, “heat” leaves it. The energy flow vector is always directed from a more heated body to a less heated one.