When placing freshly purchased products into the refrigerator, we rarely think about the complex physical processes that instantly start inside every gram of the substance. However, it is at this moment that an active exchange of energy begins between the body (product) and the environment (refrigerator air). Understanding how internal energy this process will change allows you not only to pass an exam in physics, but also to competently organize storage, preserving the structure and taste of food.
From the point of view of thermodynamics, any object with a temperature above absolute zero has a reserve of energy. When you put a warm watermelon or fresh meat on a shelf where it is cold, the system tends to balance. The internal energy of products will inevitably decrease, losing heat to a colder environment. This process is not instantaneous and depends on many factors, including the thermal conductivity of the product itself and the intensity of air circulation.
It is important to understand that the change in internal energy is not just “cooling”. This is a combination of changes in the kinetic energy of movement of molecules and the potential energy of their interaction. In the liquids and solids that form the basis of our products, the dominant factor is the speed of the chaotic movement of particles. The lower the temperature, the slower they move, and the lower the energy reserve of the system.
Thermodynamic essence of the cooling process
The fundamental law that describes what is happening is first law of thermodynamics. It states that the change in the internal energy of a system is equal to the sum of the work done by external forces and the amount of heat transferred to the system. In the case of a refrigerator, external forces (compressor) perform work to remove heat, and the product itself loses this heat. Formally, this can be described by the formula where the change in energy is equal to the difference between the initial and final states.
Since the temperature of the product is initially higher than the temperature inside the fridge compartment, heat spontaneously flows from the product to the air. Internal energy U decreases. This decrease is quantitatively equal to the amount of heat Qgiven by the product. The minus sign in the equation indicates precisely the loss of energy by the body.
The speed of this process directly depends on the temperature difference. In the first minutes, when the contrast between warm soup and cold air is maximum, heat transfer occurs most intensely. As the temperature of the product approaches the refrigerator temperature, the rate of change in internal energy drops, tending to zero at the moment of complete thermal equilibrium.
Why does soup take longer to cool than water?
Soup contains fats and protein structures that have a different heat capacity and thermal conductivity compared to pure water, which slows down the process of temperature equalization.
Microscopic picture: what happens to molecules
To better understand how the internal energy will change, you need to go down to the level of atoms and molecules. Internal energy consists of the kinetic energy of the chaotic movement of particles and the potential energy of their interaction. When cooling products, it is the kinetic energythat first decreases. Molecules of water, fats and proteins begin to move more slowly, their vibrations become less amplitude.
This decrease in activity is directly related to the preservation of food. Enzymatic reactions and bacterial activity directly depend on the speed of molecular movement. By slowing down molecules, we slow down the biochemical processes of decay and fermentation. However, there is also a danger here: if the cooling occurs too sharply for some structures, damage to the cell walls is possible.
- 🧊 Reduced speed: The root mean square speed of molecules decreases in proportion to the root of the absolute temperature.
- 📉 Decreasing amplitude: Vibrations of atoms in the crystal lattices of the solid components of the product become less pronounced.
- 🔗 Changing bonds: During phase transitions (freezing), the potential energy of interaction of molecules changes, new bonds are formed.
Particular attention should be paid to water, which makes up the majority of most products. As the temperature decreases, hydrogen bonds between water molecules become more stable. This precedes a phase change into ice if the temperature drops below freezing. At this moment, the internal energy drops abruptly, as the latent heat of melting (crystallization) is released.
Heat transfer mechanisms inside the chamber
The process of heat removal from products is carried out through three main heat transfer mechanisms. Understanding their differences helps you position products correctly for the most efficient cooling. The internal energy of the product will decrease faster if all three mechanisms are involved simultaneously.
The first mechanism is thermal conductivity. It is relevant when the product is on a shelf or in a container. Heat is transferred from the inner layers of the product to the surface and then to the air or shelf. Metal refrigerator shelves have high thermal conductivity and “suck” energy faster than glass or plastic ones.
⚠️ Attention: Do not place hot pots directly on plastic shelves or thin plastic drawers. Local overheating can lead to deformation of the material, since the thermal conductivity of plastic is low, and it does not have time to dissipate energy.
The second mechanism is convection. Cold air, falling down, flows around the product, picks up heat and is carried upward (or forced by a fan in systems No Frost). It is convection that provides the main volume of heat exchange for products that do not lie tightly on the surface. In refrigerators with forced air circulation, this process is much more intense.
The third mechanism is thermal radiation. All bodies emit electromagnetic waves. A cold product absorbs less radiation from the walls than it emits if it is warmer than its surroundings. Although the contribution of this mechanism at such small temperature differences is small compared to convection, it also contributes to the overall energy balance.
Phase transitions and latent heat
The most interesting processes occur when the temperature of the product crosses the point of phase transition, most often the freezing point of water (0°C). If you place a bottle of water or minced meat in the freezer, its internal energy will continue to decrease, but the temperature may remain constant for some time.
This phenomenon is explained by the release latent heat of crystallization. Energy is spent not on reducing the speed of molecules (temperature), but on restructuring the structure of matter, the formation of a crystal lattice of ice. During this period, the internal energy drops sharply, since the potential energy of interaction of molecules decreases during the formation of stable bonds.
| Stage of the process | Change in temperature | Change in internal energy | Basic mechanism |
|---|---|---|---|
| Cooling above 0°C | Decreases | Decreases (kinetic part) | Convection and thermal conductivity |
| Crystallization (0°C) | Does not change | Decreases sharply (potential part) | Phase transition |
| Ice cooling | Decreases | Decreases | Thermal conductivity |
For products with high water content, this stage is critically important. The formation of large ice crystals can damage the cellular structure of fruit or meat, causing loss of juice when defrosted. Quick freezing (shock) minimizes this effect, since the crystals do not have time to grow to large sizes.
☑️ Rules for safe freezing
The influence of packaging on heat transfer
Packaging plays the role of additional thermal resistance. When placing food in the refrigerator, we often leave it in plastic, foil or plastic containers. These materials affect how quickly the internal energy of the contents changes. The air in the bag is an excellent heat insulator.
If the product is tightly wrapped in several layers of polyethylene, the cooling process slows down. The internal energy will leave more slowly, since the heat must first pass through the layer of still air and the film itself. This can be useful for preventing “weathering”, but is harmful if you need to quickly cool a perishable product.
Metal foil, on the contrary, can create a screen effect by reflecting thermal radiation, but in a refrigerator, convection is more important. However, if the foil fits tightly to the product, it acts as a good conductor of heat, accelerating temperature equalization across the surface. Vacuum packaging (vacuum packed) eliminates convection inside the package, leaving only thermal conductivity through the walls and contents.
⚠️ Attention: Hermetically sealed glass jars with hot liquid are strictly prohibited from being placed in the refrigerator. A sharp change in the internal energy of glass and liquid can lead to rupture of the container due to uneven compression.
Practical aspects of storage and energy efficiency
From the point of view of refrigerator operation, loading it with warm food is stressful for the compressor. The refrigerator must compensate for the internal energy that the products gave up, plus the energy that came from outside through the walls. The more warm foods you put in, the more electricity the device will use to restore the temperature.
The optimal strategy is to first cool the food to room temperature before putting it in the refrigerator. This reduces the load on equipment and prevents a local increase in temperature in the chamber, which could negatively affect the reserves already stored there. The internal energy of the products should decrease gradually and evenly.
It is also worth considering that different products have different heat capacity. Water has a very high heat capacity, so a watermelon or a bottle of milk will take longer to cool than a piece of meat or bread of the same mass. Understanding this helps plan the loading of the refrigeration chamber.
Is it possible to speed up cooling?
Yes, if you increase the contact area with cold air (put it in a flat dish) or use forced airflow (fan), but in domestic conditions it is easier to leave gaps between products.
Frequently asked questions (FAQ)
Will the internal energy increase if you put the product in the refrigerator?
No, the internal energy of the product will decrease. The product transfers heat to the colder environment of the refrigerator. The internal energy of the air in the refrigerator and the refrigerant, which carries this heat outside, increases.
Does the change in internal energy depend on the packaging method?
The final change in internal energy itself (the difference between the initial and final temperatures) does not depend on the packaging. However, the speed with which this change will occur directly depends on the thermal insulation properties of the packaging.
What happens to the internal energy when water in a product freezes?
When freezing, the temperature does not change, but the internal energy continues to decrease. This occurs due to a decrease in the potential energy of molecules during the formation of a crystal lattice of ice (latent heat is released).
Is it harmful for the refrigerator to put a hot pan?
Yes, it is harmful. A sudden release of a large amount of thermal energy will force the compressor to wear out and can lead to melting of neighboring products and the formation of condensation, which increases the risk of corrosion and ice formation.