When you place a warm product in the refrigerator, a fundamental physical process starts heat transfer, which often becomes the subject of questions in school tests and physics olympiads. Internal energy a body, consisting of the sum of the kinetic energy of movement of molecules and the potential energy of their interaction, directly depends on the temperature of the substance. As soon as the object finds itself in an environment with a lower temperature, an intensive process of heat removal begins.
From the point of view of thermodynamics, the answer to the question “how the internal energy has changed” is clear: it has decreased. This happens because the product molecules, having a high speed of movement, collide with colder air molecules inside the chamber and the walls of the package, transferring part of their energy to them. As a result, the average kinetic energy of the product molecules decreases, which we perceive as a decrease in temperature.
However, the process is not always linear, especially if the product contains water, which can freeze under certain conditions. In this case, the change in internal energy consists of two stages: first, the temperature decreases to the point of phase transition, and then the latent heat of crystallization occurs without a change in temperature, but with a further decrease in the internal energy of the system. Understanding these nuances is important not only for passing tests, but also for proper food storage.
⚠️ Attention: Sharp cooling of hot foods can disrupt the temperature regime in the fridge compartment, causing the compressor to work with overload. Always cool food to room temperature before serving.
The physical essence of the cooling process
To accurately answer the test question about where the energy goes, it is necessary to consider the heat transfer mechanism. In a household refrigerator, the main method of heat removal is convection i thermal conductivity. Cold air, falling down, washes the surface of the food, taking away thermal energy. The product molecules slow down and are kinetic energy transformed into thermal energy of the refrigerant circulating in the evaporator.
Important note that the change in internal energy ($\Delta U$) at a constant volume is equal to the amount of heat transferred ($Q$), taken with the opposite sign, since the work of external forces in this case is minimal or absent. The formula for calculating the change in energy with a change in temperature is $\Delta U = c \cdot m \cdot \Delta T$, where $c$ is the specific heat capacity, $m$ is the mass, and $\Delta T$ is the change in temperature. Since the final temperature is lower than the initial temperature, $\Delta T$ is negative, therefore, the energy decreases.
If the product contains a significant amount of water, the process may be complicated by a phase transition. When water freezes, the internal energy continues to decrease, even if the thermometer shows a stable temperature of 0°C (or slightly lower, taking into account dissolved substances). Energy is spent on restructuring the crystal lattice of ice, which makes this stage critical for preserving the tissue structure of the product.
Thermodynamics of phase transitions in products
The greatest changes in the structure of a product occur during the phase transition of a liquid into a solid state. At this moment internal energy decreases due to a change in the potential energy of interaction of molecules, although the kinetic energy (temperature) remains constant. This phenomenon is often called "latent heat", and it requires a significant amount of energy from the refrigeration unit to remove the heat.
Different foods have different freezing points due to the presence of dissolved salts, sugars and proteins. For example, pure water freezes at 0°C, and meat juice or salty solution freezes at lower temperatures, sometimes down to -2...-3°C. During this period, the molecules line up in ordered crystalline structures, which leads to a decrease in the entropy of the system and a further drop in internal energy.
If the cooling process occurs too quickly, for example, in the Super Freezemode, ice crystals form small and do not damage the cell walls. Slow cooling produces large crystals that can tear tissue, causing loss of juice when defrosted. Thus, the rate of change in internal energy directly affects the quality of the product after storage.
Why does water expand when it freezes?
When it becomes a solid state, water molecules form a crystal lattice with voids, which increases the volume. This is a unique property of water that is important to consider when storing liquids in glass containers.
Practical application of knowledge for storage
Understanding how energy and temperature changes helps optimize refrigeration chamber loading. Proper distribution of products reduces energy consumption and extends the life of the compressor. The refrigerator operates more efficiently when air circulates freely around the food, ensuring uniform heat dissipation.
You should not fill the chamber completely "to capacity". There should be a gap between the products for convection currents. If you load a lot of warm food at the same time, the internal energy of the air in the chamber will increase sharply, and the temperature sensors will send a signal to the maximum power of the compressor. This may cause ice to form on the evaporator.
Use sealed containers. They not only prevent products from drying out (sublimation), but also regulate heat exchange. Metal utensils conduct heat faster than plastic or glass, so cooling in metal will be more intense, but can lead to local hypothermia of neighboring products.
Comparison of processes in different zones of the refrigerator
Temperature zones in the refrigerator create different conditions for changing the internal energy of products. In the freezer, the process goes to deep minus, completely stopping biochemical reactions. In the main finishing, cooling occurs only to +2...+5°C, keeping food fresh, but not frozen.
In the freshness zone (zero zone), the temperature is close to 0°C, but the humidity is higher. Here, the internal energy of the products is reduced minimally, sufficient to inhibit the proliferation of bacteria, but without the risk of freezing the cell sap. This is the ideal balance for meat and fish intended for quick cooking.
The temperature regime on the refrigerator door is most unstable due to frequent opening. Here, the internal energy of the food may fluctuate, which is undesirable for perishable food. Therefore, door shelves are best used for sauces, drinks and other products with a long shelf life.
Table of changes in parameters during cooling
For clarity, let's look at how the physical parameters of a typical product (for example, a piece of meat or water) at different stages of being in the refrigerator.
| Process stage | Product temperature | Internal energy | State of molecules |
|---|---|---|---|
| Start of cooling | High (+20°C and above) | Maximum | Chaotic fast movement |
| Cooling to 0°C | Decreases to 0°C | Decreases | Movement speed decreases |
| Crystallization | Stable (about 0°C) | Sharply decreases | Formation of crystalline lattice |
| Freezing | Reduced to -18°C | Minimal | Vibrations at lattice nodes |
Typical errors and misconceptions
One of the common mistakes in tests and in everyday life is the opinion that the cold is “transferred” to the product. In physics, cold does not exist as a substance. There is only warmth. The product becomes cold not because cold was added to it, but because it gave up its heat to the environment of the refrigerator through heat exchange.
Temperature and amount of heat are also often confused. A small piece of ice and a large piece of frozen meat may have the same temperature, but their internal energy will be radically different due to their different masses. The amount of heat that needs to be removed to cool them will also be different.
Some believe that in a vacuum the product will cool faster. In fact, in a vacuum there is no convection, and heat removal occurs only through radiation, which is much slower. Therefore, food in tightly closed vacuum bags inside the refrigerator takes longer to cool than open ones.
☑️ Correct loading of the refrigerator
⚠️ Attention: If you notice that food in one part of the refrigerator freezes and spoils in the other, the air circulation may be impaired or the temperature sensor is faulty. Check that the ventilation holes are not blocked by food.
Answers to frequently asked questions
Why does the internal energy decrease if the mass of the product has not changed?
Internal energy depends not only on mass, but also on temperature (speed of movement of molecules) and state of aggregation. When cooling, the speed of the molecules decreases, and accordingly, their kinetic energy, which makes up most of the internal energy, also decreases.
Where does the energy that the products give up go?
The energy is transferred to the air inside the refrigerator, and then through the walls of the evaporator to the refrigerant. The refrigerant, evaporating, carries this heat into the condenser (the grill at the back of the refrigerator), where it is dissipated into the surrounding room.
Does packaging affect the rate of change of internal energy?
Yes, significantly. Materials with low thermal conductivity (foam, thick plastic, wood) slow down the heat transfer process, keeping the product temperature stable longer. Metal packaging accelerates cooling.
Can internal energy increase in a refrigerator?
In a working refrigerator - no, unless there is a phase transition with the release of heat (crystallization), but even then the total energy of the “product” system decreases. An increase in energy is only possible if the thermostat or compressor malfunctions, when the temperature in the chamber begins to rise.