When you place freshly prepared hot soup or freshly purchased vegetables on the refrigerator shelf, a complex physical process is launched, which in school The course is studied in the section “Thermal Phenomena”. For an 8th grade student, it is important to understand that at this moment there is a change in the state of the substance at the molecular level, although outwardly we only see a decrease in temperature. Internal energy The system is made up of the energy of movement and interaction of the particles that make up the body.
At the moment when the products find themselves in an environment with a lower temperature, an active heat exchange process begins. Energy does not disappear without a trace; it is transferred from a more heated body (product) to a less heated one (the air in the chamber). It is this transfer of energy that is the key point explaining why cooling food loses its thermal properties.
The answer to the question of how exactly the parameters of the system have changed lies in the fundamental laws of thermodynamics. In short, when the temperature drops internal energy decreases. This is a basic rule that you need to remember to successfully solve problems and understand the processes that occur in your kitchen every day.
The physical essence of internal energy
To better understand the process, it is necessary to determine what exactly we call internal energy in the context of 8th grade physics. This is the sum of the kinetic energy of the chaotic movement of molecules and the potential energy of their interaction. When you put a warm watermelon in the refrigerator, you do not change its chemical composition, but you radically change the speed of the “dance” of its molecules.
The kinetic energy of molecules directly depends on temperature. The higher the temperature of the product, the faster its particles move. Accordingly, upon cooling, the speed of movement of molecules decreases, which leads to a decrease in the kinetic component of internal energy. This process occurs until the temperatures of the product and the environment are equal.
The potential energy of interaction between molecules can also change, especially if a phase transition occurs, for example, the freezing of water in the product. However, in most everyday situations, when we talk about cooling, the main contribution to the change is made by the temperature factor. Heat capacity the substance determines how quickly or slowly this process will occur.
⚠️ Attention: Do not confuse internal energy with temperature. Temperature is a measure of average kinetic energy, and internal energy is the total energy reserve of all particles. A large piece of meat at 20°C has more internal energy than a small pea at the same temperature, although the temperatures are equal.
Heat exchange mechanism in the refrigerator compartment
The process of energy change is impossible without a heat transfer mechanism. In a refrigerator, the main types of heat exchange are conduction and convection. When a product touches a shelf or package, thermal conductivity works: energy is transferred through direct contact of particles.
At the same time, convection works - the transfer of energy by air currents. Cold air, falling down, flows around the food, picks up heat and carries it to the evaporator. That is why it is important not to completely clutter the refrigerator so that air circulation is not disturbed. If the air stands still, cooling will proceed slowly and unevenly.
The third type of heat transfer - radiation - in this case plays a secondary role, but is also present. All bodies radiate energy, and a warm product emits more than it absorbs from the cold walls, which also contributes to the loss of internal energy. The sum of these processes leads to rapid cooling.
- 🌡️ Thermal conductivity —energy transfer upon contact with shelves and adjacent products.
- 💨 Convection —the main mechanism for the movement of cold air around the product.
- ☀️ Radiation - energy transfer in the form of electromagnetic waves, less significant at low temperatures.
The effectiveness of this process depends on the temperature difference. The hotter the product was initially, the more intense the heat exchange occurs in the first minutes. However, it is not recommended to place boiling pots in a modern refrigerator, as this creates excess load on the compressor.
Quantitative calculation of energy changes
In 8th grade physics, to calculate the amount of heat that a product gives off, the formula Q = cm(t₂ - t₁) is used. Here c is the specific heat capacity of the substance, m is the mass, and t is the initial and final temperatures. The amount of heat Q is numerically equal to the change in internal energy if no mechanical work is performed.
Consider an example: you put 1 kg of water at a temperature of 80°C in the refrigerator and cooled it to 4°C. The specific heat capacity of water is approximately 4200 J/(kg °C). Substituting the values into the formula, we get: Q = 4200 1 (4 - 80) = -319,200 J. The minus sign just indicates that the energy has decreased.
It is important to understand that different products cool at different rates due to different heat capacities. Fats, proteins and water have different ratios. That is why meat broth cools more slowly than just water, and butter can remain soft for a long time even at low temperatures.
| Substance | Specific heat capacity, J/(kg °C) | Cooling rate |
|---|---|---|
| Water | 4200 | Slow (high heat capacity) |
| Meat (beef) | ~3500 | Medium |
| Sunflower oil | ~1700 | Fast (low heat capacity) |
| Aluminum (foil) | 920 | Very fast |
The table shows that water is an excellent heat accumulator. Foods with a high water content (vegetables, fruits, soups) will release energy longer than dry or fatty foods. This explains why a watermelon in the refrigerator takes longer to cool than a piece of cheese.
Why doesn't ice in the freezer get colder indefinitely?
Ice in the freezer cools to the temperature of the evaporator. As soon as the temperatures are equalized, the heat exchange stops, and the internal energy of the ice stops decreasing, remaining constant as long as the thermostat is operating.
The influence of aggregate states on energy
The case is of particular interest when the products do not just cool down, but freeze. If you put water or a product with a high moisture content in the freezer, a phase transition begins when it reaches 0°C (for water). The temperature at this moment does not change, but the internal energy continues to decrease.
This happens because crystallization energy is released. Water molecules line up in a crystal lattice of ice, and the potential energy of their interaction decreases. This process requires the removal of a significant amount of energy, called the specific heat of fusion (or crystallization).
For water, this value is 330,000 J/kg. This means that turning 1 kg of water into ice at the same temperature (0°C) requires the same amount of energy as it takes to heat 1 kg of water from 0°C to almost 80°C. Therefore, freezing food is an energy-intensive process for a refrigerator.
- ❄️ When freezing, the temperature of the product does not change until all the water turns into ice.
- 📉 The internal energy during a phase transition decreases due to the potential energy of the molecules.
- 🧊 Ice at -18°C has lower internal energy than water at +4°C, despite the similarity of the state of aggregation (solid/liquid).
⚠️ Attention: During sudden freezing, large ice crystals form inside the cells of products, which can rupture the cell walls. This changes the structure of the product after defrosting, making it mushy.
Practical significance for food storage
Understanding the physics of the process helps not only in studying, but also in the proper storage of food. Knowing that internal energy decreases when heat is transferred, we understand: the greater the temperature difference between the product and the air, the faster the cooling occurs. However, too hot food interferes with the operation of the refrigerator.
Modern refrigerators operate on the principle of maintaining a given temperature. When you put in a warm product, sensors detect an increase in temperature and the compressor turns on more actively. The internal energy of the air in the chamber temporarily increases until the refrigerator removes excess heat to the outside through the condenser (grid at the back).
For uniform cooling, it is important to take into account the thermal conductivity of the packaging. A glass jar of compote will take longer to cool than a plastic container due to the different thermal conductivities of the materials. Metal utensils speed up the process, but are rarely used in refrigerators due to the risk of corrosion and reaction with food.
☑️ Rules for loading food
Common mistakes and misconceptions
Many people mistakenly believe that “cold” passes from the refrigerator to the product. This is not true from a physics point of view. Cold is the absence of heat. The internal energy of the product decreases not because cold was “pumped into it,” but because it “gave” its heat to the environment.
Another misconception is related to opening the door. When you open the refrigerator, warm air flows inside. Its internal energy is higher than that of the air in the chamber. The refrigerator has to expend electrical energy to reduce the internal energy of this new volume of air, returning the system to equilibrium.
Some people believe that food will cool faster in a vacuum. In fact, in a vacuum there is no convection, and heat exchange occurs only due to radiation and weak thermal conductivity through the supports. Therefore, in a vacuum, products cool slower than in air, but faster than in a thermos (where the vacuum serves to preserve heat).
FAQ: Questions and answers on the topic
Why does internal energy decrease if the mass of the product does not change?
Internal energy depends on temperature, which is a measure of the speed of movement of molecules. When cooling, the speed of molecules decreases, which means their kinetic energy, which makes up the bulk of the internal energy, also decreases, even if the mass (number of molecules) remains the same.
Where does the energy that the products give up go?
Energy does not disappear. It is transferred to the air inside the refrigerator, and then the refrigerant (freon) takes this heat and, through a system of tubes, takes it outside, into the room, where it is dissipated through the rear grille of the refrigerator.
Will the internal energy change if the product simply lies in the switched off refrigerator?
Yes, it will change, but only until the temperatures equalize. If the temperature in the room is +22°C, and in the turned off refrigerator it also became +22°C, then the product with a temperature of +22°C will not change its internal energy, since heat exchange will stop.
Does the color of the package affect the rate of energy change?
Under refrigerator conditions, where the main mechanism is convection and thermal conductivity, color has little effect. However, dark surfaces absorb and emit thermal energy (radiation) better, so theoretically there is a difference, but it is insignificant compared to air temperature.
Can internal energy increase in a refrigerator?
In itself - no, since the refrigerator removes heat. But if a fan or motor is running in a refrigerator, they do work, which ultimately turns into heat. However, for products lying on the shelf, the vector of energy change is always aimed at decreasing (cooling).