We learn physics and teach children without leaving the kitchen

2024 Author: Seth Attwood | [email protected]. Last modified: 2023-12-16 15:55

We spend 1-2 hours in the kitchen every day. Someone less, someone more. That being said, we rarely think about physical phenomena when we cook breakfast, lunch or dinner. But there can be no greater concentration of them in everyday conditions than in the kitchen, in the apartment. A good opportunity to explain physics to children!

1. Diffusion

We are constantly faced with this phenomenon in the kitchen. Its name is derived from the Latin diffusio - interaction, dispersal, distribution.

This is the process of mutual penetration of molecules or atoms of two adjoining substances. The diffusion rate is proportional to the cross-sectional area of the body (volume), and the difference in concentrations, temperatures of the mixed substances. If there is a temperature difference, then it sets the direction of propagation (gradient) - from hot to cold. As a result, spontaneous alignment of the concentrations of molecules or atoms occurs.

This phenomenon can be observed in the kitchen when odors spread. Thanks to the diffusion of gases, sitting in another room, you can understand what is cooking. As you know, natural gas is odorless and an additive is added to it to make it easier to detect the leakage of domestic gas.

An odorant such as ethyl mercaptan adds a pungent odor. If the burner does not light up the first time, then we can smell a specific smell, which we know from childhood as the smell of household gas.

And if you throw grains of tea or a teabag into boiling water and do not stir, you can see how the tea infusion spreads in the volume of pure water.

This is the diffusion of liquids. An example of diffusion in a solid would be the salting of tomato, cucumber, mushroom, or cabbage. Salt crystals in water break down into Na and Cl ions, which, moving chaotically, penetrate between the molecules of substances in the composition of vegetables or mushrooms.

2. Change of state of aggregation

Few of us noticed that in a left glass of water, after a few days, the same part of the water evaporates at room temperature as when boiling for 1-2 minutes. And when we freeze food or water for ice cubes in the refrigerator, we don’t think about how this happens.

Meanwhile, these most common and common kitchen phenomena are easily explained. A liquid has an intermediate state between solids and gases.

At temperatures other than boiling or freezing, the forces of attraction between molecules in a liquid are not as strong or weak as in solids and gases. Therefore, for example, only receiving energy (from the sun's rays, air molecules at room temperature), the liquid molecules from the open surface gradually pass into the gas phase, creating a vapor pressure above the liquid surface.

The evaporation rate increases with an increase in the surface area of the liquid, an increase in temperature, and a decrease in external pressure. If the temperature is increased, then the vapor pressure of this liquid reaches the external pressure. The temperature at which this occurs is called the boiling point. The boiling point decreases with decreasing external pressure. Therefore, in mountainous areas, water boils faster.

Conversely, when the temperature drops, water molecules lose their kinetic energy to the level of the forces of attraction between themselves. They no longer move chaotically, which allows the formation of a crystal lattice like that of solids. The 0 ° C temperature at which this occurs is called the freezing point of water.

When frozen, water expands. Many people could get acquainted with this phenomenon when they put a plastic bottle with a drink in the freezer for quick cooling and forgot about it, and then the bottle was bursting. When cooled to a temperature of 4 ° C, an increase in the density of water is first observed, at which its maximum density and minimum volume are reached. Then, at temperatures from 4 to 0 ° C, a rearrangement of bonds in the water molecule occurs, and its structure becomes less dense.

At a temperature of 0 ° C, the liquid phase of water changes to solid. After the water completely freezes and turns into ice, its volume grows by 8, 4%, which leads to the bursting of the plastic bottle. The liquid content in many products is low, so they do not increase in volume so noticeably when frozen.

3. Absorption and adsorption

These two almost inseparable phenomena, called from the Latin sorbeo (to absorb), are observed, for example, when heating water in a kettle or saucepan. A gas that does not chemically act on a liquid can nevertheless be absorbed by it on contact with it. This phenomenon is called absorption.

When gases are absorbed by solid fine-grained or porous bodies, most of them densely accumulate and are retained on the surface of pores or grains and are not distributed throughout the volume. In this case, the process is called adsorption. These phenomena can be observed when boiling water - bubbles separate from the walls of a saucepan or kettle when heated.

The air released from water contains 63% nitrogen and 36% oxygen. In general, atmospheric air contains 78% nitrogen and 21% oxygen.

Table salt in an uncovered container can become wet due to its hygroscopic properties - the absorption of water vapor from the air. And baking soda acts as an adsorbent when placed in the refrigerator to remove odors.

4. Manifestation of Archimedes' law

When we are ready to cook the chicken, we fill the pot with water about half or ¾, depending on the size of the chicken. By immersing the carcass in a pot of water, we notice that the weight of the chicken in the water is noticeably reduced, and the water rises to the edges of the pot.

This phenomenon is explained by the buoyancy force or Archimedes' law. In this case, a buoyant force acts on a body immersed in a liquid, equal to the weight of the liquid in the volume of the submerged part of the body. This force is called the force of Archimedes, as is the law itself, which explains this phenomenon.

5. Surface tension

Many people remember the experiments with films of liquids, which were shown in physics lessons at school. A small wire frame with one movable side was dipped in soapy water and then pulled out. The forces of surface tension in the film formed along the perimeter lifted the lower movable part of the frame. To keep it motionless, a weight was suspended from it when the experiment was repeated.

This phenomenon can be observed in a colander - after use, water remains in the holes in the bottom of these kitchen utensils. The same phenomenon can be observed after washing the forks - there are also streaks of water on the inner surface between some of the teeth.

The physics of liquids explains this phenomenon as follows: liquid molecules are so close to each other that the forces of attraction between them create surface tension in the plane of the free surface. If the force of attraction of water molecules of the liquid film is weaker than the force of attraction to the surface of the colander, then the water film breaks.

Also, the forces of surface tension are noticeable when we pour cereals or peas, beans into a saucepan with water, or add round grains of pepper. Some grains will remain on the surface of the water, while most will sink to the bottom under the weight of the rest. If you press lightly on the floating grains with the tip of your finger or a spoon, they will overcome the surface tension of the water and sink to the bottom.

6. Wetting and spreading

Spilled liquid may form small stains on a grease-coated stove, and a single puddle on the table. The thing is that the liquid molecules in the first case are more attracted to each other than to the surface of the plate, where there is a fatty film not wetted by water, and on a clean table the attraction of water molecules to the molecules of the table surface is higher than the attraction of water molecules to each other. As a result, the puddle spreads.

This phenomenon is also related to the physics of liquids and is related to surface tension. As you know, a soap bubble or liquid droplets have a spherical shape due to surface tension forces.

In a droplet, liquid molecules are attracted to each other more strongly than to gas molecules, and tend to the inside of the liquid droplet, reducing its surface area. But, if there is a solid wetted surface, then a part of the drop upon contact is stretched along it, because the molecules of the solid attract the molecules of the liquid, and this force exceeds the force of attraction between the molecules of the liquid.

The degree of wetting and spreading over a solid surface will depend on which force is greater - the force of attraction of molecules of a liquid and molecules of a solid between themselves or the force of attraction of molecules inside a liquid.

Since 1938, this physical phenomenon has been widely used in industry, in the production of household goods, when Teflon (polytetrafluoroethylene) material was synthesized in the DuPont laboratory.

Its properties are used not only in the manufacture of non-stick cookware, but also in the manufacture of waterproof, water-repellent fabrics and coatings for clothes and shoes. Teflon is recognized by the Guinness Book of Records as the world's most slippery substance. It has very low surface tension and adhesion (sticking), it is not wetted with water, grease or many organic solvents.

7. Thermal conductivity

One of the most common phenomena in the kitchen that we can observe is the heating of a kettle or water in a saucepan. Thermal conductivity is the transfer of heat through the movement of particles when there is a difference (gradient) in temperature. Among the types of thermal conductivity, there is also convection.

In the case of identical substances, the thermal conductivity of liquids is less than that of solids, and higher than that of gases. The thermal conductivity of gases and metals increases with increasing temperature, and that of liquids decreases. We are constantly faced with convection, whether we stir soup or tea with a spoon, or open a window, or turn on the ventilation to ventilate the kitchen.

Convection - from the Latin convectiō (transfer) - a type of heat transfer when the internal energy of a gas or liquid is transferred by jets and streams. Distinguish between natural convection and forced. In the first case, layers of liquid or air are themselves mixed when heated or cooled. And in the second case, there is a mechanical mixing of a liquid or gas - with a spoon, fan or in another way.

8. Electromagnetic radiation

A microwave oven is sometimes called a microwave oven, or microwave oven. The main element of every microwave oven is a magnetron, which converts electrical energy into microwave electromagnetic radiation with a frequency of up to 2.45 gigahertz (GHz). Radiation heats food by interacting with its molecules.

The products contain dipole molecules containing positive electrical and negative charges on their opposite parts.

These are molecules of fats, sugar, but most of all dipole molecules are in water, which is found in almost any product. The microwave field, constantly changing its direction, makes the molecules vibrate with high frequency, which line up along the lines of force so that all the positive charged parts of the molecules "look" in one direction or the other. Molecular friction arises, energy is released, which heats the food.

9. Induction

In the kitchen, you can increasingly find induction cookers, which are based on this phenomenon. English physicist Michael Faraday discovered electromagnetic induction in 1831 and since then it has been impossible to imagine our life without it.

Faraday discovered the occurrence of an electric current in a closed loop due to a change in the magnetic flux passing through this loop. A school experience is known when a flat magnet moves inside a spiral-shaped circuit of a wire (solenoid), and an electric current appears in it. There is also a reverse process - an alternating electric current in a solenoid (coil) creates an alternating magnetic field.

A modern induction cooker works on the same principle. Under a glass-ceramic heating panel (neutral to electromagnetic oscillations) of such a stove there is an induction coil through which an electric current flows with a frequency of 20-60 kHz, creating an alternating magnetic field that induces eddy currents in a thin layer (skin layer) of the bottom of a metal dish.

The electrical resistance heats up the dishes. These currents are no more dangerous than red-hot dishes on ordinary stoves. Cookware should be steel or cast iron with ferromagnetic properties (attract a magnet).

10. Refraction of light

The angle of incidence of light is equal to the angle of reflection, and the propagation of natural light or light from lamps is explained by a dual, wave-particle nature: on the one hand, these are electromagnetic waves, and on the other, particles-photons, which move at the maximum speed possible in the Universe.

In the kitchen, you can observe such an optical phenomenon as the refraction of light. For example, when there is a transparent vase with flowers on the kitchen table, the stems in the water seem to shift at the boundary of the water surface relative to their continuation outside the liquid. The fact is that water, like a lens, refracts the rays of light reflected from the stems in the vase.

A similar thing is observed in a transparent glass of tea, in which a spoon is dipped. You can also see a distorted and enlarged image of beans or cereals at the bottom of a deep pot of clear water.