Abstract: The water level gauge in junior high school physics and the thermometer in high school physics have been studied in-depth by analogy, so that students can easily leap from familiar knowledge to unfamiliar knowledge, and then deeply understand and master the structure of the electrometer. And the principle breaks through the difficulties of electrometer teaching, and it is worth learning from the majority of high school teachers.
In high school, the principles of thermometers and electrometers have always been confused by students. The reason is basically because they don’t understand that thermometers are actually thermal containers and electrometers are actually capacitors, and they don’t understand that they are connected in parallel with the object to be measured. Relationship, do not understand that the basis of measurement is the corresponding balance principle. In order to solve this problem, we organized the teaching by analogy with the water level gauge in junior high school physics that students are familiar with. Teaching practice has proved that this method is better for students to understand the principles of thermometers and electrometers. It is specially sorted out and shared with you, and everyone is invited to criticize and correct.
１ Principle of water level gauge
As shown in Figure 1, a thin vertical transparent glass tube connected to the lower part of a large bucket (or boiler) is a water level gauge. The liquid level in the water level gauge is equal to the liquid level in the bucket (or boiler). , You can understand the water level in the bucket or boiler.
1.1 “Small water container with water level gauge
Although the water level gauge is very thin, there is no doubt that it is filled with water when it works, that is to say, it is a water container, it will share a part of the water in the bucket (or boiler) to be tested; however, The water level gauge is indeed very thin. Although the water level in it may be very high, the amount of water divided by it is completely negligible relative to the water volume in the bucket (or boiler). Therefore, when the water level gauge is working, it is (Or boiler) the influence of water volume can be ignored. (Recommendation of related products in this article: side-mounted magnetic flap level gauge)
1.2 Principle of Connecting Device
The lower end of the water level gauge is connected to the lower part of the water bucket (or boiler). The pressure produced by the water in the water level gauge at the bottom is equal to the pressure produced by the water in the bucket (or boiler) at the lower part of the connection. Due to the temperature and density of the water on both sides In the same way, the upper air chambers are also connected, so the water level on both sides must be the same. This is the principle of connected devices learned in junior high school. The work of the water level gauge is carried out on the basis of the principle of the communicating device. It is worth mentioning that if the density of the liquid in the water level gauge and the bucket (or boiler) is different (generally this is not the case), it can be known that the liquid level in the water level gauge is equal to the pressure produced by the two liquids at the connecting position The height will be different from the liquid level in the bucket (or boiler). However, in any case, the pressure generated by the two liquids at the bottom of the connection is equal.
２ Thermometer principle
Common thermometers include alcohol thermometers and mercury thermometers. As shown in Figure 2, these two types of thermometers have a glass bulb at the bottom and a thin glass tube at the top. The glass bulb is filled with red alcohol or mercury. The bottom glass bubble is in full contact with the object to be measured for a period of time. After the length of the liquid column in the thin glass tube is stable, directly compare the length of the liquid column with the temperature scale engraved on the glass tube to read the object to be measured temperature.
2.1 The small thermal container of the thermometer
The alcohol or mercury filled in the thermometer will expand after absorbing heat. Experience has shown that within a certain temperature range, the volume of a certain amount of these two liquids has a good linear relationship with the temperature. Therefore, as long as At the corresponding volume (length of the liquid column), engrave the corresponding temperature value, and the temperature of the liquid in the thermometer can be read directly according to the length of the liquid column.
When the thermometer is in full contact with the object to be measured, it will absorb heat from the object to be measured that is higher than the temperature of the thermometer. However, because the amount of liquid in the thermometer is extremely small, and because the glass tube that indicates the number is very thin, it is at the temperature When the temperature rises to the same as the temperature of the object to be measured, according to Q=cmΔt, the heat absorbed is very small, so it hardly affects the temperature of the object to be measured. However, for the sensitive sensor of human skin, the skin will obviously feel the loss of heat at the contact point (feeling the thermometer is cold) at the beginning of the contact, but because the heat capacity of the thermometer is very small, it can quickly rise to the same temperature. If the skin temperature is the same, the skin no longer feels cold with the thermometer.
2.2 The law of heat balance
When two objects are in contact, heat will spontaneously transfer from the high-temperature object to the low-temperature object. When the heat transfer reaches a certain level, the heat no longer flows in a macroscopic manner. At this time, we say that the two objects have reached a thermal equilibrium state; Experiments show that when the two objects are in thermal equilibrium with the third object, when the two objects are in contact, they are already in a thermal equilibrium relationship. This is the law of thermal equilibrium, also known as the zeroth law of thermodynamics.
In order to describe the common attributes of several objects that reach thermal equilibrium, we introduce the concept of temperature and define that the temperature of several objects that reach thermal equilibrium is the same, and heat transfer occurs between objects with different temperatures. Then it is defined that the temperature of the heat provider is higher and the heat is received. The temperature on one side is low.
When the thermometer is in full contact with the object to be measured, that is, after sufficient heat transfer, the final result must be the same temperature. Therefore, by using the expansion volume of the liquid in the thermometer, the temperature of the liquid in the thermometer can be read. That is, the temperature of the object to be measured.
３ The principle of electrometer
As shown in Figure 3, the electrometer is composed of a circular metal cylinder and a thinner metal rod insulated from the cylinder. The lower end of the metal rod has a metal pointer that can freely rotate around a fixed axis. When the electrometer is connected in parallel with the capacitor under test and stabilized, the voltage between the two plates of the capacitor under test can be read using the deflection angle of the pointer of the electrometer.
3.1″ Capacitor small electrometer
As shown in Figure 3, from the structure of the electrometer, it can be seen that the electrometer is a conductive metal cylinder and a thin metal rod (together with a pointer) that are close to each other but insulated. Therefore, it is a capacitor.
When the electrometer is working, it needs to be connected in parallel with the capacitor to be tested, and part of the charge on the electrode plate of the capacitor to be tested is transferred, so that the metal cylinder and the thin metal rod (together with the pointer) carry the same amount of heterogeneous charges, and the thin rod is After the pointers are charged with the same charge, they will repel each other, thereby showing the opening angle. When the potential difference between the two electrodes of the electrometer increases, it can be seen from Q = CU that the metal cylinder or thin metal rod (together with the pointer) is more charged. Larger, the greater the repulsive force between the thin rod and the pointer, and the larger the pointer opening angle.
Because the surface area of the thin rod and pointer is small, and the distance from the metal cylinder is very large (compared to paper capacitors and electrolytic capacitors), the capacitance of the electrometer is very small. When it is connected in parallel with the capacitor under test, The amount of charge transferred from the capacitor under test can be ignored relative to the amount of charge carried by the capacitor under test.
3.2 Principle of electrostatic balance
When two conductors are connected or in direct contact through a wire, the free charge will spontaneously move in the direction of lower potential energy. When the free charge no longer moves directionally, it means that the free charge is no matter where the two connected conductors are. On one, the potential energy is the same, that is, the potential of the two connected conductors are equal, which is the principle of electrostatic balance.
As shown in Figure 4, when the metal cylinder of the electrometer is connected to one plate of the capacitor under test (both are grounded, connected through the earth, and the earth potential is taken as zero), the thin metal rod is connected to the other plate of the capacitor under test. Connected, after a very short time, the connected conductors can reach electrostatic balance, so that the potential difference between the two ends of the electrometer is equal to the potential difference between the two ends of the capacitor under test. Read the potential difference between the two poles of the electrometer from the opening angle of the pointer of the electrometer, and also read the voltage between the two poles of the capacitor to be measured in parallel with it.
4 summary comparison
Table 1 shows the types of water level gauges, thermometers, and electrometers.
From the above summary, the basis of the work of the three instruments is to achieve a balance of strength (pressure, temperature, electric potential) with the object to be measured, and in this process, it is almost inevitable that a corresponding extension of the object to be measured must occur. The transfer of the amount of water (water, heat, electric charge), which will affect the corresponding intensity of the object to be measured. However, due to the small capacity of the three instruments themselves, it may be possible to achieve a balance with the object to be measured. The value of the corresponding extension quantity obtained is almost negligible relative to the corresponding extension quantity of the object to be measured. Therefore, the indications of the three measuring instruments (the intensity of the instrument itself) are very important. Close to the corresponding intensity of the object to be measured before the measurement, so as to achieve the purpose of measurement.
Post time: 15-10-21