Useful physical knowledge points, ohmmeter, scale principle and scale method for both junior and senior high school
In the junior high school stage, the ohmmeter is not required for teaching, but the working principle and calibration method of the ohmmeter can be understood by using the knowledge of junior high school mathematics and physics, and it does not exceed the knowledge scope of junior high school students. From the point of view of resistance, an ohmmeter is a variable resistance that can read the resistance value, and it is also a measuring instrument modified from a small-range ammeter; the working principle is the same as that of a motor, that is, the energized conductor is subjected to force in the magnetic field effect. The figure below shows the working principle of the ohmmeter. G is a 10mA small-range ammeter, R0 is a fixed-value resistor, the power supply voltage is 1.5V, A and B are red and black test leads, and the resistance to be tested can be connected between A and B. Zero method. Connect the two test leads A and B directly, which means that there is no resistance connected between A and B. Adjust the sliding rheostat (knob) so that the pointer of the ammeter is deflected to the maximum position (full deflection). At this time, the total resistance connected to the circuit is: 1.5v/0.01A= 150 ohms. After that, keep the position of the sliding rheostat unchanged. Special reminder: The resistance value of 150 ohms includes the resistance value of the varistor connected to the line, the internal resistance of the small range ammeter, the resistance value of the fixed value resistor R0, the internal resistance of the power supply, and even the resistance of the wire. These resistances do not have to be taken into account when measuring resistance later. When the pointer of the ammeter is deflected to the maximum, the full bias current of the ammeter is 0.01 ampere, or 10 mA. scale method. When the current is expressed to the full bias current, it corresponds to RAB=0 ohm, therefore, 0 ohm is marked at 10mA. The calibration method of the remaining resistance values is based on the calculation formula: I=1.5V/(150+RAB)Ω, the current value I and RAB are in a one-to-one correspondence, and each current value corresponds to a value of RAB. For example, when the current is At 5mA, RAB=150 ohms, mark 150 ohms at the 5mA scale line. A conclusion can also be drawn from the formula: the scale line on the ohmmeter is uneven, because the dependent variable I and the independent variable RAB are not proportional. For example, the picture above is an ohmmeter specially used to measure resistance. When using, connect the resistance to be measured between A and B. In order to read the resistance value connected to A and B directly from the dial of the ammeter, it is necessary to mark the current value on the dial as the resistance value measured between A and B. When the two ends of A and B are directly connected together (that is, the connection resistance between AB is 0), adjust the sliding piece of the sliding rheostat to start sliding from the maximum resistance value until the pointer of the ammeter is fully offset, and then keep the resistance R . The resistance value remains unchanged. Set the power supply voltage to be 3V and the ammeter mileage to be 0.6A. 1. When no resistance is connected between AB, what is the total resistance R in the circuit? On which side of the dial is the pointer of the ammeter at this time? 2. How much resistance value should be marked on the scale line in the middle of the ohmmeter dial through calculation? 3. Please analyze and explain whether the scale value on the dial of the ohmmeter is uniform? In practical applications, ammeter, voltmeter, and ohmmeter are assembled in one meter, which is a multimeter. Picture A above is the working principle diagram of a single-range ammeter, picture B above is the working principle diagram of an ohmmeter, and picture C above is a working principle diagram of a single-range voltmeter. We can combine these three meters into a simple multimeter. S is a single-pole multi-throw switch, which is equivalent to a multi-gear knob. According to your understanding of the principles of voltmeter, ammeter and ohmmeter, talk about the purpose of the electricity meter between the two terminals, and explain how you judge the range of the electricity meter for each purpose.