In the circuit as shown in the figure, the power supply voltage is 9 V, the range of voltmeter is 0 ~ 3 V, the range of ammeter is 0 ~ 0.6 a, the setting resistance R1 = 20 Ω, and the sliding rheostat is marked with "15 Ω & nbsp; Calculation: ① when the reading of ammeter is 0.4 a, the voltage at both ends of resistance R1. ② the maximum resistance value of sliding rheostat R2 allowed to be connected into the circuit. ③ in order to make each ammeter reach the maximum value respectively in the process of moving slide, the appropriate fixed value resistance R is selected to replace the fixed value resistance R1 to find out the value range of r that meets the requirements

In the circuit as shown in the figure, the power supply voltage is 9 V, the range of voltmeter is 0 ~ 3 V, the range of ammeter is 0 ~ 0.6 a, the setting resistance R1 = 20 Ω, and the sliding rheostat is marked with "15 Ω & nbsp; Calculation: ① when the reading of ammeter is 0.4 a, the voltage at both ends of resistance R1. ② the maximum resistance value of sliding rheostat R2 allowed to be connected into the circuit. ③ in order to make each ammeter reach the maximum value respectively in the process of moving slide, the appropriate fixed value resistance R is selected to replace the fixed value resistance R1 to find out the value range of r that meets the requirements

① The voltage at both ends of resistance R1 is U1 = i1r1 = 0.4A × 20 Ω = 8V; answer: the voltage at both ends of resistance R1 is 8V. ② when the resistance of resistance R2 is the largest, the voltage at both ends of resistance R1 is U1 = u-u2 = 9v-3v = 6V, and the circuit current is I = I2 = I1 = u1r1 = 6v20 Ω = 0.3A; the maximum resistance of sliding rheostat R2 = u2i2 = 3v0.3a = 10 Ω; answer: the maximum resistance of sliding rheostat R2 is 10 Ω ③ When the ammeter reaches the maximum range, the maximum current of the circuit is 0.6A, and the total resistance of the circuit is the minimum, r = UI = 9v0.6a = 15 Ω. If the resistance of the sliding rheostat connected to the circuit is zero, the maximum resistance of the resistance R is r = r = 15 Ω. When the voltmeter reaches the maximum range, the voltage at both ends of the sliding rheostat is 3V, and the voltage at both ends of the resistance R is ur = U-U, sliding = 9v-3v = 6V, and the sliding rheostat is allowed The maximum current allowed is 2a, and the maximum current allowed by ammeter is 0.6A, then the maximum current is 0.6A, and the resistance R = URI = 60.6a = 10 Ω. Thus, the minimum resistance of constant resistance R = 10 Ω, and the value range of R is 10 Ω ~ 15 Ω. Answer: the value range of R is 10 Ω≤ R ≤ 15 Ω

When measuring resistance with V · a method, if the power supply voltage is 3 V and the measured resistance is 20 Ω, the range of voltmeter and ammeter should be ()
A. 0-3 V, 0-3 a B. 0-15 V, 0-3 a C. 0-3 V, 0-0.6 a D. 0-15 V, 0-0.6 a

In the experiment of measuring resistance by voltammetry, the constant resistance and sliding rheostat are connected in series, and the maximum power supply voltage is 3V, so the maximum voltage at both ends of the constant resistance will not exceed 3V of the power supply voltage, so the range of the voltmeter can be 0 ~ 3V; according to the formula, the maximum current in the circuit is calculated: I = ur = 3v20 Ω = 0.15A, so the range of the ammeter can be 0 ~ 0.6A, so C is selected