In the circuit, r = 10 Ω, the power supply voltage remains unchanged. If a resistor with R1 = 30 Ω is connected in series in the circuit, the indication of the ammeter will change to I1 = 0.25A 1. What is the power supply voltage equal to 2. If the indication of I2 is 0.4A, how to connect the circuit in the diagram? (R1 removed) what is the resistance value?

In the circuit, r = 10 Ω, the power supply voltage remains unchanged. If a resistor with R1 = 30 Ω is connected in series in the circuit, the indication of the ammeter will change to I1 = 0.25A 1. What is the power supply voltage equal to 2. If the indication of I2 is 0.4A, how to connect the circuit in the diagram? (R1 removed) what is the resistance value?

Series circuit, the current is equal, so the power supply voltage U = I1 * (R + R1) = 0.25 * 40 = 10V
R2 = u / i2-r = 10 / 0.4-10 = 15 Ω

As shown in the figure, the indication of voltmeter V is 6V, and that of ammeter A is 1a. If there are only two resistors R1 and R2 in the dotted line box
As shown in the figure, the indication of voltmeter V is 6V, and that of ammeter A is 1a. If there are only two resistors R1 and R2 in the dotted line box, one of them R1 = 10 Ω
(1) How are the resistances R1 and R2 connected through calculation?
(2) What is the other resistance R2 in the dotted box?

1) Because the total resistance R = u / I = 6 / 1 = 6 ohm

In the circuit, a 5 ohm resistance is connected in series with an ammeter, and the indication of the ammeter is 1a. Can a voltmeter with a range of 0-3v be used to measure the voltage at both ends of the resistance? Why?

An R = 5 ohm resistor is connected in series with an ammeter, and the indication of the ammeter is I = 1a,
The voltage at both ends of the resistor is u = IR = 5V > 3V,
Therefore, it is not possible to use a voltmeter with a range of 0 - 3V to measure the voltage at both ends of the resistor

Use ammeter and voltmeter to measure resistance and set the function of resistance
A classmate did the experiment of "measuring resistance with ammeter and voltmeter". In addition to the resistance to be measured, constant value resistance, ammeter, voltmeter, key, wire and dry battery, his existing experimental equipment
What is the function of constant resistance series connection in circuit

Protection circuit
Because may appear misoperation, will burn power and ammeter. This will protect the circuit from burning