A physics problem: there are four resistances, R1 = 1, R2 = 2, R3 = 3, R4 = 4, e = 4V, r = 0.2, total current ——R1——R3—— | | | ——R2——R4—— | | ——||————— R1 and R3 are in one branch, R2 and R4 are in one branch, that is to say, they are connected in series and then in parallel, and there is another line between R1 and R3 and between R2 and R4 The general circuit diagram is like this. The lines in the diagram are all connected Is the middle line useless?

A physics problem: there are four resistances, R1 = 1, R2 = 2, R3 = 3, R4 = 4, e = 4V, r = 0.2, total current ——R1——R3—— | | | ——R2——R4—— | | ——||————— R1 and R3 are in one branch, R2 and R4 are in one branch, that is to say, they are connected in series and then in parallel, and there is another line between R1 and R3 and between R2 and R4 The general circuit diagram is like this. The lines in the diagram are all connected Is the middle line useless?

What you add is very important: the line in the middle is not useless. It just changes the current
Because this wire makes the current after passing through R 1 and R 2 receive the obstruction of R 3 and R 4, it is correct that R 1 and R 2 are connected in parallel, and R 3 and R 4 are connected in series
If you don't understand, you can say that the nodes at both ends of the line move towards the middle of the line, because the node movement does not affect the resistance and current changes

In figure a, close the switch s, the voltage is 4V, the current is 0.4A; in Figure B, close the switch s, the voltage is 6V. Find: (1) the resistance of R1 and R2; (2) the current in Figure B

(1) In figure a, R1 = u1i = 4v0.4a = 10 Ω, the voltage at both ends of R2 U2 = u-u1 = 6v-4v = 2V, R2 = u2i = 2v0.4a = 5 Ω. Answer: the resistance values of R1 and R2 are 10 Ω and 5 Ω respectively. (2) in Figure B, I1 = ur1 = 6v10 Ω = 0.6ai2 = UR2 = 6v5 Ω = 1.2A, the total current of the circuit is I = I1 + I2 = 0.6A + 1.2A = 1.8A

In the circuit shown in the figure, the power supply voltage is constant, R1 = 10 Ω. When the switch S is closed, the current representation is 0.2A, and the voltage representation is 4V

According to the circuit diagram, R1 and R2 are connected in series. The voltmeter measures the voltage at both ends of R2 and the ammeter measures the current in the circuit