As shown in the figure, R1 = 10 Ω, R2 = 20 Ω, R3 = 30 Ω, the power supply voltage is constant, if the switch S1 is closed and S2 is open, the reading of the ammeter is 0.3A, find: (1) what is the power supply voltage? (2) When S1 and S2 are disconnected, what is the reading of the ammeter? (3) When S1 and S2 are closed, what is the reading of the ammeter?

As shown in the figure, R1 = 10 Ω, R2 = 20 Ω, R3 = 30 Ω, the power supply voltage is constant, if the switch S1 is closed and S2 is open, the reading of the ammeter is 0.3A, find: (1) what is the power supply voltage? (2) When S1 and S2 are disconnected, what is the reading of the ammeter? (3) When S1 and S2 are closed, what is the reading of the ammeter?

① When S1 is closed and S2 is open, R2 access circuit: u = i2r2 = 0.3A × 20 Ω = 6V; ② when S1 and S2 are open, r1r2 series access circuit: I1 = ur1 + R2 = 6v10 Ω + 20 Ω = 0.2A; ③ when S1 and S2 are closed, R2R3 parallel access circuit: i3 = UR2 = 6v20 Ω = 0.3A A: (1) power supply voltage 6V