In the circuit shown in the figure, R1 = 30 Ω, R2 = 10 Ω. After the switch S is closed, the indication of ammeter A is 0.4A. Calculate the power supply voltage, the current through R2 and the current in the main circuit

It can be seen from the circuit diagram that two resistors are connected in parallel, and the ammeter measures the current of R1 branch. (1) voltage of power supply: u = U2 = U1 = i1r1 = 0.4A × 30 Ω = 12V; (2) current through R2: I2 = u2r2 = 12v10 Ω = 1.2A, main current: I = I1 + I2 = 0.4A + 1.2A = 1.6A

In the circuit shown in the figure, the power supply voltage remains unchanged. In the figure a, close the switch s, the voltmeter indication is 4V, and the current indication is 0.2A
In Figure B, close the switch s, and the voltmeter is 6V
(2) Current representation in Figure B

In figure a, R1 and R2 are connected in series, the ammeter measures the total current and the voltmeter measures the R1 voltage; in Figure B, R1 and R2 are connected in parallel, the ammeter measures the total current and the voltmeter measures the total voltage; because the resistance in Figure A and figure B and the power supply do not change, the power supply voltage U = 6V, R1 = U1 / ia = 4 / 0.4 = 10 Ω; R2 = (u-u1) / ia = (6-4) / 0.4 = 5 Ω; therefore, the parallel resistance of r1r2 is r = R1 * R2 / (R1 + R2) = 10 * 5 / (10 + 5) = 10 / 3 Ω; IB = u / r = 6 * 3 / 10 = 1.8A

A single LED (white light) uses a 12V safe power supply. The current of each light is 0.02A. How many ohm resistance does it use to light up,
LED (white light) normally lights up when four LEDs are connected in series. I want to know how many ohm resistance should be added when one LED is connected in series, two LEDs are connected in series, three LEDs are connected in series, four LEDs are connected in series, and five LEDs are connected in series,

The current of the series circuit is equal, and the shunt resistance can use (supply voltage - led working voltage) / current
The rated voltage of white LED is 3 V and the current is 0.02 a
such as
One (12-3) / 0.02 = 450 Ω
Three times (12-3 * 3) / 0.02 = 150 Euro
When four are connected in series, when five are connected in parallel, the voltage is not enough

A 4 ohm resistor and a ohm resistor are connected in series to a 12 V power supply, and the current in the circuit is 1.2 A; the voltage at both ends of the circuit is changed to 24 V, and the total resistance is Ohm

1.6Ω 2.10Ω
The current of series circuit is equal
12/1.2=10
The total resistance is 10 Ω
The other resistance is 10-4 = 6 Ω
Changing the voltage has nothing to do with the resistance, so it's still 10 Ω

In the circuit shown in the figure, R1 = 30 Ω, R2 = 10 Ω. After the switch S is closed, the indication of ammeter A is 0.4A. Calculate the power supply voltage, the current through R2 and the current in the main circuit