A bulb marked with "pz220-40", the normal luminous current of the lamp is a, and the resistance is a

A bulb marked with "pz220-40", the normal luminous current of the lamp is a, and the resistance is a

Pz220-40 means the rated voltage is 220 V and 40 W
Normal lighting means under rated condition
So the rated current is I = P / u = 40 / 220 ≈ 0.18182a
The resistance R = u ^ 2 / P = 220 * 220 / 40 = 1210 Ω

Draw the circuit diagram of measuring the resistance of small bulb by voltammetry

: because the resistance of the small bulb is relatively small, it is generally much smaller than the internal resistance of the voltmeter, but not much different from the resistance of the ammeter. In order to minimize the measurement error, the ammeter external connection method should be adopted, as shown in the figure

1. In the circuit diagram of measuring resistance by voltammetry, the closed switch light is not on, and the voltage indication is about equal to the power supply voltage;

There may be the following reasons:
1. Lamp open circuit (such as filament broken, poor contact of lamp holder);
2. The position of the voltmeter and the ammeter are interchanged
Because the resistance of the voltmeter is very large, after closing the switch, the resistance in the voltmeter shares more voltage than the sliding rheostat, so the indication of the voltmeter is almost equal to the supply voltage

It is known that resistance R1 and resistance R2 are connected in series in the circuit, and the power supply voltage (electromotive force) is constant. When the resistance value of R2 changes, how the power will change
The correct answer to this question is that no matter how R2 changes, P2 decreases. Why

It can be simply considered: because R1 does not change, the internal resistance R of the power supply and R1 are regarded as the new internal resistance R of the power supply (the following R are all new internal resistance of the power supply, that is, including the resistance value of R1), only R2 is regarded as the resistance closed loop of the external circuit, and Ohm's law is the power expression of E = IR + U = I (R + R2) circuit