A third day physics power problem! If an electric kettle is marked with the word "220 V 800 W", how much power is consumed when it works normally for 20 minutes? What is the rated current? What is the resistance when it works normally? (for detailed process, speed ~~~~~~~~~~~

Electric energy 800W * 0.33 = 266wh = 0.266 degree
Rated current 800 / 220 = 3.64A
Resistance 220 ^ 2 / 800 = 60.5 Ω
Please accept

The electric power of a light bulb in the laboratory is 0.625w when it normally emits light, and the current in the filament is 0.25A?

P = I square * r, so r = 0.625 / (0.25 * 0.25) = 10 Ω
P = u square / R, so u square = 0.625 * 10 = 6.25, so u = 2.5
If it helps, please take it~

The problem of physical electric power in grade three of junior high school
A constant resistance R is connected in series with sliding rheostat R2. When the sliding rheostat slides from left to right, the electric power of resistance R varies between 9W and 36W. The detailed process is needed to find out the electric power of R when the slide slides to the middle
It is not the power of the sliding rheostat, but the power of the constant resistance R when the slide of the sliding rheostat slides to the middle

It can be seen from the title that when the power of resistance R is 9W and 36W, the resistance value in the circuit corresponds to R + R2 and R. let the circuit power supply be u, according to Ohm's law, we can get: (R2 + R) / r = 2 (based on the ratio of power on resistance R is 1:4)
So R2 = R
When the slider slides to the middle, the resistance value in the circuit is 1.5R, and the power on R is
9*（4/9）=4w

Is it possible that the power factor can not be equal to 1 if the voltage and current waveforms are different? Why?

Yes!
Different waveforms of voltage and current indicate different harmonic contents
For example, the voltage is a sine wave, and the current contains harmonics (such as rectifying its input). According to the orthogonality of the sine function, the average power of the voltage and current at different frequencies is zero, that is, the harmonic current does not work. Since there is reactive current, the power factor is less than 1
The above examples show that when the harmonic component of the current is more than the voltage, there is reactive current, which indicates that the power factor is less than 1. This is easy to understand. According to the symmetry of voltage and current in the power calculation formula, when the harmonic component of the voltage is more than the current, the power factor is also less than 1
The active power is less than or equal to the product of RMS of fundamental voltage and RMS of fundamental current, while the apparent power is equal to the product of RMS of voltage and RMS of current (approximately equal to RMS of current). Since RMS of voltage is greater than RMS of voltage, the power factor is less than 1

A third day physics power problem! If an electric kettle is marked with the word "220 V 800 W", how much power is consumed when it works normally for 20 minutes? What is the rated current? What is the resistance when it works normally? (for detailed process, speed ~~~~~~~~~~~