Formula of voltage and current
Connect a resistor to an AC power supply, as shown on the right. The relationship between voltage and current can be determined according to Ohm's law. That is:
AC electric circuit diagram of pure resistance
Ohm's law of pure resistance AC circuit
On the surface of the above formula, the basic property of the AC pure resistance circuit is that the instantaneous value of the current is proportional to the instantaneous value of the voltage at both ends of the resistance.
The relationship between the effective value U at both ends of the resistance and the current effective value I flowing through the resistance can be derived from Ohm's law.
When the size of the resistance is fixed, the voltage increases and the current increases. The voltage is zero and the current is zero. That is, the sinusoidal curve of the current is in accordance with the waveform of the sinusoidal curve of the voltage. So the voltage and current are in the same phase in the resistance load circuit.
AC power formula
Because the voltage and current of AC circuit change with time. At any time, the instantaneous product of voltage instantaneous value u and current instantaneous value I is instantaneous power. It is represented by "P":
Calculation formula for electric power of pure resistance circuit of sinusoidal alternating current
From the above formula, we can know that the instantaneous power of the resistance element is made up of two parts. The first part is Changshu urir, and the second part is the variable alternating current power with the amplitude of urir and the angular frequency of 2 Omega changing according to the cosine law.
AC electric power waveform diagram
As shown by the dotted line in the image on the right image, the P is the waveform that changes the power over time. It is always greater than zero in a period, and the surface resistance element always absorbs electrical energy, that is, power consumption.
Although the instantaneous power can change the surface power within a period of time, it is not convenient to measure and calculate the instantaneous power, which is of little practical significance. Generally speaking, the power of a circuit refers to the average value of instantaneous power in a cycle. It is called the average or active power. It is represented by the capital letter "P".
Calculation formula of active power of alternating current
Its unit is Vata. It can be seen from the upper form. When the voltage and current are represented by the effective value, the expression of the average power in the pure resistance circuit has the same form as the DC circuit.