Direct current, abbreviated as DC, refers to the current that does not change its direction. In a DC circuit, the current always flows in the same direction, and in an external circuit, it always flows from the positive to the negative. Such as batteries, DC generators, etc. Direct current is characterized by the fact that the voltage is proportional to the current. Direct current is divided into two main categories: constant current and pulsating direct current. Constant current means that the magnitude and direction do not change with time within a certain range, such as dry cell batteries. Pulsating direct current refers to the direction that does not change, but the magnitude can change with time, such as 50Hz alternating current after the diode rectification is a typical pulsating direct current.
Alternating current, abbreviated as AC, refers to an electric current that periodically changes in magnitude and direction. Generally, it changes periodically according to the sine waveform, and in fact there are triangular waves and square waves. The mains power in life is alternating current in the form of a sinusoidal waveform, such as an alternator. Alternating current is characterized by a nonlinear relationship between voltage and current, depending on the frequency and phase of the alternating current.
The main difference between direct current and alternating current is the direction and stability of the current. It is manifested in the following aspects.
1.Current direction: The direction of the current of direct current is constant and always flows in one direction. Whereas, the direction of the current of alternating current is alternating and periodically flows in both directions.
2.Stability: The voltage and current of direct current are constant and do not change over time. The voltage and current of alternating current change periodically over time, and its magnitude and direction will change.
3.Application scenario: DC is usually used in low-voltage, low-power equipment, such as mobile phones, computers and other electronic devices. Alternating current is widely used in domestic and industrial fields, such as lighting, heating, electric motors, etc.
4.Energy conversion: Direct current has high efficiency in the energy conversion process and is suitable for long-distance transmission. Alternating current, on the other hand, is less efficient in the energy conversion process, but its voltage can be easily raised and lowered, making it suitable for household and industrial use.
5.Generation: Direct current is usually generated by batteries, fuel cells, and other devices. Alternating current is generated by equipment such as generators and transformers.
6. There are differences in the relevant calculations.
1) According to P=IU, in DC circuits, power=current voltage, in AC circuits, power=iucos. The calculation method of the active power in the pure resistive AC circuit and the power in the DC circuit is exactly the same, and the voltage and current are calculated with the effective values.
The RMS value of AC current refers to the amount of heat generated by AC current on the resistance r in a period of AC change is equivalent to the amount of heat generated by the DC current on the resistance, and the value of this DC current is the RMS value of the AC current. The RMS value of sinusoidal alternating current is equal to 0707 times. In general, unless otherwise specified, the magnitude of alternating current refers to the effective value. For example, 220 volts of urban domestic electricity represents the effective value, and its peak value is about 311 volts.
2) Ohm's law. In AC circuits, the relationship between the peak value or RMS value of voltage and current is similar to Ohm's law in DC circuits, and the equation is u=iz or i=u z, where z and u are both RMS values of AC and z is impedance. Since voltage and current have phase differences depending on the component, the relationship between the RMS values of voltage and current is generally not a simple proportional quantity.
7. The advantages are different.
1) Compared with direct current, the advantages of alternating current are mainly manifested in power generation and distribution.
a. Power generation: The alternator can convert other forms of energy such as mechanical energy, chemical energy, and internal energy into electrical energy very economically and conveniently. Such as hydropower, wind energy, oil, natural gas, garbage, etc.
b. Power distribution: AC power supply and AC substation are much cheaper than DC power supply and DC converter station of the same power.
c. Transformer: AC power can be easily boosted and stepped down through the transformer, which brings great convenience to the distribution of electric energy.
2) Advantages of direct current.
a. In DC circuits, a constant electric field can be formed. The power supply can provide a constant electromotive force that does not change with time.
b. When transmitting the same power, the wire used in DC transmission is only 2 3 l 2 of AC transmission.
c. In cable transmission lines, there is no capacitive current in DC transmission, while there is capacitive current in AC transmission line, which causes loss. A coaxial capacitor is formed between the cable core and the earth, and a 200kV cable has a capacitance of about 0 per kilometer2 f, it will consume 2 hours of electricity per year6×107kwh。
8. The relationship between DC and AC. Any non-harmonic alternating current can be decomposed into a series of simple harmonic sine and cosine alternating currents.
In short, there are obvious differences between direct current and alternating current in terms of current direction, stability, application scenarios, energy conversion, generation methods, advantages and disadvantages, etc.