direct current (direct current)
An electric current whose magnitude and direction do not change with time. Also known as constant current. The circuit through is called direct current circuit, which is a closed conductive circuit composed of DC power supply and resistance. In the circuit, a constant electric field is formed, outside the power supply, the positive charge flows from the high potential to the low potential through the resistance, in the power supply, by the non-electrostatic force of the power supply, to overcome the electrostatic force, and then from the low potential to the high potential, so the cycle, forming a closed current line. Therefore, in a direct current circuit, the role of the power supply is to provide a constant electromotive force that does not change with time to replenish energy for Joule heat consumed on the resistance.
In relatively simple DC circuits, the relationship between the electromotive force, resistance, current, and voltage at any two points of the power supply can be derived from Ohm's law and the definition of the electromotive force. Complex DC networks can be solved according to G.R. Kirchhoff's equations. It consists of two parts: the node current equation and the loop voltage equation. The former points out that for any node (the intersection of 3 or more branches), the algebraic sum of each current flowing into and out of the node is zero, which is the requirement of constant condition; the latter points out that for any closed loop (grid), the algebraic sum of each part of the voltage drop is zero, which is the result of the loop theorem of electrostatic field. The two forms a complete system of equations.
The instrument that measures current, voltage, resistance, electromotive force and other physical quantities in the direct current circuit is called DC instrument. Commonly used are galvanometer, ammeter, voltmeter, bridge, potential difference meter and so on.
Dc power supply has chemical cells, fuel cells, thermal cells, solar cells, DC generators and so on. Direct current is mainly used in various electronic instruments, electrolysis, electroplating, DC electric drive and so on. The experiment of electrolysis of water can also be carried out by using direct current. When the negative electrode is inserted into water, the water can be electrolyzed to hydrogen, and the positive electrode can be electrolyzed to oxygen.
In power transmission, after the 1880s, because it was not easy to increase the low voltage of direct current to high voltage for long-distance transmission, DC transmission gave way to AC transmission. Since the 1960s, due to the use of high voltage, high power converter to convert direct current into alternating current, DC transmission system has been paid attention to again and obtained new development.
[Edit paragraph] What is the difference between direct current and alternating current
Alternating current is a type of electricity that varies in size and direction with time.
Alternating current is issued by the alternator, in the process of power generation, multiple pairs of magnetic poles are evenly distributed in a circle at a certain Angle, so that in the process of power generation, each coil will cut the magnetic field line, because of the multiple pairs of magnetic poles, each pair of magnetic field line is cut by the voltage, current is changed according to the law of the string, so it can constantly produce stable current. The frequency of alternating current is generally 50 Hertz, which is 50 times per second. There are other frequencies, of course. Such as square waves in electronic lines, triangular, etc., but the alternating current of these waveforms is not generated by the conductor cutting the magnetic force line, but by the capacitor charging and discharging, and the switching transistor is generated when it works.
The direction of direct current does not change with time. Usually divided into and constant current. There are AC components, such as about 300 volts in the power supply circuit of the color TV is the component can be removed by capacitance. Constant current is more ideal, the size and direction are unchanged.