Maintenance Engineer of Power and Environment Specialty of China Data Center - Basic Circuit Knowledge

electric current
The directional motion of an electric charge is called an electric current. The directional movement of free electrons in metal conductors under the action of electric field force, and the movement of positive and negative ions in electrolyte in the opposite direction under the action of electric field force are all called currents.
Current is a physical phenomenon, and also a physical quantity representing the strength of the directional motion of charged particles. The current is equal in magnitude to the ratio of the charge q passing through the cross-section of the conductor and the time t used for passing through these charge quantities. Expressed as
I = 
Where, q - the amount of electric charge passing through the cross section of conductor, unit: coulomb, symbol: C;
T - time used for passing charge quantity q, unit: second, symbol: s;
I - current, unit: ampere, symbol: A.
In the SI system, the common units of current are milliampere (mA) and microamp（ μ A)：
1A= 103mA＝106 μ A
The direction of positive charge directional movement is specified as the current direction.
In circuit calculation, a current direction (pseudo current direction) is usually assumed as the reference direction, and the reference direction of current is indicated in the circuit diagram with arrows. If the calculated current is positive, then the true direction of the current is consistent with the reference direction. If the calculated current is negative, then the true direction of the current is opposite to the reference direction.
The current is a scalar, and the direction of the current only indicates the direction of the charge's directional movement. The magnitude and direction of the current do not change with time, which is called DC current, as shown in Figure 1.1-1 (a). The magnitude of the current changes with time, but the direction does not change with time. It is called pulsating current, as shown in Figure 1.1 - 1 (b). The current whose magnitude and direction change with time is called AC current, as shown in Fig. 1-1 (c).

Textbook for Operation and Maintenance Engineer of Power and Environment Specialty of Data Center - Basic Circuit Knowledge