What is Electric Current?

What is Electric Current?

Electric current is nothing but the rate of flow of electric charge through a conductor with respect to time. It is caused by drift of free electrons through a conductor to a particular direction. As we all know, the measuring unit of electric change is Coulomb and the unit of time is second, the measuring unit of electric current is Coulombs per second and this logical unit of current has a specific name Ampere after the famous French scientist André-Marie Ampere.

If total Q Coulomb charge passes through a conductor by time t, then electric current I = Q / t coulomb par second or Ampere.

For better understanding, let give an example, suppose total 100 coulombs of charge is transferred through a conductor in 50 seconds. What is the electric current?

As the electric current is nothing but the rate at which charge is transferred per unit of time, it would be ratio of total charge transferred to the required time for that. Hence, here electric current I = 100 coulombs / 50 second = 2 Amperes.

'Ampere' is Sl unit of current.
Definition of Electric Current

While a potential difference is applied across a conductor, electrical charge flows through it and electrical electric current is the measure of the quantity of the electrical charge flowing through the conductor per unit time.

PER UNIT CALCULATION

PER UNIT CALCULATION

a per-unit system is the expression of system quantities as fractions of a defined base unit quantity. Calculations are simplified because quantities expressed as per-unit do not change when they are referred from one side of a transformer to the other. This can be a pronounced advantage in power system analysis where large numbers of transformers may be encountered. Moreover, similar types of apparatus will have the impedance lying within a narrow numerical range when expressed as a per-unit fraction of the equipment rating, even if the unit size varies widely. Conversion of per-unit quantities to volts, ohms, or amperes requires a knowledge of the base that the per-unit quantities were referenced to. The per-unit system is used in power flow, short circuit evaluation, motor starting studies etc.

The main idea of a per unit system is to absorb large differences in absolute values into base relationships. Thus, representations of elements in the system with per unit values become more uniform.

There are several reasons for using a per-unit system:

• Similar apparatus (generators, transformers, lines) will have similar per-unit impedance and losses expressed on their own rating, regardless of their absolute size. Because of this, per-unit data can be checked rapidly for gross errors. A per unit value out of normal range is worth looking into for potential errors.
• Manufacturers usually specify the impedance of apparatus in per unit values.
• Use of the constant ${\displaystyle \textstyle {\sqrt {3}}}$ is reduced in three-phase calculations.
• Per-unit quantities are the same on either side of a transformer, independent of voltage level
• By normalizing quantities to a common base, both hand and automatic calculations are simplified.
• It improves numerical stability of automatic calculation methods.
• Per unit data representation yields important information about relative magnitudes.

The PU system deals primarily with values of power, voltage, current, and impedance. Any two variables are selected to be base values. The selection of two base values then fixes the other values. A per unit quantity then becomes the ratio of a selected parameter to a selected base value. Basically,

Per Unit = Present Value/ Base Value

Relationship between units