Electrical Engineering ⇒ Topic : Current

Current is defined as the rate of flow of charge through a conductor. If a total charge q passes through a conductor in time t (rate of flow of charge is constant) then current I = q/t. The unit of charge is coulomb and unit of time is second, the unit of current is :

I = Coulomb / Second = Ampere

If the rate of flow of charge is not constant, the instantaneous value of current is given by

i = dq / dt

As water always falls from higher to lower levels, the current also always flows from higher potential to lower potential. Current flow through an element is represented by an arrow and a value (variable). Arrow represents a reference direction and value represents the current flow in the reference direction as shown in Figure

Representation of current.

Figure below shows another example. In Figure (a), 5 A current is flowing from left to right whereas in Figure (b), 5 A current is flowing from right to left. These two cases are identical to each other.

Two-way representation of the same current.

Unit of current

Current ; The flow of electron in a circuit 

current is a scalar quantity not a vector

Current

The flow of electrons through a conductor is called electric current. In order to determine the quantity of current flowing through the conductor, it is necessary to use a unit to measure the current flow. Ampere is the unit of electric current.The symbol for ampere is A. In 1950, the International Electrotechnical Commission adopted ampere, namely, the unit of current as the basic electrical unit. It is defined as "that current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in a vacuum, would produce between these conductors a force of 2 x 10^-7 newton per metre of length".

Current

The absolute value of ampere is determined by means of a current balance in which the force between the fixed and moving coils carrying the current is balanced by the force of gravity acting on a known mass. The fixed and the moving coils are made to known dimensions and from these dimensions and the relative position of the coils, the value of the current required to exert a certain force can be calculated.

The flow of electrons through a conductor is called electric current. In order to determine the quantity of current flowing through the conductor, it is necessary to use a unit to measure the current flow. Ampere is the unit of electric current.The symbol for ampere is A. In 1950, the International Electrotechnical Commission adopted ampere, namely, the unit of current as the basic electrical unit. It is defined as "that current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in a vacuum, would produce between these conductors a force of 2 x 10⁷ newton per metre of length".

There are free electrons available in all semiconductive and conductive materials.These free electrons move at random in all directions within the structure in the absence of external pressure or voltage. If a certain amount of voltage is applied across the material, all the free electrons move in one direction depending on the polarity of the applied voltage, as shown in Fig.(a).

         This movement of electrons from one end of the material to the other end constitutes an electric current, denoted by either I or i. The conventional direction of current flow is opposite to the flow of -ve charges, i.e. the electrons.

Current is defined as the rate of flow of electrons in a conductive or semiconductive material. It is measured by the number of electrons that flow past a point in unit time. Expressed mathematically,

where I is the current, Q is the charge of electrons, and t is the time, or

where dq is the small change in charge, and dt is the small change in time.In practice, the unit ampere is used to measure current, denoted by A. One ampere is equal to one coulomb per second. One coulomb is the charge carried by 6.25 X 10¹⁸ electrons. For example, an ordinary 80 W domestic ceiling fan on 230 V supply takes a current of approximately 0.35 A. This means that electricity is passing through the fan at the rate of 0.35 coulomb every second, i.e. 2.187 X 10¹⁸ electrons are passing through the fan in every second; or simply, the current is 0.35 A.