The flow of charge in define direction is called electric
current. It is measured by the time rate of flow of charge
through a conductor (or material). In the words, electric
current is the flow of charge through any cross-section of
the conductor per second. If q is the charge flowing through
any cross-section of the conductor in time t, then,
Electric current, I = q / t
When a cell is connected across the ends of conductor (see the
fig), electric field is set up in the conductor. The electric
field exerts force on the free electrons, causing the free
Electrons to drift towards the positive terminal of the cell.
This Constitutes electric current in the conductor. As convention
the direction of flow of positive charge is taken as the direction
of current flow. If moving charge are negative ,as with free
flow of negative , as with free electrons in metal, then direction
of current is opposite to the flow of negative charge. This
convention does not make any difference in the actual operation
of the electrical appliances.
If the rate of flow of charge
varies with time, then current at any time(i.e. instantaneous current) is given by;
= dq / dt
where dq is a small charge passing through any cross-section
of the conductor is small dt.
Unit of electric current:
The SI unit of charge is one coulomb and that of time is one
second . Therefore, SI unit of current is 1 coulomb/sec which
has been given a special name of ampere. If q = IC and t =
1s, then I =1/1 = 1 ampere.
One ampere of current is said to flow through a wire if at any
cross-section, one coulomb of charge flow in one second .
Thus, if 5 ampere current is flowing through a wire , it means
that 5 C per second flows past any cross-section of the wire.
C = charge on 6.25×1018
electrons. Thus when we say that current through a wire is
1A, it means that 6.25×1018electrons per second
flow past any cross-section of wire. Therefore:
I = q / t = ne / t
where e = -1.6×10-19C