ELECTRIC CURRENT

An electric current is caused by the motion of electric charge. 

If a net charge ∆Q passes through any cross section of a conductor in time ∆t, we say that an electric current I has been established through the conductor where 

   I = ∆Q/∆t

The SI unit of current is ampere and it is a current due to flow of charge at the rate of one coulomb per second.

Motion of electric charge which causes an electric current is due to the flow of charge carriers. 

• In case of metallic conductors, the charge carriers are electrons. 
• The charge carriers in electrolyte are positive and negative ions.
• In gases, the charge carriers are electrons and ions.

Current Direction

History: Early scientists regarded an electric current as a flow of positive charge from positive to negative terminal of the battery through an external circuit. 

Later on, it was found that a current in metallic conductors is actually due to the flow of negative charge carriers called electrons moving in the opposite direction i.e., from negative to positive terminal of the battery, but it is a convention to take the direction of current as the direction in which positive charges flow. 

This current is referred as conventional current. The reason is that it has been found experimentally that positive charge moving in one direction is equivalent in all external effects to a negative charge moving in the opposite direction. 

As the current is measured by its external effects so a current due to motion of negative charges, after reversing its direction of flow can be substituted by an equivalent current due to flow of positive charges.

Conventional Current 

"The conventional current in a circuit is defined as that equivalent current which passes from a point at higher potential to a point at a lower potential as if it represented a movement of positive charges."

While analyzing the electric circuit, we use the direction of the current according to the above mentioned convention. 

If we wish to refer to the motion of electrons, we use the term electronic current. 

Current Through a Metallic Conductor

In a metal, the valence electrons are not attached to individual atoms but are free to move about within the body. These electrons are known as free electrons. The free electrons are in random motion just like the molecules of a gas in a container and they act as charge carriers in metals. The speed of randomly moving electrons depends upon temperature.

If we consider any section of metallic wire, the rate at which the free electrons pass through it from right to left is the same as the rate at which they pass from left to right. As a result the current through the wire is zero. 

If the ends of the wire are connected to a battery, an electric field E will be set up at every point within the wire. The free electrons will now experience a force in the direction opposite to E. As a result of this force the free electrons acquire a motion in the direction of -E. It may be noted that the force experienced by the free electrons does not produce a net acceleration because the electrons keep on colliding with the atoms of the conductor. The overall effect of these collisions is to transfer the energy of accelerating electrons to the lattice with the result that the electrons acquire an average velocity, called the drift velocity in the direction of -E. 

The drift velocity is of the order of 10^-3 ms^-1, whereas the velocity of free electrons at room temperature due to their thermal motion is several hundred kilometres per second.

Thus, when an electric field is established in a conductor, the free electrons modify their random motion in such a way that they drift slowly in a direction opposite to the field. In other words the electrons, in addition to their violent thermal motion, acquire a constant drift velocity due to which a net directed motion of charges takes place along the wire and a current begins to flow through it. 

Steady Current

A steady current is established in a wire when a constant potential difference is maintained across it which generates the requisite electric field E along the wire.