3.2 Electric Current

Electric charges in motion constitute an electric current. Any medium having practically free electric charges, free to migrate is a conductor of electricity. The electric charge flows from higher potential energy state to lower potential energy state.

Imagine a small area held normal to the direction of the flow of charges. Both the positive and the negative charges may flow forward and backward across the area. In a given time interval \(t\), let \(q_{+}\)be the net amount (i e; forward minus backward) of positive charge that flows in the forward direction across the area. Similarly, let \(q_{-}\)be the net amount of negative charge flowing across the area in the forward direction. The net amount of charge flowing across the area in the forward direction in the time interval \(t\), then, is \(q=q_{+}-q_{-}\). This is proportional to \(t\) for steady current and the quotient
\(
I=\frac{q}{t} \dots(3.1)
\)
is defined to be the current across the area in the forward direction. (If it turns out to be a negative number, it implies a current in the backward direction.)

Positive charge flows from higher to lower potential and negative charge flows from lower to higher. Metals such as gold, silver, copper, aluminium etc. are good conductors. When charge flows in a conductor from one place to the other, then the rate of flow of charge is called electric current ( \(I\) ). When there is a transfer of charge from one point to other point in a conductor, we say that there is an electric current through the area. If the moving charges are positive, the current is in the direction of the motion of charge. If they are negative the current is opposite to the direction of motion. Currents are not always steady and hence more generally, we define the current as follows. Currents are not always steady and hence more generally, we define the current as follows. Let \(\Delta Q\) be the net charge flowing across a cross-section of a conductor during the time interval \(\Delta t\) [i.e., between times \(t\) and \((t+\Delta t)]\). Then, the current at time \(t\) across the cross-section of the conductor is defined as the value of the ratio of \(\Delta Q\) to \(\Delta t\) in the limit of \(\Delta t\) tending to zero,

\(
\text {Instantaneous current }  I(t)=\lim _{\Delta t \rightarrow 0} \frac{\Delta Q}{\Delta t} \dots(3.2)
\)

\(
\text { Average current } I_{a v}=\frac{\Delta Q}{\Delta t}
\)

In SI units, the unit of current is ampere. An ampere is defined through magnetic effects of currents that we will study in the following chapter. An ampere is typically the order of magnitude of currents in domestic appliances. An average lightning carries currents of the order of tens of thousands of amperes and at the other extreme, currents in our nerves are in microamperes.

CLASSIFICATION OF MATERIALS ACCORDING TO CONDUCTIVITY

Conductor

In some materials, the outer electrons of each atoms or molecules are only weakly bound to it. These electrons are almost free to move throughout the body of the material and are called free electrons. They are also known as conduction electrons. When such a material is placed in an electric field, the free electrons move in a direction opposite to the field. Such materials are called conductors.

Insulator

Another class of materials is called insulators in which all the electrons are tightly bound to their respective atoms or molecules. Effectively, there are no free electrons. When such a material is placed in an electric field, the electrons may slightly shift opposite to the field but they can’t leave their parent atoms or molecules and hence can’t move through long distances. Such materials are also called dielectrics.

Semiconductor

In semiconductors, the behaviour is like an insulator at low levels of temperature. But at higher temperatures, a small number of electrons are able to free themselves and they respond to the applied electric field. As the number of free electrons in a semiconductor is much smaller than that in a conductor, its behaviour is in between a conductor and an insulator and hence, the name semiconductor. A free electron in a semiconductor leaves a vacancy in its normal bound position. These vacancies also help in conduction.