In Chapter 1, all charges whether free or bound, were considered to be at rest. Charges in motion constitute an electric current. Such currents occur naturally in many situations. Lightning is one such phenomenon in which charges flow from the clouds to the earth through the atmosphere, sometimes with disastrous results. The flow of charges in lightning is not steady, but in our everyday life we see many devices where charges flow in a steady manner, like water flowing smoothly in a river. A torch and a cell-driven clock are examples of such devices. In the present chapter, we shall study some of the basic laws concerning steady electric currents.

Currents are not always steady and hence more generally, we define the current as follows. Let DQ be the net charge flowing across a cross-section of a conductor during the time interval Dt. Then, the current at time t across the cross-section of the conductor is defined as the value of the ratio of DQ to Dt in the limit of Dt tending to zero,
I(t) = lim (DQ / Dt)
As derived, in SI units, the unit of current is ampere. An ampere is aligned with practical current measurements in household electricity.

Currents can be steady or unsteady. A steady current flows at a constant rate, while an unsteady current varies with time. For unsteady currents, we need to think about the net charge flowing through a conductor's cross-section over a small interval, as defined above; whereas steady currents allow for a simpler calculation.

A basic law regarding flow of currents was discovered by G.S. Ohm in 1828. Imagine a conductor through which a current I is flowing and let V be the potential difference between the ends of the conductor. Then Ohm’s law states that V ∝ I or, V = RI, where R is the resistance of the conductor.

An electric charge will experience a force if an electric field is applied. If it is free to move, it will thus move contributing to a current. We consider only solid conductors, where the current is carried by the negatively charged electrons in a backdrop of fixed positive ions. As we analyze the movement of these charges, we find that as long as there's an electric field, electrons drift and create a current.

Electrons in a conductor, due to thermal motion, move in random directions. However, when subjected to an electric field, they acquire a drift velocity that results in a net flow of charge. This drift velocity is quite small compared to the random velocities of the electrons due to thermal energy.

The electric power dissipated in a conductor can be calculated by the product of the current flowing through it and the voltage across it. Hence, the power loss in a resistor can be represented as P = IV, which can also be expressed using Ohm’s Law as P = I²R or P = V²/R.