NCERT Book Class 12 Physics Chapter 3 Current Electricity

Chapter Three

CURRENT
ELECTRICITY

3.1 INTRODUCTION
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.

3.2 ELECTRIC CURRENT
Imagine a small area held normal to the direction of 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