Current ElectricityIntroduction
A monsoon evening in Mumbai, and the power has gone. Outside Riya's window, lightning forks down into the sea: for a split second, a huge amount of charge rushes from the clouds to the earth, sometimes with disastrous results. Inside, Riya switches on her torch and carries on reading. On the wall, the cell-driven clock keeps ticking as if nothing had happened.
Both are electric currents: charges in motion. In Chapter 1 every charge, free or bound, was at rest. Now the charges move. But the lightning and the torch move them very differently. Sort these by how the charge flows.
Torch, clock, calculator and parking light keep a gentle flow going for hours. Lightning, sparks and flashes dump their charge all at once.
Two Kinds of Flow
Here are the two currents of Riya's evening, drawn to scale. Look at the units on each axis before you compare them.
A lightning stroke: about 20 000 A, over in a few millionths of a second.
Riya's torch: about 0.3 A, minute after minute.
In this chapter we study steady currents, the calm kind on the right. What exactly is a current, and how do we measure it at one instant? Why do electrons in a wire drift so slowly, and why does a wire resist them at all? Where does the energy of a current go, and what keeps a current flowing? And how do we work out every current in a whole network of cells and resistors? Two simple rules, due to Kirchhoff, will answer the last question.