Introduction

Current electricity is the branch of physics that studies the flow of electric charge, as opposed to electrostatics (Unit 1), which studies charges that are at rest. Even with no battery connected, the free electrons inside a metal like copper are never actually still -- they are in continuous, random thermal motion, colliding constantly with the fixed positive ions of the metal lattice. Because this motion has no preferred direction, exactly as many electrons cross any imaginary cross-section moving one way as moving the other way, so there is no net transfer of charge and hence no current, even though the electrons themselves are moving extremely fast.

The origin of current electricity as a subject is credited to Alessandro Volta (1745-1827), who invented the first electric battery capable of producing a steady, continuous flow of current -- earlier static-electricity generators could only produce brief sparks or discharges, not a sustained current. Once a battery is connected across a conductor, it sets up an electric potential difference between the conductor's two ends. Exactly as water flows only when there is a difference in height between two points (and does not flow at all when both ends of a pipe are at the same height), positive charge flows from a region of higher electric potential to a region of lower electric potential, and negative charge flows the opposite way, from lower to higher potential. The battery's whole job, in this picture, is simply to create and maintain that potential difference -- it is the potential difference, not the battery as a physical object, that actually pushes the charges around the circuit.

Modern life depends overwhelmingly on this controlled flow of charge: it powers machines, communication systems, electronic devices and virtually every home appliance in daily use. This unit builds up the whole quantitative framework for describing that flow -- starting from the basic definition of current itself, through Ohm's law and resistance, combinations of resistors, cells and batteries, Kirchhoff's rules for complex networks, the heating effect of current, and finally the reversible thermoelectric effects that connect electricity and heat.