Introduction

What this section does

• This is a short orientation section, with no formula of its own to be examined on.
• Std XI covered the origin of electrical conductivity, especially in metals, and how to find the effective resistance of two or more resistors in series and in parallel.
• The point being made is a limitation: a circuit containing several complex connections of electrical components cannot easily be reduced to a single loop by the series and parallel rules. A Wheatstone network is the standard example — none of its four arms is simply in series or simply in parallel with another.
• Such circuits are analysed instead by Kirchhoff's laws, two rules formulated by Gustav Robert Kirchhoff (1824-1887). This chapter states those two laws and then applies them to three instruments: the Wheatstone bridge (and its practical forms, the metre bridge and the post office box), the potentiometer, and the moving coil galvanometer converted into an ammeter or a voltmeter.

Recall questions the chapter opens with

• There are three types of electrical conductors: good conductors (metals), semiconductors, and bad conductors (insulators).
• A semiconductor diode and a resistor do not have similar electrical properties — a resistor obeys Ohm's law and conducts equally both ways, a diode does not.
• Two or more resistors have different effective resistances in series and in parallel because in series the same current passes through each and the potential differences add, while in parallel the same potential difference acts across each and the currents add.