3.6 LIMITATIONS OF OHM’S LAW

. LIMITATIONS OF OHM’S LAW Although Ohm’s law has been found valid over a large class of materials, there do exist materials and devices used in electric circuits where the proportionality of V and I does not hold. The deviations broadly are one or more of the following types: (a) V ceases to be proportional to I (Fig. .

). (b) The relation between V and I depends on the sign of V. In other words, if I is the current for a certain V, then reversing the direction of V keeping its magnitude fixed, does not produce a current of the same magnitude as I in the opposite direction (Fig. .

). This happens, for example, in a diode which we will study in Chapter . (c) The relation between V and I is not unique, i.e., there is more than one value of V for the same current I (Fig. .

). A material exhibiting such behaviour is GaAs. Materials and devices not obeying Ohm’s law in the form of Eq. ( .

) are actually widely used in electronic circuits. In this and a few subsequent chapters, however, we will study the electrical currents in materials that obey Ohm’s law.