Q.In Joule's heating law, when R and t are constant, if H is taken along the y axis and $I^2$ along the x axis, the graph is

The law. If a current I flows through a conductor across a potential difference V for a time t, the electrical work done is $W=VIt$; when this energy is spent entirely on heating the conductor (no other useful effect such as mechanical work), the heat produced is $H=VIt$, which, using Ohm's law $V=IR$, converts into the commonly used resistor form

This relation, verified experimentally by Joule, is stated as: the heat developed in a circuit due to current flow varies directly as (i) the SQUARE of the current, (ii) the resistance, and (iii) the time for which the current flows.

Everyday and industrial applications, all built on this one law. Electric heaters, irons and toasters use heating elements made of nichrome (a nickel-chromium alloy), chosen for its high specific resistance and its ability to withstand very high, repeated heating without oxidising. Electric fuses are a deliberately weak, short length of low-melting-point wire (lead-tin alloy below 15 A, copper above it) connected in series specifically so that it melts and breaks the circuit first, before an excessive current can damage anything more valuable downstream; modern circuit breakers perform the identical protective function but can simply be reset and switched back on rather than physically replaced after tripping. **Electric f …