Question

A student uses two wires of copper and nichrome of equal length . He first joins the two wires in series and passes electric current through the combination which increases gradually . after that the two wires are connected in parallel and repeats the process of passing the current . Which wire will glow first in each case and why ?

Answer 1

Since the resistivity of nichrome is more than copper, therefore, the resistance of copper wire is less than that of nichrome wire, for the given length and diameter.
In series combination, there will be the same current through nichrome as well as copper wire. Since the rate of heat production,
$P=I^{2}R$ or P ∝ R ( for given value of I)

Therefore, in nichrome wire, the rate of heat production is more and thus nichrome wire will glow first.

In parallel combination, there will be the same potential difference through nichrome as well as copper wire. Since the rate of heat production,
$P=\frac{V^2}{R}$ or P ∝ $\frac{1}{R}$ ( for given value of V)

Therefore, in copper wire, the rate of heat production is more and thus copper wire will glow first.