A student has two wires of iron and copper of equal length and diameter. He first joins two wires in series and passes an electric current through the combination which increases gradually. After that he joins two wires in parallel and repeats the process of passing current. Which wire will glow first in each case?

Explanation

(1) In series combination, there will be same current through Iron and as well as copper wire. Since the rate of heat production, `P = I^(2) R` or `P prop R` (for the given value of 1). The resistance of iron wire is more than that of copper for the given length and diameter. Hence in iron wire, the rate of heat production increases gradually. In series combination Iron will glow first.
(2) In parallel combination of Iron and copper wire, there will be same P.D (V) across them.
Since the rate of heat production, `P = (V^(2))/(R ) "or" P prop (1)/(R )` (for the given value fo V). The resistance of Iron is more than that of copper for the given length and diameter. Hence in copper wire, the rate of heat production is more.
In parallel combination copper will glow first.