The ends Q and R of two thin wires, PQ and RS, are soldered

Last updated: 7/30/2022

The ends Q and R of two thin wires, PQ and RS, are soldered (joined) together. Initially, each of the wire has a length of 1 m 10°C. Now, the end P is maintained at 10°C, while the end S is heated and maintained at 400°C. The system is thermally insulated from its surroundings. If the thermal conductivity of wire PQ is twice that of the wire RS and the coefficient of linear thermal expansion of PQ is 1.2 × 10³ K -1 -¹, the change in length of the wire PQ is (a) 0.78 mm (b) 0.90 mm (c) 1.56 mm (d) 2.34 mm