The ends Q and R of two thin wires, PQ and RS, are soldered (joined) together. Initially each of the wires
has a length of 1 m at 10
o
C. Now the end P is maintained at 10
o
C, while the end S is heated and
maintained at 400
o
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 \times 10
-5
K
-1
, the change in length of the wire PQ is