NCERT Solutions
Class 11 Physics
Thermal Properties of Matter

Q. 10.10
A brass rod of length 50 cm and diameter 3.0 mm is joined to a steel rod of the same length and diameter, what is the change in length of the combined rod at 250 °C, if the original lengths are at 40.0 °C? Is there a ‘thermal stress’ developed at the junction? The ends of the rod are free to expand (Co-efficient of linear expansion of brass = 2.0 × 10–5 K–1, steel = 1.2 × 10–5 K–1).
Initial temperature, T1 = 40°C
Final temperature, T2 = 250°C
Change in temperature, ΔT = (T2 – T1) = 210°C
Length of the brass rod at T1, l1 = 50 cm
Diameter of the brass rod at T1, d1 = 3.0 mm
Length of the steel rod at T2, l2 = 50 cm
Diameter of the steel rod at T2, d2 = 3.0 mm
Coefficient of linear expansion of brass, α1 = 2.0 × 10–5 K–1
Coefficient of linear expansion of steel, α2 = 1.2 × 10–5 K–1
For the expansion in the brass rod, we have:
(Change in length (Δ l1)) / (Original length (l1)) = α1 ΔT
Therefore,
Δl1 = (50 x (2.1 x 10-5) x 210)
=0.2205cm
For the expansion in the steel rod, we have:
(Change in length (Δ l2)) / (Original length (l2)) = α2 ΔT
Therefore,
Δl2 = (50 x (1.2 x 10-5) x 210)
=0.126cm
Total change in the lengths of brass and steel,
Δl = Δl1 + Δl2
= 0.2205 + 0.126
= 0.346 cm
Total change in the length of the combined rod = 0.346 cm
No thermal stress is developed so the ends of rod are free to expand.