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### Thermal Properties of Matter NCERT Solutions Class 11 Physics - Solved Exercise Question 11.9

Question 11.9:
A brass wire 1.8 m long at 27 °C is held taut with little tension between two rigid supports. If the wire is cooled to a temperature of –39 °C, what is the tension developed in the wire, if its diameter is 2.0 mm? Co-efficient of linear expansion of brass = 2.0 × 10–5 K–1; Young’s modulus of brass = 0.91 × 1011 Pa.
Solution:
Initial temperature, T1 = 27°C
Length of the brass wire at T1, l = 1.8 m
Final temperature, T2 = –39°C
Diameter of the wire, d = 2.0 mm = 2 × 10–3 m
Tension developed in the wire = F
Coefficient of linear expansion of brass, α = 2.0 × 10–5 K–1
Young’s modulus of brass, Y = 0.91 × 1011 Pa
Young’s modulus is given by the relation:
γ = Stress / Strain  =  (F/A) / (∆L/L)
∆L = F X L / (A X Y)      ......(i)

Where,
F = Tension developed in the wire
A = Area of cross-section of the wire.
ΔL = Change in the length, given by the relation:
ΔL = αL(T2T1) … (ii)
Equating equations (i) and (ii), we get:
αL(T2 - T1) = FL / [ π(d/2)2 X Y ]
F = α(T2 - T1)πY(d/2)2
F = 2 X 10-5 X (-39-27) X 3.14 X 0.91 X 1011 X (2 X 10-3 / 2 )2
= -3.8 X 102 N