A steel scale measures the length of a copper wire as 80.0 cm when both are at 20°C (the calibration temperature for scale). What would be the scale read for the length of the wire when both are at 40°C? (Given αsteel = 11×10-6  per °C and αcopper = 17×10-6per0C)

With temperature rise (same 25°C for both), steel scale and copper wire both expand. Hence length of copper wire w.r.t. steel scale or apparent length of copper wire after rise in temperatureLapp =L0+ LCu'-Lsteel'        =L0+ [L0(1+αCu∆θ)-L0(1+αs∆θ)]⇒Lapp =L0+ L0(αCu-αs)∆θ              =80+ 80(17×10-6-11×10-6)×20)               = 80.0096 cm