Question

A coil of copper wire has resistance of 90 Ω at 20°C and is connected to a 230- V supply. By how much must the voltage be increased in order to maintain the current consant if the temperature of the coil rises to 60°C? Take the temperature coefficient of resistance of copper as 0.00428 from 0°C.

Answer 1

R₆₀ = 90 × 1.2568/1.0856 = 104.2Ω 

Now, current at 20°C = 230/90 = 23/9 A 

Since the wire resistance has become 104.2Ω at 60°C, the new voltage required for keeping the current constant at its previous value = 104.2 × 23/9 = 266.3 V 

∴ increase in voltage required = 266.3 − 230 = 36.3 V