Calculate the length of a constantan wire of cross-sectional area 4 π x 10-8m² and resistivity 1.1 x 10-6 Ω m required to construct a standard resistor of resistance 21 Ω. [Take π = 22/7]

A. 0.42m
B. 2.40 m
C. 6.25 m
D. 183.75 m
E. 590.00 m

Correct Answer:

Option B – 2.40 m

Explanation

R = PL/A

=> L =RA/P

=21 x 4π x 10^8/1.1 x 10^-6

= 2.40