Question

Calculate the length of a constant wire of cross sectional areas 4π x 10^8m and resistivity 1.1 x 10^-6Ωm required to construct a standard resistor of resistance 21Ω (take π=22/7)

Answer 1

Resistance of wire of length $l$, cross sectional area $S$ and resistivity $\rho$ is $R = \frac{\rho l}{S}$, from where $l = \frac{ R S}{\rho} = \frac{21 \Omega \cdot 4\pi \cdot 10^{-8}m^2}{1.1 \cdot 10^{-6} \Omega \cdot m} = 2.4 m$.