If the outer conductor of a coaxial cable has radius 2.6 mm, what should be the radius of the inner conductor so that the inductance per unit length does not exceed 50 nH per meter? Express your answer using two significant figures.

Inner radius = 2 mm

Explanation:

In a coaxial cable, series inductance per unit length is given by the formula;

L' = (µ / (2π)) •ln (R/r)

Where R is outer radius and r is inner radius.

We are given;

L' = 50 nH/m = 50 * 10^ (-9) H/m

R = 2.6mm = 2.6 * 10^ (-3) m

Meanwhile µ is magnetic constant and has a value of µ = µ_o = 4π * 10^ (-7) H/m

Plugging in the relevant values, we have;

50 * 10^ (-9) = (4π * 10^ (-7)) / (2π)) * ln (2.6 * 10^ (-3) / r)

Rearranging, we have;

(50 * 10^ (-9)) / (2 * 10^ (-7)) = ln ((2.6 * 10^ (-3)) / r)

0.25 = ln ((2.6 * 10^ (-3)) / r)

So,

e^ (0.25) = (2.6 * 10^ (-3)) / r)

1.284 = (2.6 * 10^ (-3)) / r)

Cross multiply to give;

r = (2.6 * 10^ (-3)) / 1.284)

r = 0.002 m or 2 mm