Magnetism and Matter

Given, area A = 5 cm² = 5 $\times$ 10^$-$4 m²

Number of turns per unit length, n = 4000 turns/m

Magnetic field, B = $\pi$T

Current, i = 5A

Now, magnetic intensity, H = ni = 4000 $\times$ 5 = 2 $\times$ 10⁴ Am^$-$1

As, B = $\mu$₀ (H + I) [I = intensity of magnetisation]

Substituting all the values in above equation, we get

$\pi = 4\pi \times {10^{ - 7}}((2 \times {10^4}) + I)$

${1 \over 4} \times {10^7} = (2 \times {10^4}) + I$

$250 \times {10^4} = 2 \times {10^4} + I$

$I = 250 \times {10^4} - 2 \times {10^4}$

$ \Rightarrow I = 248 \times {10^4}$ A/m

Now, pole strength, $m = I \times A = 248 \times {10^4} \times 5 \times {10^{ - 4}}$

= 1240 A-m