Define magnetisation, magnetic intensity and magnetic susceptibility. Obtain the relation between relative permeability and susceptibility.

1. 1 · Define M. Net magnetic moment per unit volume, M = m_net/V; unit A m⁻¹.
2. 2 · Define H. H = B/μ₀ − M; inside a solenoid H = nI; unit A m⁻¹.
3. 3 · Define χ. M = χH, so χ = M/H, a pure number.
4. 4 · Derive the relation. B = μ₀(H + M) = μ₀(1 + χ)H = μ₀μrH, so μr = 1 + χ.
5. 5 · Real life. Transformer cores use soft iron alloys with a very large μr.
6. 6 · Close in one line. So, B = μH with μ = μ₀μr = μ₀(1 + χ).

Scoring tip: Give the unit with each definition. Two of the three quantities share the unit A m⁻¹, and examiners check this.