Physics · Magnetism And Matter · NEET
B (magnetic field) is in tesla (T). H (magnetic intensity) and M (magnetisation) are BOTH in ampere per metre (A/m). H and M share a unit because of the relation B = μ₀(H + M): for the two quantities inside the bracket to add, they must have the same unit. So whenever you see A/m, think 'H or M', and whenever you see tesla, think 'B'. This one fact clears most unit MCQs.
Yes. χ = M/H, and since M and H both have the unit A/m, the units cancel. So χ is a pure number with NO unit and NO dimension. This is a favourite one-line NEET fact: susceptibility is dimensionless. Relative permeability μ_r = 1 + χ is dimensionless too, for the same reason.
Magnetic moment m = current × area = I × A, so its unit is A·m² and its dimensional formula is [A L²] = [M⁰ L² T⁰ A]. For a bar magnet, m = pole strength × length, which also comes out to A·m². Do not confuse this with torque (N·m). Magnetic moment is A·m²; torque is N·m.
From B = μH, we get μ = B/H = tesla ÷ (A/m) = T·m/A. So μ₀ = 4π × 10⁻⁷ T·m/A. The SAME unit is also written as N/A² or henry/metre (H/m) — they are all equal. In NEET numericals, if you multiply μ₀ (T·m/A) by a dimensionless number like (1 + χ), the answer stays in T·m/A.
Pole strength (often written m or q_m) measures how strong a magnetic pole is; its SI unit is ampere·metre (A·m). Magnetic moment = pole strength × magnetic length, so A·m × m = A·m². A common trap: pole strength is A·m, but magnetic MOMENT is A·m². The word 'moment' adds one extra metre.
An iron rod of susceptibility 599 is subjected to a magnetising field of 1200 A/m. The permeability of the material of the rod is (μ₀ = 4π × 10⁻⁷ T·m/A):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Susceptibility χ = M/H is a pure number (dimensionless). Permeability μ = B/H has the unit T·m/A. They are linked by μ = μ₀(1 + χ), so μ_r = 1 + χ.
Magnetic flux Φ is in weber (Wb), where 1 Wb = 1 T·m². Magnetic field B (flux density) is in tesla (T) = Wb/m². So flux is field × area.
Yes, both are pure numbers with no unit. Relative permeability μ_r = μ/μ₀ and susceptibility χ = M/H both come from ratios where units cancel.
B has unit tesla = kg·A⁻¹·s⁻², so its dimensional formula is [M T⁻² A⁻¹]. It follows from B = F/(qv) or F = BIL.
Yes. Magnetic intensity H and magnetisation M are both measured in ampere per metre (A/m) in SI units, which is why their ratio χ is dimensionless.
