Magnetic susceptibility (χ) is a pure number that tells how easily a material gets magnetised in a magnetic field. It is defined as χ = M / H, where M is the magnetisation produced and H is the applied magnetic intensity. Memory hook: "chi = the material's WILLINGNESS to become a magnet" — negative for diamagnetic, small positive for paramagnetic, very large for ferromagnetic.
Sign and value of χ for each material class: negative for diamagnetic, small positive for paramagnetic, very large for ferromagnetic, and zero for non-magnetic.
Your doubts, answered
Is susceptibility M/H or B/H? I keep mixing them up.
Susceptibility is χ = M / H (magnetisation over magnetic intensity), NOT B/H. B/H is a different quantity called permeability (μ). Both M and H have the same unit (A/m), so their ratio χ is just a pure number with no unit. Keep this straight: χ links M to H, while μ links B to H.
Does magnetic susceptibility have a unit?
No. Since M and H are both measured in ampere per metre (A/m), their ratio χ = M/H cancels out to a pure dimensionless number. So χ has NO unit and NO dimension. This is a favourite NEET one-liner — if a question asks for the unit of susceptibility, the answer is 'dimensionless / no unit'.
Why is χ negative for diamagnetic materials?
In a diamagnetic material the induced magnetisation M points OPPOSITE to the applied field H (the material weakly opposes the field). Since M and H point in opposite directions, χ = M/H comes out negative. Its value is small: 0 > χ ≥ −1. A superconductor is the extreme case with χ = −1 (perfect diamagnet).
What is the difference between susceptibility and relative permeability?
They describe the same material behaviour but are linked by one formula: μ_r = 1 + χ. Susceptibility χ measures the extra magnetisation from the material alone; relative permeability μ_r compares the total field inside the material to the field in vacuum. If you know one, you instantly get the other. This relation is directly tested in NEET numericals.
Can susceptibility be more than 1?
Yes — for ferromagnetic materials like iron, nickel and cobalt, χ is a very large positive number (χ >> 1, often hundreds or thousands). For paramagnetic materials χ is a small positive number (much less than 1). Only diamagnetic materials have negative χ (down to −1).
⚠️ The NEET trap ✗ Students write susceptibility as χ = B/H and give it a unit. ✓ Susceptibility is χ = M/H and is a pure DIMENSIONLESS number (no unit). B/H is permeability μ, which does have a unit (T·m/A). 🧠 χ links M to H (both A/m → cancels → no unit). μ links B to H (has a unit). Don't swap them.
Real NEET questions
NEET 2016
The magnetic susceptibility is negative for:
A · diamagnetic material only ✓
B · paramagnetic material only
C · ferromagnetic material only
D · paramagnetic and ferromagnetic materials
Solution: Susceptibility χ = M/H. In a diamagnetic material the induced magnetisation M opposes the applied field H, so M and H point opposite ways and χ becomes negative (−1 ≤ χ < 0). Paramagnetic and ferromagnetic materials have M along H, so their χ is positive. Therefore χ is negative for diamagnetic material only. Answer: A.
NEET 2024
Match List-I (Material) with List-II (Susceptibility χ). A. Diamagnetic B. Ferromagnetic C. Paramagnetic D. Non-magnetic || I. χ = 0 II. 0 > χ ≥ −1 III. χ >> 1 IV. 0 < χ < ε (small positive)
A · A-II, B-I, C-III, D-IV
B · A-III, B-II, C-I, D-IV
C · A-IV, B-III, C-II, D-I
D · A-II, B-III, C-IV, D-I ✓
Solution: Match each type to its χ range. Diamagnetic: small NEGATIVE, so 0 > χ ≥ −1 → II. Ferromagnetic: very LARGE positive, χ >> 1 → III. Paramagnetic: small POSITIVE, 0 < χ < ε → IV. Non-magnetic: χ = 0 → I. So A-II, B-III, C-IV, D-I. Answer: D.
NEET 2020
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):
A · 2.4π × 10⁻⁵ T·m/A
B · 2.4π × 10⁻⁷ T·m/A
C · 2.4π × 10⁻⁴ T·m/A ✓
D · 8.0 × 10⁻⁵ T·m/A
Solution: Step 1: Use the link between permeability and susceptibility: μ = μ₀(1 + χ). Step 2: Substitute χ = 599, so (1 + χ) = 600. Step 3: μ = 4π × 10⁻⁷ × 600 = 2400π × 10⁻⁷ = 2.4π × 10⁻⁴ T·m/A. (The magnetising field value 1200 A/m is extra data not needed here.) Answer: C.
Solved Magnetism And Matter NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a number that tells how easily a material becomes magnetised when placed in a magnetic field. A larger positive χ means the material magnetises strongly in the field's direction.
What is the formula for magnetic susceptibility?
χ = M / H, where M is the magnetisation (dipole moment per unit volume) and H is the magnetic intensity. It is a pure number with no unit.
What is the SI unit of magnetic susceptibility?
It has no unit. Because M and H are both in A/m, their ratio is dimensionless.
What is the relation between susceptibility and relative permeability?
μ_r = 1 + χ. So permeability μ = μ₀(1 + χ). Knowing χ lets you find μ instantly.
For which material is susceptibility negative?
Only for diamagnetic materials, where χ lies between 0 and −1. Paramagnetic and ferromagnetic materials have positive χ.
How does susceptibility depend on temperature?
For paramagnetic materials, Curie's law gives χ = C/T, so χ decreases as temperature rises. Diamagnetic χ is almost independent of temperature.
