Why Susceptibility Is Negative for Diamagnetic Materials
Physics · Magnetism And Matter · NEET
Susceptibility is defined as chi = M/H. In a diamagnetic material the induced magnetisation M points OPPOSITE to the applied field H, so their ratio comes out NEGATIVE (small, in the range -1 <= chi < 0). Memory hook: diamagnetic = "against magnetic" — M fights back, so the sign flips to minus.
In a diamagnetic sample the induced magnetisation M points opposite to the applied field H, so chi = M/H is negative (-1 <= chi < 0), and relative permeability mu_r = 1 + chi is slightly less than 1.
Your doubts, answered
Why exactly does M point opposite to H in a diamagnetic material?
When you switch on an external field H, it changes the electron orbital motion inside every atom. By Lenz's law, the atoms set up a tiny induced current that OPPOSES the change, creating a small magnetic moment in the reverse direction. Since M (dipole moment per unit volume) points backward while H points forward, chi = M/H is negative. This effect exists in ALL materials, but in diamagnetics there is no permanent moment to hide it, so the negative value survives.
What does a negative susceptibility physically mean?
A negative chi means the material weakly OPPOSES the field: it develops magnetisation in the reverse direction and is pushed toward regions of weaker field (repelled by a magnet). Positive chi (para/ferro) means the material lines up WITH the field and is attracted. So the sign of chi directly tells you attract vs repel.
What is the exact range of chi for diamagnetic materials?
For ordinary diamagnetics the range is -1 <= chi < 0, a small negative number close to zero (for example about -10^-5). The special limit chi = -1 belongs to a perfect diamagnet like a superconductor (Meissner effect), where the field is completely expelled. NEET 2024 tested this exact bound as '0 > chi >= -1'.
Is diamagnetic susceptibility affected by temperature?
No. Diamagnetism comes from induced orbital currents, not from aligning permanent moments, so diamagnetic chi is essentially independent of temperature. This is the opposite of paramagnetism, where chi falls as temperature rises (Curie's law, chi is proportional to 1/T).
If chi is negative, is relative permeability less than 1?
Yes. Since mu_r = 1 + chi and chi is a small negative number, mu_r is slightly LESS than 1 for diamagnetics (for example 0.99999). That is why the field B = mu_0(1+chi)H inside a diamagnetic is slightly smaller than the applied field — the material weakens the field.
⚠️ The NEET trap ✗ Susceptibility is negative for diamagnetic AND paramagnetic materials because both are weakly magnetic. ✓ chi is negative ONLY for diamagnetic materials. Paramagnetic and ferromagnetic materials have POSITIVE chi (M is parallel to H). 'Weak' does not mean 'negative' — paramagnetics are weakly magnetic but still positive. 🧠 Negative sign = against the field (diamagnetic only). Weak positive = paramagnetic. Don't lump the two weak types together.
Real NEET questions
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 chi = M/H. Step 1: For a diamagnetic material the induced magnetisation M is opposite in direction to the applied field H. Step 2: Opposite directions make the ratio M/H negative, so chi < 0 (range -1 <= chi < 0). Step 3: For paramagnetic and ferromagnetic materials M is parallel to H, so chi is positive. Therefore chi is negative only for diamagnetic materials. Answer: A.
2024
Match List-I (Material) with List-II (Susceptibility chi): A. Diamagnetic, B. Ferromagnetic, C. Paramagnetic, D. Non-magnetic; I. chi = 0, II. 0 > chi >= -1, III. chi >> 1, IV. 0 < chi < e (a small positive number).
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: Step 1: Diamagnetic -> M opposes H -> chi is a small negative number, 0 > chi >= -1, so A-II. Step 2: Ferromagnetic -> very strong alignment -> chi >> 1, so B-III. Step 3: Paramagnetic -> weak alignment with H -> small positive, 0 < chi < e, so C-IV. Step 4: Non-magnetic -> no response -> chi = 0, so D-I. Combination A-II, B-III, C-IV, D-I. Answer: D.
Solved Magnetism And Matter NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Which materials have negative magnetic susceptibility?
Only diamagnetic materials (for example bismuth, copper, water, gold). Paramagnetic and ferromagnetic materials always have positive susceptibility.
What is the value of susceptibility for a diamagnetic material?
A small negative number, typically around -10^-5, lying in the range -1 <= chi < 0. A perfect diamagnet (superconductor) reaches chi = -1.
Why is diamagnetism present in every material?
Because an applied field always distorts electron orbits and, by Lenz's law, induces an opposing moment. In para/ferro materials this weak effect is simply masked by their stronger positive response.
How is negative chi linked to being repelled by a magnet?
Negative chi means induced M opposes H, so the sample is pushed toward weaker-field regions — it is repelled. This is why a diamagnetic rod is pushed OUT of a strong field region.
Does relative permeability become less than 1 for diamagnetics?
Yes. mu_r = 1 + chi, and with chi slightly negative, mu_r is just below 1, so the field inside is slightly weaker than the applied field.
