Superconductor as a Perfect Diamagnet (Meissner Effect)
Physics · Magnetism And Matter · NEET
A superconductor is a perfect diamagnet: below its critical temperature it pushes out all magnetic field from inside, so B = 0 everywhere inside it. This expulsion is called the Meissner effect, and it forces the susceptibility to its most negative possible value, χ = −1, with relative permeability μ_r = 0. Memory hook: "Super = super repel." A superconductor is the strongest diamagnet, so it sits at the extreme edge χ = −1.
Below the critical temperature the superconductor expels the external field. Field lines curve around it instead of passing through, so B = 0 inside — this is the Meissner effect and it forces χ = −1.
Your doubts, answered
Is a superconductor diamagnetic or paramagnetic?
It is diamagnetic, and in fact the strongest possible diamagnet. Diamagnetic materials are repelled by a magnetic field and push field lines out. A superconductor pushes out the field completely, so it is called a perfect diamagnet. It is never paramagnetic (paramagnetic materials pull field lines in, χ is small and positive).
Why is the susceptibility of a superconductor exactly −1?
Inside any material, B = μ0(H + M) and M = χH. For a superconductor the field inside is fully expelled, so B = 0. Setting B = 0 gives H + M = 0, so M = −H, which means χ = M/H = −1. This is the lowest value χ can reach, so a superconductor sits at the extreme end of the diamagnetic range 0 > χ ≥ −1.
What is the difference between a normal diamagnet and a perfect diamagnet?
A normal diamagnet (like bismuth or copper) only weakly opposes the field, so χ is a very small negative number, around −10^-5. A perfect diamagnet (a superconductor) opposes it completely, so χ = −1 and B inside is exactly zero. Both repel a magnet, but the superconductor does it 100 percent.
Does the Meissner effect push field lines inside or outside?
Outside. The Meissner effect means the superconductor expels the magnetic field, so field lines bend and go around the material instead of passing through it. Because no field lines pass inside, B = 0 inside. This is easy to draw: lines curve away from the superconductor, like water flowing around a rock.
What is the permeability of a superconductor?
Zero. Since μ_r = 1 + χ and χ = −1, the relative permeability μ_r = 0, so the absolute permeability μ = μ0·μ_r = 0. A zero permeability means the material lets no magnetic flux through it, which matches B = 0 inside.
Why does a magnet float above a superconductor?
Because the superconductor expels the magnet's field (Meissner effect), it sets up currents that repel the magnet. This repulsion balances gravity, so the magnet levitates. This is a direct visual proof that a superconductor is a perfect diamagnet.
⚠️ The NEET trap ✗ Reading χ = −1 as if it were a small diamagnetic value and matching a superconductor with the ordinary diamagnetic slot in a match-the-column question. ✓ A superconductor is the extreme diamagnet with χ = −1 exactly. Ordinary diamagnetics have χ near −10^-5. In the diamagnetic range 0 > χ ≥ −1, the superconductor is the lower boundary value. 🧠 Superconductor = the diamagnet at the wall: χ can never go below −1.
Real NEET questions
2024
Match List-I with List-II. List-I (Material): A. Diamagnetic, B. Ferromagnetic, C. Paramagnetic, D. Non-magnetic. List-II (Susceptibility χ): I. χ = 0, II. 0 > χ ≥ −1, III. χ >> 1, IV. 0 < χ < ε (a small positive number). Choose the correct answer.
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 materials have negative susceptibility, from a tiny value down to the perfect-diamagnet limit −1, so 0 > χ ≥ −1. This is range II. Note the lower bound −1 is exactly the superconductor (perfect diamagnet). So A → II. Step 2: Ferromagnetic materials have very large positive susceptibility, χ >> 1, which is range III. So B → III. Step 3: Paramagnetic materials have a small positive susceptibility, 0 < χ < ε, which is range IV. So C → IV. Step 4: Non-magnetic (vacuum-like) has χ = 0, which is range I. So D → I. The match is A-II, B-III, C-IV, D-I, which is option D.
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: Step 1: Susceptibility χ = M/H tells how a material responds to a field. Step 2: Paramagnetic and ferromagnetic materials get magnetised along the field, so M is positive and χ is positive. Step 3: Diamagnetic materials get magnetised opposite to the field, so M is negative and χ is negative. A superconductor is the extreme case with χ = −1. Step 4: Only diamagnetic materials have negative χ, so the answer is option A.
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 the complete expulsion of magnetic field from inside a superconductor when it is cooled below its critical temperature, making B = 0 inside.
What are the values of χ and μ_r for a superconductor?
Susceptibility χ = −1 and relative permeability μ_r = 1 + χ = 0. So the absolute permeability μ = 0 as well.
Is the Meissner effect the same as zero resistance?
No. Zero resistance is one property of superconductors; the Meissner effect (perfect diamagnetism, field expulsion) is a separate, additional property. A superconductor shows both below its critical temperature.
Can χ ever be more negative than −1?
No. χ = −1 gives B = 0 inside, which is total expulsion. You cannot expel more than all of the field, so −1 is the lowest possible susceptibility.
Why is this concept important for NEET?
NEET often uses match-the-column and one-liner questions on susceptibility ranges. Knowing the superconductor sits at χ = −1 (the diamagnetic boundary) helps you place the diamagnetic range 0 > χ ≥ −1 correctly.
