Behaviour of Dia, Para & Ferro Materials in a Non-Uniform Magnetic Field
Physics · Magnetism And Matter · NEET
In a non-uniform magnetic field, a diamagnetic material is REPELLED and moves toward the WEAKER field region, while paramagnetic and ferromagnetic materials are ATTRACTED and move toward the STRONGER field region (near the poles). Reason: a diamagnet magnetises opposite to the field (M opposes H), so it is pushed away; para and ferro magnetise along the field, so they are pulled in. Memory hook: "Dia flies, Para/Ferro dive" — dia flies to the weak side, para and ferro dive into the strong side.
In a non-uniform field, paramagnetic/ferromagnetic samples are pulled toward the strong-field region near the pole, while diamagnetic samples are pushed toward the weak-field region.
Your doubts, answered
Does a diamagnetic material move to the strong field or the weak field?
To the WEAK field. A diamagnet magnetises in the direction OPPOSITE to the applied field (M opposes H, so its susceptibility is negative). The induced 'south' faces the approaching magnet, so it is repelled and drifts to where the field is weakest. Example: a bismuth or water sample, or a superconductor, moves away from the poles.
Why do paramagnetic and ferromagnetic materials move toward the strong field?
Because they magnetise ALONG the field (M is parallel to H, susceptibility positive). The near face becomes an opposite pole to the magnet, so they are attracted and pulled to the region of maximum field strength — right up to the poles. Ferromagnets do this very strongly (large positive susceptibility); paramagnets do it weakly.
Is it torque or force that decides this movement?
FORCE, not torque. A UNIFORM field gives zero net force (only a torque that aligns the dipole). A NON-UNIFORM field gives an unbalanced net translational force, so the sample actually moves. That is why the classic NEET wording always says 'non-uniform field' — it is telling you to think about net force and displacement, not rotation.
Why doesn't a magnet attract copper or aluminium the way it attracts iron?
Copper is diamagnetic (very slightly repelled) and aluminium is paramagnetic (very weakly attracted). Both effects are tiny — susceptibility is close to zero — so you cannot feel them by hand. Iron is ferromagnetic with susceptibility in the hundreds, so the pull is huge and obvious.
⚠️ The NEET trap ✗ In a non-uniform field a diamagnetic material moves toward the stronger field, just weakly. ✓ A diamagnetic material moves toward the WEAKER field — it is repelled. Only paramagnetic and ferromagnetic materials move toward the stronger field. 🧠 Sign of susceptibility decides direction: negative (dia) = pushed to weak side; positive (para/ferro) = pulled to strong side. Never let 'weak effect' trick you into 'moves to strong field'.
Real NEET questions
2018
A thin diamagnetic rod is placed vertically between the poles of an electromagnet. When the current is switched on, the diamagnetic rod is pushed up, out of the horizontal magnetic field, gaining gravitational potential energy. The work required to do this comes from:
A · the lattice structure of the material of the rod
B · the magnetic field
C · the current source ✓
D · the induced electric field due to the changing magnetic field
Solution: Step 1: A diamagnetic material is repelled from the strong-field region (between the poles) toward the weaker field, so the rod is pushed up and out. Step 2: The magnetic force on charges does no net work (F is perpendicular to velocity), so the field itself cannot supply the gravitational PE. Step 3: As the rod moves, it changes the flux linkage of the electromagnet; to keep the current (and hence the field) steady against this back-reaction, the current source must deliver extra energy. Therefore the work that lifts the rod comes from the current source. Answer: (C).
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: Step 1: Diamagnetic has small NEGATIVE susceptibility, 0 > χ ≥ −1 → (II); this negative sign is exactly why it is repelled to the weak-field side. Step 2: Ferromagnetic has very large positive susceptibility, χ >> 1 → (III); strongly attracted to strong field. Step 3: Paramagnetic has small positive susceptibility, 0 < χ < ε → (IV); weakly attracted. Step 4: Non-magnetic has χ = 0 → (I). Matching gives 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.
In one line, how do the three materials behave in a non-uniform field?
Diamagnetic is repelled and goes to the weak-field region; paramagnetic and ferromagnetic are attracted and go to the strong-field region (near the poles).
Which property decides the direction of motion?
The SIGN of magnetic susceptibility. Negative (diamagnetic) means pushed to weak field; positive (paramagnetic and ferromagnetic) means pulled to strong field.
What happens in a uniform field instead?
In a uniform field the net force is zero, so none of the materials move translationally. There is only a torque that tends to align the sample. Motion toward a pole needs a NON-uniform field.
Is a superconductor an exception?
No, it follows the diamagnetic rule but in the extreme. A superconductor is a perfect diamagnet (χ = −1, Meissner effect), so it is strongly repelled and expelled from the field completely.
