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Magnetic Materials

Ch. 11Std 12

Easy Overview

Why is iron magnetic but copper isn't? Why does a fridge magnet lose its strength over time? And what makes some materials float above a magnet? This chapter is about how different materials respond to magnetic fields — some love them, some ignore them, and some actively push them away.

Diamagnetism

Diamagnetic materials weakly repel magnetic fields. Water, copper, gold, and bismuth are diamagnetic. When you bring a magnet near them, they create a tiny opposing magnetic field inside. It's like they're saying 'stay away.' The effect is super weak — you won't notice it with a fridge magnet. But with a strong enough magnet, you can make a drop of water levitate. That's right — water floats above a magnet because it's diamagnetic. It looks like magic, but it's just physics.

Paramagnetism

Paramagnetic materials are weakly attracted to magnets. Aluminum, platinum, and oxygen are paramagnetic. Their atoms have unpaired electrons that act like tiny magnets, but they're all oriented randomly. When you bring an external magnet, these tiny atomic magnets try to align with it — like compass needles in a magnetic field. But the effect is weak because thermal jiggling keeps knocking them out of alignment. Cool the material down, and the alignment gets stronger.

Ferromagnetism

Ferromagnetic materials — iron, nickel, cobalt — are the strong ones. They can be permanently magnetized. Why? Because they have domains — microscopic regions where all the atomic magnets are already aligned. In an unmagnetized piece of iron, these domains point in random directions. Apply a magnetic field, and the domains that are already aligned grow, swallowing the misaligned ones. The result is a strong net magnetization. That's why you can magnetize an iron nail by stroking it with a magnet. And why heating it above a certain temperature (Curie temperature) destroys the magnetism — the domains break apart from thermal vibrations.

Hysteresis

Hysteresis is the memory of magnetic materials. When you magnetize a piece of iron and then remove the field, it doesn't return to zero magnetization — it stays partly magnetized. To demagnetize it, you need to apply a field in the opposite direction. Plotting magnetization vs applied field gives you a loop — the hysteresis loop. Materials with wide loops (hard magnets) are used for permanent magnets. Materials with narrow loops (soft magnets) are used for transformer cores — they need to magnetize and demagnetize easily without wasting energy.

Key Points

  • •Diamagnetic materials are repelled by magnets. They have no unpaired electrons. χ < 0.
  • •Paramagnetic materials are weakly attracted. They have unpaired electrons. χ > 0 but small.
  • •Ferromagnetic materials are strongly attracted. They have domains of aligned magnetic moments.
  • •Curie temperature: above this, ferromagnetic materials become paramagnetic.
  • •Hysteresis: the lag between magnetization and applied field. Area of loop = energy lost per cycle.
  • •Soft iron (narrow hysteresis) is used in transformers. Steel (wide hysteresis) is used for permanent magnets.

Practice Questions

  • Distinguish between dia-, para-, and ferromagnetic materials with examples.
  • Explain the domain theory of ferromagnetism. What is Curie temperature?
  • What is hysteresis? Draw and explain the B-H curve for a ferromagnetic material.
  • Why is soft iron used for electromagnets and transformer cores while steel is used for permanent magnets?