Class 12 Physics Magnetism and Matter Notes

5.1 Introduction

Magnetism is the study of magnetic effects of electric currents and materials. This chapter explores magnetic fields, magnetization, and the behavior of materials in a magnetic field, building on concepts of moving charges and magnetic forces.

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5.2 The Bar Magnet

• A bar magnet has two poles: North (N) and South (S).
• Magnetic field lines emerge from the north pole and enter the south pole.
• Properties of a bar magnet:
  • Like poles repel, unlike poles attract.
  • Magnetic field is strongest near the poles.
• A bar magnet behaves like a magnetic dipole, with magnetic moment:

$$\vec{m} = \text{strength} \times \text{length}$$m=strength×length

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5.3 Magnetism and Gauss’s Law

• Magnetic field lines form closed loops, unlike electric field lines.
• Gauss’s law for magnetism states:

$$\oint \vec{B} \cdot d\vec{A} = 0$$∮B⋅dA=0

• This implies no magnetic monopoles exist; every magnetic field has north and south poles.

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5.4 Magnetisation and Magnetic Intensity

• Magnetisation (M⃗\vec{M}M): Magnetic moment per unit volume of a material.
• Magnetic field intensity (H⃗\vec{H}H): Field produced in space by free currents.
• Relationship:

$$\vec{B} = \mu_0 (\vec{H} + \vec{M})$$B=μ0​(H+M)

• SI units:
  • $\vec{M}$M in A/m
  • $\vec{H}$H in A/m

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5.5 Magnetic Properties of Materials

Materials respond differently to magnetic fields:

1. Diamagnetic Materials
   • Weakly repelled by a magnetic field
   • Examples: Bismuth, Copper
2. Paramagnetic Materials
   • Weakly attracted by a magnetic field
   • Examples: Aluminum, Platinum
3. Ferromagnetic Materials
   • Strongly attracted, can retain magnetization
   • Examples: Iron, Cobalt, Nickel

• Ferromagnetic materials are widely used in permanent magnets and transformers.